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SubjectRe: [tip:x86/pti] x86/entry/64: Introduce the PUSH_AND_CLEAN_REGS macro
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On 02/12/2018 11:17 AM, tip-bot for Dominik Brodowski wrote:
> Commit-ID: 7b7b09f110f06f3c006e9b3f4590f7d9ba91345b
> Gitweb: https://git.kernel.org/tip/7b7b09f110f06f3c006e9b3f4590f7d9ba91345b
> Author: Dominik Brodowski <linux@dominikbrodowski.net>
> AuthorDate: Sun, 11 Feb 2018 11:49:45 +0100
> Committer: Ingo Molnar <mingo@kernel.org>
> CommitDate: Mon, 12 Feb 2018 08:06:36 +0100
>
> x86/entry/64: Introduce the PUSH_AND_CLEAN_REGS macro
>
> Those instances where ALLOC_PT_GPREGS_ON_STACK is called just before
> SAVE_AND_CLEAR_REGS can trivially be replaced by PUSH_AND_CLEAN_REGS.
> This macro uses PUSH instead of MOV and should therefore be faster, at
> least on newer CPUs.
>
> Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
> Signed-off-by: Dominik Brodowski <linux@dominikbrodowski.net>
> 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: Peter Zijlstra <peterz@infradead.org>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: dan.j.williams@intel.com
> Link: http://lkml.kernel.org/r/20180211104949.12992-5-linux@dominikbrodowski.net
> Signed-off-by: Ingo Molnar <mingo@kernel.org>
> ---
> arch/x86/entry/calling.h | 36 ++++++++++++++++++++++++++++++++++++
> arch/x86/entry/entry_64.S | 6 ++----
> 2 files changed, 38 insertions(+), 4 deletions(-)
>
> diff --git a/arch/x86/entry/calling.h b/arch/x86/entry/calling.h
> index a05cbb8..57b1b87 100644
> --- a/arch/x86/entry/calling.h
> +++ b/arch/x86/entry/calling.h
> @@ -137,6 +137,42 @@ For 32-bit we have the following conventions - kernel is built with
> UNWIND_HINT_REGS offset=\offset
> .endm
>
> + .macro PUSH_AND_CLEAR_REGS
> + /*
> + * Push registers and 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.
> + * Interleave XOR with PUSH for better uop scheduling:
> + */
> + pushq %rdi /* pt_regs->di */
> + pushq %rsi /* pt_regs->si */
> + pushq %rdx /* pt_regs->dx */
> + pushq %rcx /* pt_regs->cx */
> + pushq %rax /* pt_regs->ax */
> + pushq %r8 /* pt_regs->r8 */
> + xorq %r8, %r8 /* nospec r8 */

xorq's are slower than xorl's on Silvermont/Knights Landing.
I propose using xorl instead.

> + 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*/

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