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SubjectRe: [PATCH v1 3/8] x86/entry/clearregs: Clear registers for 64bit SYSCALL
On Tue, Jan 9, 2018 at 8:03 PM, Andi Kleen <andi@firstfloor.org> wrote:
> From: Andi Kleen <ak@linux.intel.com>
>
> We clear all the non argument registers for 64bit SYSCALLs
> to minimize any risk of bad speculation using user values.
>
> So far unused argument registers still leak. To be addressed
> in future patches.
>
> Signed-off-by: Andi Kleen <ak@linux.intel.com>
> ---
> arch/x86/entry/entry_64.S | 9 +++++++++
> 1 file changed, 9 insertions(+)
>
> diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
> index bbdfbdd817d6..632081fd7086 100644
> --- a/arch/x86/entry/entry_64.S
> +++ b/arch/x86/entry/entry_64.S
> @@ -236,6 +236,14 @@ GLOBAL(entry_SYSCALL_64_after_hwframe)
> pushq %r11 /* pt_regs->r11 */
> sub $(6*8), %rsp
> SAVE_EXTRA_REGS
> + /* Sanitize registers against speculation attacks */
> + /* r10 is cleared later, arguments are handled in san_args* */
> + CLEAR_R11_TO_R15

Don't need to explicitly clear R11 here. It is clobbered with current_task.

> +#ifndef CONFIG_FRAME_POINTER
> + xor %ebp, %ebp
> +#endif
> + xor %ebx, %ebx
> + xor %ecx, %ecx
>
> UNWIND_HINT_REGS extra=0
>
> @@ -263,6 +271,7 @@ entry_SYSCALL_64_fastpath:
> #endif
> ja 1f /* return -ENOSYS (already in pt_regs->ax) */
> movq %r10, %rcx
> + xor %r10, %r10

RCX is already clear, so xchgq %r10, %rcx will be simpler.

--
Brian Gerst

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