[PATCH v6 32/33] riscv: Documentation for shadow stack on riscv

[Deepak Gupta]

Adding documentation on shadow stack for user mode on riscv and kernel
interfaces exposed so that user tasks can enable it.

Signed-off-by: Deepak Gupta <debug@xxxxxxxxxxxx>
---
 Documentation/arch/riscv/index.rst   |   1 +
 Documentation/arch/riscv/zicfiss.rst | 176 +++++++++++++++++++++++++++++++++++
 2 files changed, 177 insertions(+)

diff --git a/Documentation/arch/riscv/index.rst b/Documentation/arch/riscv/index.rst
index be7237b69682..e240eb0ceb70 100644
--- a/Documentation/arch/riscv/index.rst
+++ b/Documentation/arch/riscv/index.rst
@@ -15,6 +15,7 @@ RISC-V architecture
     vector
     cmodx
     zicfilp
+    zicfiss
 
     features
 
diff --git a/Documentation/arch/riscv/zicfiss.rst b/Documentation/arch/riscv/zicfiss.rst
new file mode 100644
index 000000000000..5ba389f15b3f
--- /dev/null
+++ b/Documentation/arch/riscv/zicfiss.rst
@@ -0,0 +1,176 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+:Author: Deepak Gupta <debug@xxxxxxxxxxxx>
+:Date:   12 January 2024
+
+=========================================================
+Shadow stack to protect function returns on RISC-V Linux
+=========================================================
+
+This document briefly describes the interface provided to userspace by Linux
+to enable shadow stack for user mode applications on RISV-V
+
+1. Feature Overview
+--------------------
+
+Memory corruption issues usually result in to crashes, however when in hands of
+an adversary and if used creatively can result into variety security issues.
+
+One of those security issues can be code re-use attacks on program where
+adversary can use corrupt return addresses present on stack and chain them
+together to perform return oriented programming (ROP) and thus compromising
+control flow integrity (CFI) of the program.
+
+Return addresses live on stack and thus in read-write memory and thus are
+susceptible to corruption and allows an adversary to reach any program counter
+(PC) in address space. On RISC-V ``zicfiss`` extension provides an alternate
+stack termed as shadow stack on which return addresses can be safely placed in
+prolog of the function and retrieved in epilog. ``zicfiss`` extension makes
+following changes:
+
+- PTE encodings for shadow stack virtual memory
+  An earlier reserved encoding in first stage translation i.e.
+  PTE.R=0, PTE.W=1, PTE.X=0  becomes PTE encoding for shadow stack pages.
+
+- ``sspush x1/x5`` instruction pushes (stores) ``x1/x5`` to shadow stack.
+
+- ``sspopchk x1/x5`` instruction pops (loads) from shadow stack and compares
+  with ``x1/x5`` and if un-equal, CPU raises ``software check exception`` with
+  ``*tval = 3``
+
+Compiler toolchain makes sure that function prologue have ``sspush x1/x5`` to
+save return address on shadow stack in addition to regular stack. Similarly
+function epilogs have ``ld x5, offset(x2)`` followed by ``sspopchk x5`` to
+ensure that popped value from regular stack matches with popped value from
+shadow stack.
+
+2. Shadow stack protections and linux memory manager
+-----------------------------------------------------
+
+As mentioned earlier, shadow stack get new page table encodings and thus have
+some special properties assigned to them and instructions that operate on them
+as below:
+
+- Regular stores to shadow stack memory raises access store faults. This way
+  shadow stack memory is protected from stray inadvertant writes.
+
+- Regular loads to shadow stack memory are allowed. This allows stack trace
+  utilities or backtrace functions to read true callstack (not tampered).
+
+- Only shadow stack instructions can generate shadow stack load or shadow stack
+  store.
+
+- Shadow stack load / shadow stack store on read-only memory raises AMO/store
+  page fault. Thus both ``sspush x1/x5`` and ``sspopchk x1/x5`` will raise AMO/
+  store page fault. This simplies COW handling in kernel During fork, kernel
+  can convert shadow stack pages into read-only memory (as it does for regular
+  read-write memory) and as soon as subsequent ``sspush`` or ``sspopchk`` in
+  userspace is encountered, then kernel can perform COW.
+
+- Shadow stack load / shadow stack store on read-write, read-write-execute
+  memory raises an access fault. This is a fatal condition because shadow stack
+  should never be operating on read-write, read-write-execute memory.
+
+3. ELF and psABI
+-----------------
+
+Toolchain sets up :c:macro:`GNU_PROPERTY_RISCV_FEATURE_1_BCFI` for property
+:c:macro:`GNU_PROPERTY_RISCV_FEATURE_1_AND` in notes section of the object file.
+
+4. Linux enabling
+------------------
+
+User space programs can have multiple shared objects loaded in its address space
+and it's a difficult task to make sure all the dependencies have been compiled
+with support of shadow stack. Thus it's left to dynamic loader to enable
+shadow stack for the program.
+
+5. prctl() enabling
+--------------------
+
+:c:macro:`PR_SET_SHADOW_STACK_STATUS` / :c:macro:`PR_GET_SHADOW_STACK_STATUS` /
+:c:macro:`PR_LOCK_SHADOW_STACK_STATUS` are three prctls added to manage shadow
+stack enabling for tasks. prctls are arch agnostic and returns -EINVAL on other
+arches.
+
+* prctl(PR_SET_SHADOW_STACK_STATUS, unsigned long arg)
+
+If arg1 :c:macro:`PR_SHADOW_STACK_ENABLE` and if CPU supports ``zicfiss`` then
+kernel will enable shadow stack for the task. Dynamic loader can issue this
+:c:macro:`prctl` once it has determined that all the objects loaded in address
+space have support for shadow stack. Additionally if there is a
+:c:macro:`dlopen` to an object which wasn't compiled with ``zicfiss``, dynamic
+loader can issue this prctl with arg1 set to 0 (i.e.
+:c:macro:`PR_SHADOW_STACK_ENABLE` being clear)
+
+* prctl(PR_GET_SHADOW_STACK_STATUS, unsigned long *arg)
+
+Returns current status of indirect branch tracking. If enabled it'll return
+:c:macro:`PR_SHADOW_STACK_ENABLE`.
+
+* prctl(PR_LOCK_SHADOW_STACK_STATUS, unsigned long arg)
+
+Locks current status of shadow stack enabling on the task. User space may want
+to run with strict security posture and wouldn't want loading of objects
+without ``zicfiss`` support in it and thus would want to disallow disabling of
+shadow stack on current task. In that case user space can use this prctl to
+lock current settings.
+
+5. violations related to returns with shadow stack enabled
+-----------------------------------------------------------
+
+Pertaining to shadow stack, CPU raises software check exception in following
+condition:
+
+- On execution of ``sspopchk x1/x5``, ``x1/x5`` didn't match top of shadow
+  stack. If mismatch happens then cpu does ``*tval = 3`` and raise software
+  check exception.
+
+Linux kernel will treat this as :c:macro:`SIGSEV`` with code =
+:c:macro:`SEGV_CPERR` and follow normal course of signal delivery.
+
+6. Shadow stack tokens
+-----------------------
+Regular stores on shadow stacks are not allowed and thus can't be tampered
+with via arbitrary stray writes due to bugs. Method of pivoting / switching to
+shadow stack is simply writing to csr ``CSR_SSP`` changes active shadow stack.
+This can be problematic because usually value to be written to ``CSR_SSP`` will
+be loaded somewhere in writeable memory and thus allows an adversary to
+corruption bug in software to pivot to an any address in shadow stack range.
+Shadow stack tokens can help mitigate this problem by making sure that:
+
+- When software is switching away from a shadow stack, shadow stack pointer
+  should be saved on shadow stack itself and call it ``shadow stack token``
+
+- When software is switching to a shadow stack, it should read the
+  ``shadow stack token`` from shadow stack pointer and verify that
+  ``shadow stack token`` itself is pointer to shadow stack itself.
+
+- Once the token verification is done, software can perform the write to
+  ``CSR_SSP`` to switch shadow stack.
+
+Here software can be user mode task runtime itself which is managing various
+contexts as part of single thread. Software can be kernel as well when kernel
+has to deliver a signal to user task and must save shadow stack pointer. Kernel
+can perform similar procedure by saving a token on user shadow stack itself.
+This way whenever :c:macro:`sigreturn` happens, kernel can read the token and
+verify the token and then switch to shadow stack. Using this mechanism, kernel
+helps user task so that any corruption issue in user task is not exploited by
+adversary by arbitrarily using :c:macro:`sigreturn`. Adversary will have to
+make sure that there is a ``shadow stack token`` in addition to invoking
+:c:macro:`sigreturn`
+
+7. Signal shadow stack
+-----------------------
+Following structure has been added to sigcontext for RISC-V::
+
+    struct __sc_riscv_cfi_state {
+        unsigned long ss_ptr;
+    };
+
+As part of signal delivery, shadow stack token is saved on current shadow stack
+itself and updated pointer is saved away in :c:macro:`ss_ptr` field in
+:c:macro:`__sc_riscv_cfi_state` under :c:macro:`sigcontext`. Existing shadow
+stack allocation is used for signal delivery. During :c:macro:`sigreturn`,
+kernel will obtain :c:macro:`ss_ptr` from :c:macro:`sigcontext` and verify the
+saved token on shadow stack itself and switch shadow stack.

-- 
2.45.0