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SubjectRe: [PATCH v2 02/13] stackleak: move skip_erasing() check earlier
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On 27.04.2022 20:31, Mark Rutland wrote:
> In stackleak_erase() we check skip_erasing() after accessing some fields
> from current. As generating the address of current uses asm which
> hazards with the static branch asm, this work is always performed, even
> when the static branch is patched to jump to the return a the end of the
> function.

Nice find!

> This patch avoids this redundant work by moving the skip_erasing() check
> earlier.
>
> To avoid complicating initialization within stackleak_erase(), the body
> of the function is split out into a __stackleak_erase() helper, with the
> check left in a wrapper function. The __stackleak_erase() helper is
> marked __always_inline to ensure that this is inlined into
> stackleak_erase() and not instrumented.
>
> Before this patch, on x86-64 w/ GCC 11.1.0 the start of the function is:
>
> <stackleak_erase>:
> 65 48 8b 04 25 00 00 mov %gs:0x0,%rax
> 00 00
> 48 8b 48 20 mov 0x20(%rax),%rcx
> 48 8b 80 98 0a 00 00 mov 0xa98(%rax),%rax
> 66 90 xchg %ax,%ax <------------ static branch
> 48 89 c2 mov %rax,%rdx
> 48 29 ca sub %rcx,%rdx
> 48 81 fa ff 3f 00 00 cmp $0x3fff,%rdx
>
> After this patch, on x86-64 w/ GCC 11.1.0 the start of the function is:
>
> <stackleak_erase>:
> 0f 1f 44 00 00 nopl 0x0(%rax,%rax,1) <--- static branch
> 65 48 8b 04 25 00 00 mov %gs:0x0,%rax
> 00 00
> 48 8b 48 20 mov 0x20(%rax),%rcx
> 48 8b 80 98 0a 00 00 mov 0xa98(%rax),%rax
> 48 89 c2 mov %rax,%rdx
> 48 29 ca sub %rcx,%rdx
> 48 81 fa ff 3f 00 00 cmp $0x3fff,%rdx
>
> Before this patch, on arm64 w/ GCC 11.1.0 the start of the function is:
>
> <stackleak_erase>:
> d503245f bti c
> d5384100 mrs x0, sp_el0
> f9401003 ldr x3, [x0, #32]
> f9451000 ldr x0, [x0, #2592]
> d503201f nop <------------------------------- static branch
> d503233f paciasp
> cb030002 sub x2, x0, x3
> d287ffe1 mov x1, #0x3fff
> eb01005f cmp x2, x1
>
> After this patch, on arm64 w/ GCC 11.1.0 the start of the function is:
>
> <stackleak_erase>:
> d503245f bti c
> d503201f nop <------------------------------- static branch
> d503233f paciasp
> d5384100 mrs x0, sp_el0
> f9401003 ldr x3, [x0, #32]
> d287ffe1 mov x1, #0x3fff
> f9451000 ldr x0, [x0, #2592]
> cb030002 sub x2, x0, x3
> eb01005f cmp x2, x1
>
> While this may not be a huge win on its own, moving the static branch
> will permit further optimization of the body of the function in
> subsequent patches.
>
> Signed-off-by: Mark Rutland <mark.rutland@arm.com>
> Cc: Alexander Popov <alex.popov@linux.com>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Andy Lutomirski <luto@kernel.org>
> Cc: Kees Cook <keescook@chromium.org>
> ---
> kernel/stackleak.c | 13 +++++++++----
> 1 file changed, 9 insertions(+), 4 deletions(-)
>
> diff --git a/kernel/stackleak.c b/kernel/stackleak.c
> index ddb5a7f48d69e..753eab797a04d 100644
> --- a/kernel/stackleak.c
> +++ b/kernel/stackleak.c
> @@ -70,7 +70,7 @@ late_initcall(stackleak_sysctls_init);
> #define skip_erasing() false
> #endif /* CONFIG_STACKLEAK_RUNTIME_DISABLE */
>
> -asmlinkage void noinstr stackleak_erase(void)
> +static __always_inline void __stackleak_erase(void)

Are you sure that __stackleak_erase() doesn't need asmlinkage and noinstr as well?

> {
> /* It would be nice not to have 'kstack_ptr' and 'boundary' on stack */
> unsigned long kstack_ptr = current->lowest_stack;
> @@ -78,9 +78,6 @@ asmlinkage void noinstr stackleak_erase(void)
> unsigned int poison_count = 0;
> const unsigned int depth = STACKLEAK_SEARCH_DEPTH / sizeof(unsigned long);
>
> - if (skip_erasing())
> - return;
> -
> /* Check that 'lowest_stack' value is sane */
> if (unlikely(kstack_ptr - boundary >= THREAD_SIZE))
> kstack_ptr = boundary;
> @@ -125,6 +122,14 @@ asmlinkage void noinstr stackleak_erase(void)
> current->lowest_stack = current_top_of_stack() - THREAD_SIZE/64;
> }
>
> +asmlinkage void noinstr stackleak_erase(void)
> +{
> + if (skip_erasing())
> + return;
> +
> + __stackleak_erase();
> +}
> +
> void __used __no_caller_saved_registers noinstr stackleak_track_stack(void)
> {
> unsigned long sp = current_stack_pointer;

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