Re: [PATCH v5 10/14] arm64: preserve x18 when CPU is suspended

Mark Rutland
2019-11-15T14:27:17Z

On Tue, Nov 05, 2019 at 03:56:04PM -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>
> ---
> 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

For a moment I thought this might impact the alignment of the array, but
I see cpu_suspend_ctx is force-aligned to 16 bytes anyway, and since
commit cabe1c81ea5be983 the only impact would be a performance thing.

Reviewed-by: Mark Rutland <mark.rutland@arm.com>

Mark.

> #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]
> msr tpidr_el0, x2
> msr tpidrro_el0, x3
> msr contextidr_el1, x4
> --
> 2.24.0.rc1.363.gb1bccd3e3d-goog
>