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SubjectRe: [PATCH v3 2/3] x86/entry: Clear registers for 64bit exceptions/interrupts

* Dominik Brodowski <linux@dominikbrodowski.net> wrote:

> 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

Makes sense and it's also more readable - could you send this patch on top of
tip:x86/pti or tip:master, once I've pushed out the latest version (within the
next few hours)?

Thanks,

Ingo

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