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Subject[tip:x86/pti] x86/entry/64: Clear registers for exceptions/interrupts, to reduce speculation attack surface
Commit-ID:  3ac6d8c787b835b997eb23e43e09aa0895ef7d58
Gitweb: https://git.kernel.org/tip/3ac6d8c787b835b997eb23e43e09aa0895ef7d58
Author: Dan Williams <dan.j.williams@intel.com>
AuthorDate: Mon, 5 Feb 2018 17:18:11 -0800
Committer: Ingo Molnar <mingo@kernel.org>
CommitDate: Tue, 6 Feb 2018 11:47:44 +0100

x86/entry/64: Clear registers for exceptions/interrupts, to reduce speculation attack surface

Clear the 'extra' registers on entering the 64-bit kernel for exceptions
and interrupts. The common registers are not cleared since they are
likely clobbered well before they can be exploited in a speculative
execution attack.

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/151787989146.7847.15749181712358213254.stgit@dwillia2-desk3.amr.corp.intel.com
[ Made small improvements to the changelog and the code comments. ]
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
arch/x86/entry/calling.h | 19 +++++++++++++++++++
arch/x86/entry/entry_64.S | 6 +++++-
2 files changed, 24 insertions(+), 1 deletion(-)

diff --git a/arch/x86/entry/calling.h b/arch/x86/entry/calling.h
index 3f48f69..f4b129d 100644
--- a/arch/x86/entry/calling.h
+++ b/arch/x86/entry/calling.h
@@ -147,6 +147,25 @@ For 32-bit we have the following conventions - kernel is built with
UNWIND_HINT_REGS offset=\offset
.endm

+ /*
+ * Sanitize registers of values that a speculation attack
+ * might otherwise want to exploit. The lower registers are
+ * likely clobbered well before they could be put to use in
+ * a speculative execution gadget:
+ */
+ .macro CLEAR_REGS_NOSPEC
+ xorl %ebp, %ebp
+ xorl %ebx, %ebx
+ xorq %r8, %r8
+ xorq %r9, %r9
+ xorq %r10, %r10
+ xorq %r11, %r11
+ xorq %r12, %r12
+ xorq %r13, %r13
+ xorq %r14, %r14
+ xorq %r15, %r15
+ .endm
+
.macro POP_EXTRA_REGS
popq %r15
popq %r14
diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index 065a71b..9e48002 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -575,6 +575,7 @@ END(irq_entries_start)
ALLOC_PT_GPREGS_ON_STACK
SAVE_C_REGS
SAVE_EXTRA_REGS
+ CLEAR_REGS_NOSPEC
ENCODE_FRAME_POINTER

testb $3, CS(%rsp)
@@ -1133,6 +1134,7 @@ ENTRY(xen_failsafe_callback)
ALLOC_PT_GPREGS_ON_STACK
SAVE_C_REGS
SAVE_EXTRA_REGS
+ CLEAR_REGS_NOSPEC
ENCODE_FRAME_POINTER
jmp error_exit
END(xen_failsafe_callback)
@@ -1178,6 +1180,7 @@ ENTRY(paranoid_entry)
cld
SAVE_C_REGS 8
SAVE_EXTRA_REGS 8
+ CLEAR_REGS_NOSPEC
ENCODE_FRAME_POINTER 8
movl $1, %ebx
movl $MSR_GS_BASE, %ecx
@@ -1230,8 +1233,8 @@ ENTRY(error_entry)
cld
SAVE_C_REGS 8
SAVE_EXTRA_REGS 8
+ CLEAR_REGS_NOSPEC
ENCODE_FRAME_POINTER 8
- xorl %ebx, %ebx
testb $3, CS+8(%rsp)
jz .Lerror_kernelspace

@@ -1428,6 +1431,7 @@ ENTRY(nmi)
pushq %r14 /* pt_regs->r14 */
pushq %r15 /* pt_regs->r15 */
UNWIND_HINT_REGS
+ CLEAR_REGS_NOSPEC
ENCODE_FRAME_POINTER

/*
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