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Subject[tip:x86/pti] x86/entry/64/compat: Clear registers for compat syscalls, to reduce speculation attack surface
Commit-ID:  6b8cf5cc9965673951f1ab3f0e3cf23d06e3e2ee
Gitweb: https://git.kernel.org/tip/6b8cf5cc9965673951f1ab3f0e3cf23d06e3e2ee
Author: Dan Williams <dan.j.williams@intel.com>
AuthorDate: Mon, 5 Feb 2018 17:18:17 -0800
Committer: Ingo Molnar <mingo@kernel.org>
CommitDate: Tue, 6 Feb 2018 11:47:57 +0100

x86/entry/64/compat: Clear registers for compat syscalls, to reduce speculation attack surface

At entry userspace may have populated registers with values that could
otherwise be useful in a speculative execution attack. Clear them to
minimize the kernel's attack surface.

Originally-From: Andi Kleen <ak@linux.intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Cc: <stable@vger.kernel.org>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/151787989697.7847.4083702787288600552.stgit@dwillia2-desk3.amr.corp.intel.com
[ Made small improvements to the changelog. ]
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
arch/x86/entry/entry_64_compat.S | 30 ++++++++++++++++++++++++++++++
1 file changed, 30 insertions(+)

diff --git a/arch/x86/entry/entry_64_compat.S b/arch/x86/entry/entry_64_compat.S
index 98d5358..fd65e01 100644
--- a/arch/x86/entry/entry_64_compat.S
+++ b/arch/x86/entry/entry_64_compat.S
@@ -85,15 +85,25 @@ ENTRY(entry_SYSENTER_compat)
pushq %rcx /* pt_regs->cx */
pushq $-ENOSYS /* pt_regs->ax */
pushq $0 /* pt_regs->r8 = 0 */
+ xorq %r8, %r8 /* nospec r8 */
pushq $0 /* pt_regs->r9 = 0 */
+ xorq %r9, %r9 /* nospec r9 */
pushq $0 /* pt_regs->r10 = 0 */
+ xorq %r10, %r10 /* nospec r10 */
pushq $0 /* pt_regs->r11 = 0 */
+ xorq %r11, %r11 /* nospec r11 */
pushq %rbx /* pt_regs->rbx */
+ xorl %ebx, %ebx /* nospec rbx */
pushq %rbp /* pt_regs->rbp (will be overwritten) */
+ xorl %ebp, %ebp /* nospec rbp */
pushq $0 /* pt_regs->r12 = 0 */
+ xorq %r12, %r12 /* nospec r12 */
pushq $0 /* pt_regs->r13 = 0 */
+ xorq %r13, %r13 /* nospec r13 */
pushq $0 /* pt_regs->r14 = 0 */
+ xorq %r14, %r14 /* nospec r14 */
pushq $0 /* pt_regs->r15 = 0 */
+ xorq %r15, %r15 /* nospec r15 */
cld

/*
@@ -214,15 +224,25 @@ GLOBAL(entry_SYSCALL_compat_after_hwframe)
pushq %rbp /* pt_regs->cx (stashed in bp) */
pushq $-ENOSYS /* pt_regs->ax */
pushq $0 /* pt_regs->r8 = 0 */
+ xorq %r8, %r8 /* nospec r8 */
pushq $0 /* pt_regs->r9 = 0 */
+ xorq %r9, %r9 /* nospec r9 */
pushq $0 /* pt_regs->r10 = 0 */
+ xorq %r10, %r10 /* nospec r10 */
pushq $0 /* pt_regs->r11 = 0 */
+ xorq %r11, %r11 /* nospec r11 */
pushq %rbx /* pt_regs->rbx */
+ xorl %ebx, %ebx /* nospec rbx */
pushq %rbp /* pt_regs->rbp (will be overwritten) */
+ xorl %ebp, %ebp /* nospec rbp */
pushq $0 /* pt_regs->r12 = 0 */
+ xorq %r12, %r12 /* nospec r12 */
pushq $0 /* pt_regs->r13 = 0 */
+ xorq %r13, %r13 /* nospec r13 */
pushq $0 /* pt_regs->r14 = 0 */
+ xorq %r14, %r14 /* nospec r14 */
pushq $0 /* pt_regs->r15 = 0 */
+ xorq %r15, %r15 /* nospec r15 */

/*
* User mode is traced as though IRQs are on, and SYSENTER
@@ -338,15 +358,25 @@ ENTRY(entry_INT80_compat)
pushq %rcx /* pt_regs->cx */
pushq $-ENOSYS /* pt_regs->ax */
pushq $0 /* pt_regs->r8 = 0 */
+ xorq %r8, %r8 /* nospec r8 */
pushq $0 /* pt_regs->r9 = 0 */
+ xorq %r9, %r9 /* nospec r9 */
pushq $0 /* pt_regs->r10 = 0 */
+ xorq %r10, %r10 /* nospec r10 */
pushq $0 /* pt_regs->r11 = 0 */
+ xorq %r11, %r11 /* nospec r11 */
pushq %rbx /* pt_regs->rbx */
+ xorl %ebx, %ebx /* nospec rbx */
pushq %rbp /* pt_regs->rbp */
+ xorl %ebp, %ebp /* nospec rbp */
pushq %r12 /* pt_regs->r12 */
+ xorq %r12, %r12 /* nospec r12 */
pushq %r13 /* pt_regs->r13 */
+ xorq %r13, %r13 /* nospec r13 */
pushq %r14 /* pt_regs->r14 */
+ xorq %r14, %r14 /* nospec r14 */
pushq %r15 /* pt_regs->r15 */
+ xorq %r15, %r15 /* nospec r15 */
cld

/*
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