[PATCH 3.16 67/86] x86/speculation/mds: Clear CPU buffers on exit to user

From: Ben Hutchings
Date: Thu May 16 2019 - 12:02:32 EST

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3.16.68-rc1 review patch. If anyone has any objections, please let me know.

------------------

From: Thomas Gleixner <tglx@xxxxxxxxxxxxx>

commit 04dcbdb8057827b043b3c71aa397c4c63e67d086 upstream.

Add a static key which controls the invocation of the CPU buffer clear
mechanism on exit to user space and add the call into
prepare_exit_to_usermode() and do_nmi() right before actually returning.

Add documentation which kernel to user space transition this covers and
explain why some corner cases are not mitigated.

Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Reviewed-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
Reviewed-by: Borislav Petkov <bp@xxxxxxx>
Reviewed-by: Frederic Weisbecker <frederic@xxxxxxxxxx>
Reviewed-by: Jon Masters <jcm@xxxxxxxxxx>
Tested-by: Jon Masters <jcm@xxxxxxxxxx>
[bwh: Backported to 3.16: Add an assembly macro equivalent to
mds_user_clear_cpu_buffers() and use this in the system call exit path,
as we don't have prepare_exit_to_usermode()]
Cc: Dominik Brodowski <linux@xxxxxxxxxxxxxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxx>
Cc: x86@xxxxxxxxxx
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
--- a/Documentation/x86/mds.rst
+++ b/Documentation/x86/mds.rst
@@ -97,3 +97,55 @@ According to current knowledge additiona
itself are not required because the necessary gadgets to expose the leaked
data cannot be controlled in a way which allows exploitation from malicious
user space or VM guests.
+
+Mitigation points
+-----------------
+
+1. Return to user space
+^^^^^^^^^^^^^^^^^^^^^^^
+
+ When transitioning from kernel to user space the CPU buffers are flushed
+ on affected CPUs when the mitigation is not disabled on the kernel
+ command line. The migitation is enabled through the static key
+ mds_user_clear.
+
+ The mitigation is invoked in prepare_exit_to_usermode() which covers
+ most of the kernel to user space transitions. There are a few exceptions
+ which are not invoking prepare_exit_to_usermode() on return to user
+ space. These exceptions use the paranoid exit code.
+
+ - Non Maskable Interrupt (NMI):
+
+ Access to sensible data like keys, credentials in the NMI context is
+ mostly theoretical: The CPU can do prefetching or execute a
+ misspeculated code path and thereby fetching data which might end up
+ leaking through a buffer.
+
+ But for mounting other attacks the kernel stack address of the task is
+ already valuable information. So in full mitigation mode, the NMI is
+ mitigated on the return from do_nmi() to provide almost complete
+ coverage.
+
+ - Double fault (#DF):
+
+ A double fault is usually fatal, but the ESPFIX workaround, which can
+ be triggered from user space through modify_ldt(2) is a recoverable
+ double fault. #DF uses the paranoid exit path, so explicit mitigation
+ in the double fault handler is required.
+
+ - Machine Check Exception (#MC):
+
+ Another corner case is a #MC which hits between the CPU buffer clear
+ invocation and the actual return to user. As this still is in kernel
+ space it takes the paranoid exit path which does not clear the CPU
+ buffers. So the #MC handler repopulates the buffers to some
+ extent. Machine checks are not reliably controllable and the window is
+ extremly small so mitigation would just tick a checkbox that this
+ theoretical corner case is covered. To keep the amount of special
+ cases small, ignore #MC.
+
+ - Debug Exception (#DB):
+
+ This takes the paranoid exit path only when the INT1 breakpoint is in
+ kernel space. #DB on a user space address takes the regular exit path,
+ so no extra mitigation required.
--- a/arch/x86/include/asm/nospec-branch.h
+++ b/arch/x86/include/asm/nospec-branch.h
@@ -262,6 +262,8 @@ DECLARE_STATIC_KEY_FALSE(switch_to_cond_
DECLARE_STATIC_KEY_FALSE(switch_mm_cond_ibpb);
DECLARE_STATIC_KEY_FALSE(switch_mm_always_ibpb);

+DECLARE_STATIC_KEY_FALSE(mds_user_clear);
+
#include <asm/segment.h>

/**
@@ -287,5 +289,31 @@ static inline void mds_clear_cpu_buffers
asm volatile("verw %[ds]" : : [ds] "m" (ds) : "cc");
}

+/**
+ * mds_user_clear_cpu_buffers - Mitigation for MDS vulnerability
+ *
+ * Clear CPU buffers if the corresponding static key is enabled
+ */
+static inline void mds_user_clear_cpu_buffers(void)
+{
+ if (static_branch_likely(&mds_user_clear))
+ mds_clear_cpu_buffers();
+}
+
#endif /* __ASSEMBLY__ */
+
+#ifdef __ASSEMBLY__
+.macro MDS_USER_CLEAR_CPU_BUFFERS
+#ifdef CONFIG_JUMP_LABEL
+ STATIC_JUMP_IF_FALSE .Lmds_skip_clear_\@, mds_user_clear, def=0
+#endif
+#ifdef CONFIG_X86_64
+ verw mds_clear_cpu_buffers_ds(%rip)
+#else
+ verw mds_clear_cpu_buffers_ds
+#endif
+.Lmds_skip_clear_\@:
+.endm
+#endif
+
#endif /* _ASM_X86_NOSPEC_BRANCH_H_ */
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -58,6 +58,12 @@ DEFINE_STATIC_KEY_FALSE(switch_mm_cond_i
/* Control unconditional IBPB in switch_mm() */
DEFINE_STATIC_KEY_FALSE(switch_mm_always_ibpb);

+/* Control MDS CPU buffer clear before returning to user space */
+DEFINE_STATIC_KEY_FALSE(mds_user_clear);
+
+/* For use by asm MDS_CLEAR_CPU_BUFFERS */
+const u16 mds_clear_cpu_buffers_ds = __KERNEL_DS;
+
#ifdef CONFIG_X86_32

static double __initdata x = 4195835.0;
--- a/arch/x86/kernel/nmi.c
+++ b/arch/x86/kernel/nmi.c
@@ -29,6 +29,7 @@
#include <asm/mach_traps.h>
#include <asm/nmi.h>
#include <asm/x86_init.h>
+#include <asm/nospec-branch.h>

#define CREATE_TRACE_POINTS
#include <trace/events/nmi.h>
@@ -522,6 +523,9 @@ nmi_restart:
write_cr2(this_cpu_read(nmi_cr2));
if (this_cpu_dec_return(nmi_state))
goto nmi_restart;
+
+ if (user_mode(regs))
+ mds_user_clear_cpu_buffers();
}
NOKPROBE_SYMBOL(do_nmi);

--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -55,6 +55,7 @@
#include <asm/fixmap.h>
#include <asm/mach_traps.h>
#include <asm/alternative.h>
+#include <asm/nospec-branch.h>

#ifdef CONFIG_X86_64
#include <asm/x86_init.h>
@@ -258,6 +259,14 @@ dotraplinkage void do_double_fault(struc
normal_regs->orig_ax = 0; /* Missing (lost) #GP error code */
regs->ip = (unsigned long)general_protection;
regs->sp = (unsigned long)&normal_regs->orig_ax;
+
+ /*
+ * This situation can be triggered by userspace via
+ * modify_ldt(2) and the return does not take the regular
+ * user space exit, so a CPU buffer clear is required when
+ * MDS mitigation is enabled.
+ */
+ mds_user_clear_cpu_buffers();
return;
}
#endif
--- a/arch/x86/kernel/entry_32.S
+++ b/arch/x86/kernel/entry_32.S
@@ -443,6 +443,7 @@ sysenter_after_call:
testl $_TIF_ALLWORK_MASK, %ecx
jne sysexit_audit
sysenter_exit:
+ MDS_USER_CLEAR_CPU_BUFFERS
/* if something modifies registers it must also disable sysexit */
movl PT_EIP(%esp), %edx
movl PT_OLDESP(%esp), %ecx
@@ -531,6 +532,7 @@ syscall_exit:
jne syscall_exit_work

restore_all:
+ MDS_USER_CLEAR_CPU_BUFFERS
TRACE_IRQS_IRET
restore_all_notrace:
#ifdef CONFIG_X86_ESPFIX32
--- a/arch/x86/kernel/entry_64.S
+++ b/arch/x86/kernel/entry_64.S
@@ -475,6 +475,7 @@ sysret_check:
movl TI_flags+THREAD_INFO(%rsp,RIP-ARGOFFSET),%edx
andl %edi,%edx
jnz sysret_careful
+ MDS_USER_CLEAR_CPU_BUFFERS
CFI_REMEMBER_STATE
/*
* sysretq will re-enable interrupts:
@@ -870,6 +871,7 @@ retint_swapgs: /* return to user-space
* The iretq could re-enable interrupts:
*/
DISABLE_INTERRUPTS(CLBR_ANY)
+ MDS_USER_CLEAR_CPU_BUFFERS
TRACE_IRQS_IRETQ
/*
* This opens a window where we have a user CR3, but are
@@ -1384,7 +1386,7 @@ paranoid_userspace:
GET_THREAD_INFO(%rcx)
movl TI_flags(%rcx),%ebx
andl $_TIF_WORK_MASK,%ebx
- jz paranoid_kernel
+ jz paranoid_userspace_done
movq %rsp,%rdi /* &pt_regs */
call sync_regs
movq %rax,%rsp /* switch stack for scheduling */
@@ -1406,6 +1408,9 @@ paranoid_schedule:
DISABLE_INTERRUPTS(CLBR_ANY)
TRACE_IRQS_OFF
jmp paranoid_userspace
+paranoid_userspace_done:
+ MDS_USER_CLEAR_CPU_BUFFERS
+ jmp paranoid_kernel
CFI_ENDPROC
END(paranoid_exit)

--- a/arch/x86/ia32/ia32entry.S
+++ b/arch/x86/ia32/ia32entry.S
@@ -188,6 +188,7 @@ sysenter_dispatch:
testl $_TIF_ALLWORK_MASK,TI_flags+THREAD_INFO(%rsp,RIP-ARGOFFSET)
jnz sysexit_audit
sysexit_from_sys_call:
+ MDS_USER_CLEAR_CPU_BUFFERS
andl $~TS_COMPAT,TI_status+THREAD_INFO(%rsp,RIP-ARGOFFSET)
/* clear IF, that popfq doesn't enable interrupts early */
andl $~0x200,EFLAGS-R11(%rsp)
@@ -362,6 +363,7 @@ cstar_dispatch:
testl $_TIF_ALLWORK_MASK,TI_flags+THREAD_INFO(%rsp,RIP-ARGOFFSET)
jnz sysretl_audit
sysretl_from_sys_call:
+ MDS_USER_CLEAR_CPU_BUFFERS
andl $~TS_COMPAT,TI_status+THREAD_INFO(%rsp,RIP-ARGOFFSET)
RESTORE_ARGS 0,-ARG_SKIP,0,0,0
movl RIP-ARGOFFSET(%rsp),%ecx

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