Re: [PATCH v2 02/13] stackleak: move skip_erasing() check earlier

[Alexander Popov <alex.popov@linux.com>]

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;