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SubjectRe: [PATCH v3 2/3] x86/entry: Clear registers for 64bit exceptions/interrupts
On Mon, Feb 05, 2018 at 05:18:11PM -0800, Dan Williams wrote:
> From: Andi Kleen <ak@linux.intel.com>
>
> Clear the 'extra' registers on entering the 64bit 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.

Could the clever trick from patch 3/3 (interleave xorq with movq) be
used here as well? Something like below (untested)? This includes removing
the (seemingly?) unused SAVE_C_REGS_EXCEPT_* macros, which probably needs to
become a spearate patch.

Thanks,
Dominik


diff --git a/arch/x86/entry/calling.h b/arch/x86/entry/calling.h
index 3f48f695d5e6..7bdee6d14f0a 100644
--- a/arch/x86/entry/calling.h
+++ b/arch/x86/entry/calling.h
@@ -101,49 +101,36 @@ For 32-bit we have the following conventions - kernel is built with
addq $-(15*8), %rsp
.endm

- .macro SAVE_C_REGS_HELPER offset=0 rax=1 rcx=1 r8910=1 r11=1
- .if \r11
+ .macro SAVE_C_REGS offset=0
+ xorq %r11, %r11 /* nospec r11 */
movq %r11, 6*8+\offset(%rsp)
- .endif
- .if \r8910
movq %r10, 7*8+\offset(%rsp)
+ xorq %r10, %r10 /* nospec r10 */
movq %r9, 8*8+\offset(%rsp)
+ xorq %r9, %r9 /* nospec r9 */
movq %r8, 9*8+\offset(%rsp)
- .endif
- .if \rax
+ xorq %r8, %r8 /* nospec r8 */
movq %rax, 10*8+\offset(%rsp)
- .endif
- .if \rcx
movq %rcx, 11*8+\offset(%rsp)
- .endif
movq %rdx, 12*8+\offset(%rsp)
movq %rsi, 13*8+\offset(%rsp)
movq %rdi, 14*8+\offset(%rsp)
UNWIND_HINT_REGS offset=\offset extra=0
.endm
- .macro SAVE_C_REGS offset=0
- SAVE_C_REGS_HELPER \offset, 1, 1, 1, 1
- .endm
- .macro SAVE_C_REGS_EXCEPT_RAX_RCX offset=0
- SAVE_C_REGS_HELPER \offset, 0, 0, 1, 1
- .endm
- .macro SAVE_C_REGS_EXCEPT_R891011
- SAVE_C_REGS_HELPER 0, 1, 1, 0, 0
- .endm
- .macro SAVE_C_REGS_EXCEPT_RCX_R891011
- SAVE_C_REGS_HELPER 0, 1, 0, 0, 0
- .endm
- .macro SAVE_C_REGS_EXCEPT_RAX_RCX_R11
- SAVE_C_REGS_HELPER 0, 0, 0, 1, 0
- .endm

.macro SAVE_EXTRA_REGS offset=0
movq %r15, 0*8+\offset(%rsp)
+ xorq %r15, %r15 /* nospec r15 */
movq %r14, 1*8+\offset(%rsp)
+ xorq %r14, %r14 /* nospec r14 */
movq %r13, 2*8+\offset(%rsp)
+ xorq %r13, %r13 /* nospec r13 */
movq %r12, 3*8+\offset(%rsp)
+ xorq %r12, %r12 /* nospec r12*/
movq %rbp, 4*8+\offset(%rsp)
+ xorl %ebp, %ebp /* nospec rbp */
movq %rbx, 5*8+\offset(%rsp)
+ xorl %ebx, %ebx /* nospec rbx */
UNWIND_HINT_REGS offset=\offset
.endm

diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index c752abe89d80..de19a46f40b2 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -1218,7 +1218,6 @@ ENTRY(error_entry)
SAVE_C_REGS 8
SAVE_EXTRA_REGS 8
ENCODE_FRAME_POINTER 8
- xorl %ebx, %ebx
testb $3, CS+8(%rsp)
jz .Lerror_kernelspace

@@ -1405,15 +1404,25 @@ ENTRY(nmi)
pushq %rcx /* pt_regs->cx */
pushq %rax /* pt_regs->ax */
pushq %r8 /* pt_regs->r8 */
+ xorq %r8, %r8 /* nospec r8 */
pushq %r9 /* pt_regs->r9 */
+ xorq %r9, %r9 /* nospec r9 */
pushq %r10 /* pt_regs->r10 */
+ xorq %r10, %r10 /* nospec r10 */
pushq %r11 /* pt_regs->r11 */
+ 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*/
UNWIND_HINT_REGS
ENCODE_FRAME_POINTER

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