Re: [PATCH v6 10/15] arm64: preserve x18 when CPU is suspended

Will Deacon
2020-01-16T18:28:45Z

On Fri, Dec 06, 2019 at 02:13:46PM -0800, Sami Tolvanen wrote:
> Don't lose the current task's shadow stack when the CPU is suspended.
>
> Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
> Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
> Reviewed-by: Kees Cook <keescook@chromium.org>
> Reviewed-by: Mark Rutland <mark.rutland@arm.com>
> ---
> arch/arm64/include/asm/suspend.h | 2 +-
> arch/arm64/mm/proc.S | 14 ++++++++++++++
> 2 files changed, 15 insertions(+), 1 deletion(-)
>
> diff --git a/arch/arm64/include/asm/suspend.h b/arch/arm64/include/asm/suspend.h
> index 8939c87c4dce..0cde2f473971 100644
> --- a/arch/arm64/include/asm/suspend.h
> +++ b/arch/arm64/include/asm/suspend.h
> @@ -2,7 +2,7 @@
> #ifndef __ASM_SUSPEND_H
> #define __ASM_SUSPEND_H
>
> -#define NR_CTX_REGS 12
> +#define NR_CTX_REGS 13
> #define NR_CALLEE_SAVED_REGS 12
>
> /*
> diff --git a/arch/arm64/mm/proc.S b/arch/arm64/mm/proc.S
> index fdabf40a83c8..5c8219c55948 100644
> --- a/arch/arm64/mm/proc.S
> +++ b/arch/arm64/mm/proc.S
> @@ -49,6 +49,8 @@
> * cpu_do_suspend - save CPU registers context
> *
> * x0: virtual address of context pointer
> + *
> + * This must be kept in sync with struct cpu_suspend_ctx in <asm/suspend.h>.
> */
> ENTRY(cpu_do_suspend)
> mrs x2, tpidr_el0
> @@ -73,6 +75,11 @@ alternative_endif
> stp x8, x9, [x0, #48]
> stp x10, x11, [x0, #64]
> stp x12, x13, [x0, #80]
> + /*
> + * Save x18 as it may be used as a platform register, e.g. by shadow
> + * call stack.
> + */
> + str x18, [x0, #96]
> ret
> ENDPROC(cpu_do_suspend)
>
> @@ -89,6 +96,13 @@ ENTRY(cpu_do_resume)
> ldp x9, x10, [x0, #48]
> ldp x11, x12, [x0, #64]
> ldp x13, x14, [x0, #80]
> + /*
> + * Restore x18, as it may be used as a platform register, and clear
> + * the buffer to minimize the risk of exposure when used for shadow
> + * call stack.
> + */
> + ldr x18, [x0, #96]
> + str xzr, [x0, #96]

Mumble, mumble, spectre-v4.

But I think it's fairly hopeless trying to fix that everywhere it crops up,
so:

Acked-by: Will Deacon <will@kernel.org>

Will