[PATCH v5] Documentation: Add section about CPU vulnerabilities for Spectre

[Tim Chen]

Add documentation for Spectre vulnerability and the mitigation mechanisms:

- Explain the problem and risks
- Document the mitigation mechanisms
- Document the command line controls
- Document the sysfs files

Co-developed-by: Andi Kleen <ak@xxxxxxxxxxxxxxx>
Signed-off-by: Andi Kleen <ak@xxxxxxxxxxxxxxx>
Co-developed-by: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
Signed-off-by: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
Reviewed-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
---
 Documentation/admin-guide/hw-vuln/index.rst   |   1 +
 Documentation/admin-guide/hw-vuln/spectre.rst | 697 ++++++++++++++++++
 Documentation/userspace-api/spec_ctrl.rst     |   2 +
 3 files changed, 700 insertions(+)
 create mode 100644 Documentation/admin-guide/hw-vuln/spectre.rst

diff --git a/Documentation/admin-guide/hw-vuln/index.rst b/Documentation/admin-guide/hw-vuln/index.rst
index ffc064c1ec68..49311f3da6f2 100644
--- a/Documentation/admin-guide/hw-vuln/index.rst
+++ b/Documentation/admin-guide/hw-vuln/index.rst
@@ -9,5 +9,6 @@ are configurable at compile, boot or run time.
 .. toctree::
    :maxdepth: 1
 
+   spectre
    l1tf
    mds
diff --git a/Documentation/admin-guide/hw-vuln/spectre.rst b/Documentation/admin-guide/hw-vuln/spectre.rst
new file mode 100644
index 000000000000..42936f223a53
--- /dev/null
+++ b/Documentation/admin-guide/hw-vuln/spectre.rst
@@ -0,0 +1,697 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+Spectre Side Channels
+=====================
+
+Spectre is a class of side channel attacks that exploit branch prediction
+and speculative execution on modern CPUs to read memory, possibly
+bypassing access controls. Speculative execution side channel exploits
+do not modify memory but attempt to infer privileged data in the memory.
+
+This document covers Spectre variant 1 and Spectre variant 2.
+
+Affected processors
+-------------------
+
+Speculative execution side channel methods affect a wide range of modern
+high performance processors, since most modern high speed processors
+use branch prediction and speculative execution.
+
+The following CPUs are vulnerable:
+
+    - Intel Core, Atom, Pentium, and Xeon processors
+
+    - AMD Phenom, EPYC, and Zen processors
+
+    - IBM POWER and zSeries processors
+
+    - Higher end ARM processors
+
+    - Apple CPUs
+
+    - Higher end MIPS CPUs
+
+    - Likely most other high performance CPUs. Contact your CPU vendor for details.
+
+Whether a processor is affected or not can be read out from the Spectre
+vulnerability files in sysfs. See :ref:`spectre_sys_info`.
+
+Related CVEs
+------------
+
+The following CVE entries describe Spectre variants:
+
+   =============   =======================  =================
+   CVE-2017-5753   Bounds check bypass      Spectre variant 1
+   CVE-2017-5715   Branch target injection  Spectre variant 2
+   =============   =======================  =================
+
+Problem
+-------
+
+CPUs use speculative operations to improve performance. That may leave
+traces of memory accesses or computations in the processor's caches,
+buffers, and branch predictors. Malicious software may be able to
+influence the speculative execution paths, and then use the side effects
+of the speculative execution in the CPUs' caches and buffers to infer
+privileged data touched during the speculative execution.
+
+Spectre variant 1 attacks take advantage of speculative execution of
+conditional branches, while Spectre variant 2 attacks use speculative
+execution of indirect branches to leak privileged memory.
+See :ref:`[1] <spec_ref1>` :ref:`[5] <spec_ref5>` :ref:`[7] <spec_ref7>`
+:ref:`[10] <spec_ref10>` :ref:`[11] <spec_ref11>`.
+
+Spectre variant 1 (Bounds Check Bypass)
+---------------------------------------
+
+The bounds check bypass attack :ref:`[2] <spec_ref2>` takes advantage
+of speculative execution that bypass conditional branch instructions
+used for memory access bounds check (e.g. checking if the index of an
+array results in memory access within a valid range). This results in
+memory accesses to invalid memory (with out-of-bound index) that are
+done speculatively before validation checks resolve. Such speculative
+memory accesses can leave side effects, creating side channels which
+leak information to the attacker.
+
+There are some extensions of Spectre variant 1 attacks for reading data
+over the network, see :ref:`[12] <spec_ref12>`. However such attacks
+are difficult, low bandwidth, fragile, and are considered low risk.
+
+Spectre variant 2 (Branch Target Injection)
+-------------------------------------------
+
+The branch target injection attack takes advantage of speculative
+execution of indirect branches :ref:`[3] <spec_ref3>`.  The indirect
+branch predictors inside the processor used to guess the target of
+indirect branches can be influenced by an attacker, causing gadget code
+to be speculatively executed, thus exposing sensitive data touched by
+the victim. The side effects left in the CPU's caches during speculative
+execution can be measured to infer data values.
+
+.. _poison_btb:
+
+In Spectre variant 2 attacks, the attacker can steer speculative indirect
+branches in the victim to gadget code by poisoning the branch target
+buffer of a CPU used for predicting indirect branch addresses. Such
+poisoning could be done by indirect branching into existing code,
+with the address offset of the indirect branch under the attacker's
+control. Since the branch prediction on impacted hardware does not
+fully disambiguate branch address and uses the offset for prediction,
+this could cause privileged code's indirect branch to jump to a gadget
+code with the same offset.
+
+The most useful gadgets take an attacker-controlled input parameter (such
+as a register value) so that the memory read can be controlled. Gadgets
+without input parameters might be possible, but the attacker would have
+very little control over what memory can be read, reducing the risk of
+the attack revealing useful data.
+
+One other variant 2 attack vector is for the attacker to poison the
+return stack buffer (RSB) :ref:`[13] <spec_ref13>` to cause speculative
+subroutine return instruction execution to go to an gadget.  An attacker's
+imbalanced subroutine call instructions might "poison" entries in the
+return stack buffer which are later consumed by a victim's subroutine
+return instructions.  This attack can be mitigated by flushing the return
+stack buffer on context switch, or virtual machine (VM) exit.
+
+On systems with simultaneous multi-threading (SMT), attacks are possible
+from from the sibling thread, as level 1 cache and branch target buffer
+(BTB) may be shared between hardware threads in a CPU core.  A malicious
+program running on the sibling thread may influence its peer's BTB to
+steer its indirect branch speculations to gadget code, and measure the
+speculative execution's side effects left in level 1 cache to infer the
+victim's data.
+
+Attack scenarios
+----------------
+
+The following list of attack scenarios have been anticipated, but may
+not cover all possible attack vectors.
+
+1. A user process attacking the kernel
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+   The attacker passes a parameter to the kernel via a register or
+   via a known address in memory during a syscall. Such parameter may
+   be used later by the kernel as an index to an array or to derive
+   a pointer for a Spectre variant 1 attack.  The index or pointer
+   is invalid, but bound checks are bypassed in the code branch taken
+   for speculative execution. This could cause privileged memory to be
+   accessed and leaked.
+
+   For kernel code that has been identified where data pointers could
+   potentially be influenced for Spectre attacks, new "nospec" accessor
+   macros are used to prevent speculative loading of data.
+
+   Spectre variant 2 attacker can :ref:`poison <poison_btb>` the branch
+   target buffer (BTB) before issuing syscall to launch an attack.
+   After entering the kernel, the kernel could use the poisoned branch
+   target buffer on indirect jump and jump to gadget code in speculative
+   execution.
+
+   If an attacker tries to control the memory addresses leaked during
+   speculative execution, he would also need to pass a parameter to the
+   gadget, either through a register or a known address in memory. After
+   the gadget has executed, he can measure the side effect.
+
+   The kernel can protect itself against consuming poisoned branch
+   target buffer entries by using return trampolines (also known as
+   "retpoline") :ref:`[3] <spec_ref3>` :ref:`[9] <spec_ref9>` for all
+   indirect branches. Return trampolines trap speculative execution paths
+   to prevent jumping to gadget code during speculative execution.
+   x86 CPUs with Enhanced Indirect Branch Restricted Speculation
+   (Enhanced IBRS) available in hardware should use the feature to
+   mitigate Spectre variant 2 instead of retpoline. Enhanced IBRS is
+   more efficient than retpoline.
+
+   There may be gadget code in firmware which could be exploited with
+   Spectre variant 2 attack by a rogue user process. To mitigate such
+   attacks on x86, Indirect Branch Restricted Speculation (IBRS) feature
+   is turned on before the kernel invokes any firmware code.
+
+2. A user process attacking another user process
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+   A malicious user process can try to attack another user process,
+   either via a context switch on the same hardware thread, or from the
+   sibling hyperthread sharing a physical processor core on simultaneous
+   multi-threading (SMT) system.
+
+   Spectre variant 1 attacks generally require passing parameters
+   between the processes, which needs a data passing relationship, such
+   as remote procedure calls (RPC).  Those parameters are used in gadget
+   code to derive invalid data pointers accessing privileged memory in
+   the attacked process.
+
+   Spectre variant 2 attacks can be launched from a rogue process by
+   :ref:`poisoning <poison_btb>` the branch target buffer.  This can
+   influence the indirect branch targets for a victim process that either
+   runs later on the same hardware thread, or running concurrently on
+   a sibling hardware thread sharing the same physical core.
+
+   A user process can protect itself against Spectre variant 2 attacks
+   by using the prctl() syscall to disable indirect branch speculation
+   for itself.  An administrator can also cordon off an unsafe process
+   from polluting the branch target buffer by disabling the process's
+   indirect branch speculation. This comes with a performance cost
+   from not using indirect branch speculation and clearing the branch
+   target buffer.  When SMT is enabled on x86, for a process that has
+   indirect branch speculation disabled, Single Threaded Indirect Branch
+   Predictors (STIBP) :ref:`[4] <spec_ref4>` are turned on to prevent the
+   sibling thread from controlling branch target buffer.  In addition,
+   the Indirect Branch Prediction Barrier (IBPB) is issued to clear the
+   branch target buffer when context switching to and from such process.
+
+   On x86, the return stack buffer is stuffed on context switch.
+   This prevents the branch target buffer from being used for branch
+   prediction when the return stack buffer underflows while switching to
+   a deeper call stack. Any poisoned entries in the return stack buffer
+   left by the previous process will also be cleared.
+
+   User programs should use address space randomization to make attacks
+   more difficult (Set /proc/sys/kernel/randomize_va_space = 1 or 2).
+
+3. A virtualized guest attacking the host
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+   The attack mechanism is similar to how user processes attack the
+   kernel.  The kernel is entered via hyper-calls or other virtualization
+   exit paths.
+
+   For Spectre variant 1 attacks, rogue guests can pass parameters
+   (e.g. in registers) via hyper-calls to derive invalid pointers to
+   speculate into privileged memory after entering the kernel.  For places
+   where such kernel code has been identified, nospec accessor macros
+   are used to stop speculative memory access.
+
+   For Spectre variant 2 attacks, rogue guests can :ref:`poison
+   <poison_btb>` the branch target buffer or return stack buffer, causing
+   the kernel to jump to gadget code in the speculative execution paths.
+
+   To mitigate variant 2, the host kernel can use return trampolines
+   for indirect branches to bypass the poisoned branch target buffer,
+   and flushing the return stack buffer on VM exit.  This prevents rogue
+   guests from affecting indirect branching in the host kernel.
+
+   To protect host processes from rogue guests, host processes can have
+   indirect branch speculation disabled via prctl().  The branch target
+   buffer is cleared before context switching to such processes.
+
+4. A virtualized guest attacking other guest
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+   A rogue guest may attack another guest to get data accessible by the
+   other guest.
+
+   Spectre variant 1 attacks are possible if parameters can be passed
+   between guests.  This may be done via mechanisms such as shared memory
+   or message passing.  Such parameters could be used to derive data
+   pointers to privileged data in guest.  The privileged data could be
+   accessed by gadget code in the victim's speculation paths.
+
+   Spectre variant 2 attacks can be launched from a rogue guest by
+   :ref:`poisoning <poison_btb>` the branch target buffer or the return
+   stack buffer. Such poisoned entries could be used to influence
+   speculation execution paths in the victim guest.
+
+   Linux kernel mitigates attacks to other guests running in the same
+   CPU hardware thread by flushing the return stack buffer on VM exit,
+   and clearing the branch target buffer before switching to a new guest.
+
+   If SMT is used, Spectre variant 2 attacks from an untrusted guest
+   in the sibling hyperthread can be mitigated by the administrator,
+   by turning off the unsafe guest's indirect branch speculation via
+   prctl().  A guest can also protect itself by turning on microcode
+   based mitigations (such as IBPB or STIBP on x86) within the guest.
+
+.. _spectre_sys_info:
+
+Spectre system information
+--------------------------
+
+The Linux kernel provides a sysfs interface to enumerate the current
+mitigation status of the system for Spectre: whether the system is
+vulnerable, and which mitigations are active.
+
+The sysfs file showing Spectre variant 1 mitigation status is:
+
+   /sys/devices/system/cpu/vulnerabilities/spectre_v1
+
+The possible values in this file are:
+
+  =======================================  =================================
+  'Mitigation: __user pointer sanitation'  Protection in kernel on a case by
+                                           case base with explicit pointer
+                                           sanitation.
+  =======================================  =================================
+
+However, the protections are put in place on a case by case basis,
+and there is no guarantee that all possible attack vectors for Spectre
+variant 1 are covered.
+
+The spectre_v2 kernel file reports if the kernel has been compiled with
+retpoline mitigation or if the CPU has hardware mitigation, and if the
+CPU has support for additional process-specific mitigation.
+
+This file also reports CPU features enabled by microcode to mitigate
+attack between user processes:
+
+1. Indirect Branch Prediction Barrier (IBPB) to add additional
+   isolation between processes of different users.
+2. Single Thread Indirect Branch Predictors (STIBP) to add additional
+   isolation between CPU threads running on the same core.
+
+These CPU features may impact performance when used and can be enabled
+per process on a case-by-case base.
+
+The sysfs file showing Spectre variant 2 mitigation status is:
+
+   /sys/devices/system/cpu/vulnerabilities/spectre_v2
+
+The possible values in this file are:
+
+  - Kernel status:
+
+  ====================================  =================================
+  'Not affected'                        The processor is not vulnerable
+  'Vulnerable'                          Vulnerable, no mitigation
+  'Mitigation: Full generic retpoline'  Software-focused mitigation
+  'Mitigation: Full AMD retpoline'      AMD-specific software mitigation
+  'Mitigation: Enhanced IBRS'           Hardware-focused mitigation
+  ====================================  =================================
+
+  - Firmware status: Show if Indirect Branch Restricted Speculation (IBRS) is
+    used to protect against Spectre variant 2 attacks when calling firmware (x86 only).
+
+  ========== =============================================================
+  'IBRS_FW'  Protection against user program attacks when calling firmware
+  ========== =============================================================
+
+  - Indirect branch prediction barrier (IBPB) status for protection between
+    processes of different users. This feature can be controlled through
+    prctl() per process, or through kernel command line options. This is
+    an x86 only feature. For more details see below.
+
+  ===================   ========================================================
+  'IBPB: disabled'      IBPB unused
+  'IBPB: always-on'     Use IBPB on all tasks
+  'IBPB: conditional'   Use IBPB on SECCOMP or indirect branch restricted tasks
+  ===================   ========================================================
+
+  - Single threaded indirect branch prediction (STIBP) status for protection
+    between different hyper threads. This feature can be controlled through
+    prctl per process, or through kernel command line options. This is x86
+    only feature. For more details see below.
+
+  ====================  ========================================================
+  'STIBP: disabled'     STIBP unused
+  'STIBP: forced'       Use STIBP on all tasks
+  'STIBP: conditional'  Use STIBP on SECCOMP or indirect branch restricted tasks
+  ====================  ========================================================
+
+  - Return stack buffer (RSB) protection status:
+
+  =============   ===========================================
+  'RSB filling'   Protection of RSB on context switch enabled
+  =============   ===========================================
+
+Full mitigation might require an microcode update from the CPU
+vendor. When the necessary microcode is not available, the kernel will
+report vulnerability.
+
+Turning on mitigation for Spectre variant 1 and Spectre variant 2
+-----------------------------------------------------------------
+
+1. Kernel mitigation
+^^^^^^^^^^^^^^^^^^^^
+
+   For the Spectre variant 1, vulnerable kernel code (as determined
+   by code audit or scanning tools) are annotated on a case by case
+   basis to use nospec accessor macros for bounds clipping :ref:`[2]
+   <spec_ref2>` to avoid any usable disclosure gadgets. However, it may
+   not cover all attack vectors for Spectre variant 1.
+
+   For Spectre variant 2 mitigation, the compiler turns indirect calls or
+   jumps in the kernel into equivalent return trampolines (retpolines)
+   :ref:`[3] <spec_ref3>` :ref:`[9] <spec_ref9>` to go to the target
+   addresses.  Speculative execution paths under retpolines are trapped
+   in an infinite loop to prevent any speculative execution jumping to
+   a gadget.
+
+   To turn on retpoline mitigation on a vulnerable CPU, the kernel
+   needs to be compiled with a gcc compiler that supports the
+   -mindirect-branch=thunk-extern -mindirect-branch-register options.
+   If the kernel is compiled with a Clang compiler, the compiler needs
+   to support -mretpoline-external-thunk option.  The kernel config
+   CONFIG_RETPOLINE needs to be turned on, and the CPU needs to run with
+   the latest updated microcode.
+
+   On Intel Skylake-era systems the mitigation covers most, but not all,
+   cases. See :ref:`[3] <spec_ref3>` for more details.
+
+   On CPUs with hardware mitigation for Spectre variant 2 (e.g. Enhanced
+   IBRS on x86), retpoline is automatically disabled at run time.
+
+   The retpoline mitigation is turned on by default on vulnerable
+   CPUs. It can be forced on or off by the administrator
+   via the kernel command line and sysfs control files. See
+   :ref:`spectre_mitigation_control_command_line`.
+
+   On x86, indirect branch restricted speculation is turned on by default
+   before invoking any firmware code to prevent Spectre variant 2 exploits
+   using the firmware.
+
+   Using kernel address space randomization (CONFIG_RANDOMIZE_SLAB=y
+   and CONFIG_SLAB_FREELIST_RANDOM=y in the kernel configuration) makes
+   attacks on the kernel generally more difficult.
+
+2. User program mitigation
+^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+   User programs can mitigate Spectre variant 1 using LFENCE or "bounds
+   clipping". For more details see :ref:`[2] <spec_ref2>`.
+
+   For Spectre variant 2 mitigation, individual user programs
+   can be compiled with return trampolines for indirect branches.
+   This protects them from consuming poisoned entries in the branch
+   target buffer left by malicious software.  Alternatively, the
+   programs can disable their indirect branch speculation via prctl()
+   (See :ref:`Documentation/userspace-api/spec_ctrl.rst <set_spec_ctrl>`)
+   On x86, this will turn on STIBP to guard against attacks from the
+   sibling thread when the user program is running, and use IBPB to
+   flush the branch target buffer when switching to/from the program.
+
+   Restricting indirect branch speculation on a user program will
+   also prevent the program from launching a variant 2 attack
+   on x86.  All sand-boxed SECCOMP programs have indirect branch
+   speculation restricted by default.  Administrators can change
+   that behavior via the kernel command line and sysfs control files.
+   See :ref:`spectre_mitigation_control_command_line`.
+
+   Programs that disable their indirect branch speculation will have
+   more overheads and run slower.
+
+   User programs should use address space randomization
+   (/proc/sys/kernel/randomize_va_space = 1 or 2) to make attacks more
+   difficult.
+
+3. VM mitigation
+^^^^^^^^^^^^^^^^
+
+   Within the kernel, Spectre variant 1 attacks from rogue guests are
+   mitigated on a case by case basis in VM exit paths. Vulnerable code
+   uses nospec accessor macros for "bounds clipping", to avoid any
+   usable disclosure gadgets.  However, this may not cover all variant
+   1 attack vectors.
+
+   For Spectre variant 2 attacks from rogue guests to the kernel, the
+   Linux kernel uses retpoline or Enhanced IBRS to prevent consumption of
+   poisoned entries in branch target buffer left by rogue guests.  It also
+   flushes the return stack buffer on every VM exit to prevent a return
+   stack buffer underflow so poisoned branch target buffer could be used,
+   or attacker guests leaving poisoned entries in the return stack buffer.
+
+   To mitigate guest-to-guest attacks in the same CPU hardware thread,
+   the branch target buffer is sanitized by flushing before switching
+   to a new guest on a CPU.
+
+   The above mitigations are turned on by default on vulnerable CPUs.
+
+   To mitigate guest-to-guest attacks from sibling thread when SMT is
+   in use, an untrusted guest running in the sibling thread can have
+   its indirect branch speculation disabled by administrator via prctl().
+
+   The kernel also allows guests to use any microcode based mitigation
+   they chose to use (such as IBPB or STIBP on x86) to protect themselves.
+
+.. _spectre_mitigation_control_command_line:
+
+Mitigation control on the kernel command line
+---------------------------------------------
+
+Spectre variant 2 mitigation can be disabled or force enabled at the
+kernel command line.
+
+	nospectre_v2
+
+		[X86] Disable all mitigations for the Spectre variant 2
+		(indirect branch prediction) vulnerability. System may
+		allow data leaks with this option, which is equivalent
+		to spectre_v2=off.
+
+
+        spectre_v2=
+
+		[X86] Control mitigation of Spectre variant 2
+		(indirect branch speculation) vulnerability.
+		The default operation protects the kernel from
+		user space attacks.
+
+		on
+			unconditionally enable, implies
+			spectre_v2_user=on
+		off
+			unconditionally disable, implies
+		        spectre_v2_user=off
+		auto
+			kernel detects whether your CPU model is
+		        vulnerable
+
+		Selecting 'on' will, and 'auto' may, choose a
+		mitigation method at run time according to the
+		CPU, the available microcode, the setting of the
+		CONFIG_RETPOLINE configuration option, and the
+		compiler with which the kernel was built.
+
+		Selecting 'on' will also enable the mitigation
+		against user space to user space task attacks.
+
+		Selecting 'off' will disable both the kernel and
+		the user space protections.
+
+		Specific mitigations can also be selected manually:
+
+		retpoline
+					replace indirect branches
+		retpoline,generic
+					google's original retpoline
+		retpoline,amd
+					AMD-specific minimal thunk
+
+		Not specifying this option is equivalent to
+		spectre_v2=auto.
+
+For user space mitigation:
+
+        spectre_v2_user=
+
+		[X86] Control mitigation of Spectre variant 2
+		(indirect branch speculation) vulnerability between
+		user space tasks
+
+		on
+			Unconditionally enable mitigations. Is
+			enforced by spectre_v2=on
+
+		off
+			Unconditionally disable mitigations. Is
+			enforced by spectre_v2=off
+
+		prctl
+			Indirect branch speculation is enabled,
+			but mitigation can be enabled via prctl
+			per thread. The mitigation control state
+			is inherited on fork.
+
+		prctl,ibpb
+			Like "prctl" above, but only STIBP is
+			controlled per thread. IBPB is issued
+			always when switching between different user
+			space processes.
+
+		seccomp
+			Same as "prctl" above, but all seccomp
+			threads will enable the mitigation unless
+			they explicitly opt out.
+
+		seccomp,ibpb
+			Like "seccomp" above, but only STIBP is
+			controlled per thread. IBPB is issued
+			always when switching between different
+			user space processes.
+
+		auto
+			Kernel selects the mitigation depending on
+			the available CPU features and vulnerability.
+
+		Default mitigation:
+		If CONFIG_SECCOMP=y then "seccomp", otherwise "prctl"
+
+		Not specifying this option is equivalent to
+		spectre_v2_user=auto.
+
+		In general the kernel by default selects
+		reasonable mitigations for the current CPU. To
+		disable Spectre variant 2 mitigations boot with
+		spectre_v2=off. Spectre variant 1 mitigations
+		cannot be disabled.
+
+Mitigation selection guide
+--------------------------
+
+1. Trusted userspace
+^^^^^^^^^^^^^^^^^^^^
+
+   If all userspace applications are from trusted sources and do not
+   execute externally supplied untrusted code, then the mitigations can
+   be disabled.
+
+2. Protect sensitive programs
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+   For security-sensitive programs that have secrets (e.g. crypto
+   keys), protection against Spectre variant 2 can be put in place by
+   disabling indirect branch speculation when the program is running
+   (See :ref:`Documentation/userspace-api/spec_ctrl.rst <set_spec_ctrl>`).
+
+3. Sandbox untrusted programs
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+   Untrusted programs that could be a source of attacks can be cordoned
+   off by disabling their indirect branch speculation when they are run
+   (See :ref:`Documentation/userspace-api/spec_ctrl.rst <set_spec_ctrl>`).
+   This prevents untrusted programs from polluting the branch target
+   buffer.  All programs running in SECCOMP sandboxes have indirect
+   branch speculation restricted by default. This behavior can be
+   changed via the kernel command line and sysfs control files. See
+   :ref:`spectre_mitigation_control_command_line`.
+
+3. High security mode
+^^^^^^^^^^^^^^^^^^^^^
+
+   All Spectre variant 2 mitigations can be forced on
+   at boot time for all programs (See the "on" option in
+   :ref:`spectre_mitigation_control_command_line`).  This will add
+   overhead as indirect branch speculations for all programs will be
+   restricted.
+
+   On x86, branch target buffer will be flushed with IBPB when switching
+   to a new program. STIBP is left on all the time to protect programs
+   against variant 2 attacks originating from programs running on
+   sibling threads.
+
+   Alternatively, STIBP can be used only when running programs
+   whose indirect branch speculation is explicitly disabled,
+   while IBPB is still used all the time when switching to a new
+   program to clear the branch target buffer (See "ibpb" option in
+   :ref:`spectre_mitigation_control_command_line`).  This "ibpb" option
+   has less performance cost than the "on" option, which leaves STIBP
+   on all the time.
+
+References on Spectre
+---------------------
+
+Intel white papers:
+
+.. _spec_ref1:
+
+[1] `Intel analysis of speculative execution side channels <https://newsroom.intel.com/wp-content/uploads/sites/11/2018/01/Intel-Analysis-of-Speculative-Execution-Side-Channels.pdf>`_.
+
+.. _spec_ref2:
+
+[2] `Bounds check bypass <https://software.intel.com/security-software-guidance/software-guidance/bounds-check-bypass>`_.
+
+.. _spec_ref3:
+
+[3] `Deep dive: Retpoline: A branch target injection mitigation <https://software.intel.com/security-software-guidance/insights/deep-dive-retpoline-branch-target-injection-mitigation>`_.
+
+.. _spec_ref4:
+
+[4] `Deep Dive: Single Thread Indirect Branch Predictors <https://software.intel.com/security-software-guidance/insights/deep-dive-single-thread-indirect-branch-predictors>`_.
+
+AMD white papers:
+
+.. _spec_ref5:
+
+[5] `AMD64 technology indirect branch control extension <https://developer.amd.com/wp-content/resources/Architecture_Guidelines_Update_Indirect_Branch_Control.pdf>`_.
+
+.. _spec_ref6:
+
+[6] `Software techniques for managing speculation on AMD processors <https://developer.amd.com/wp-content/resources/90343-B_SoftwareTechniquesforManagingSpeculation_WP_7-18Update_FNL.pdf>`_.
+
+ARM white papers:
+
+.. _spec_ref7:
+
+[7] `Cache speculation side-channels <https://developer.arm.com/support/arm-security-updates/speculative-processor-vulnerability/download-the-whitepaper>`_.
+
+.. _spec_ref8:
+
+[8] `Cache speculation issues update <https://developer.arm.com/support/arm-security-updates/speculative-processor-vulnerability/latest-updates/cache-speculation-issues-update>`_.
+
+Google white paper:
+
+.. _spec_ref9:
+
+[9] `Retpoline: a software construct for preventing branch-target-injection <https://support.google.com/faqs/answer/7625886>`_.
+
+MIPS white paper:
+
+.. _spec_ref10:
+
+[10] `MIPS: response on speculative execution and side channel vulnerabilities <https://www.mips.com/blog/mips-response-on-speculative-execution-and-side-channel-vulnerabilities/>`_.
+
+Academic papers:
+
+.. _spec_ref11:
+
+[11] `Spectre Attacks: Exploiting Speculative Execution <https://spectreattack.com/spectre.pdf>`_.
+
+.. _spec_ref12:
+
+[12] `NetSpectre: Read Arbitrary Memory over Network <https://arxiv.org/abs/1807.10535>`_.
+
+.. _spec_ref13:
+
+[13] `Spectre Returns! Speculation Attacks using the Return Stack Buffer <https://www.usenix.org/system/files/conference/woot18/woot18-paper-koruyeh.pdf>`_.
diff --git a/Documentation/userspace-api/spec_ctrl.rst b/Documentation/userspace-api/spec_ctrl.rst
index 1129c7550a48..7ddd8f667459 100644
--- a/Documentation/userspace-api/spec_ctrl.rst
+++ b/Documentation/userspace-api/spec_ctrl.rst
@@ -49,6 +49,8 @@ If PR_SPEC_PRCTL is set, then the per-task control of the mitigation is
 available. If not set, prctl(PR_SET_SPECULATION_CTRL) for the speculation
 misfeature will fail.
 
+.. _set_spec_ctrl:
+
 PR_SET_SPECULATION_CTRL
 -----------------------
 
-- 
2.20.1