[klibc] [PATCH] : store 4 bytes in arm64 errno

[Ben Hutchings]

On Mon, 2022-08-15 at 02:05 -0700, Greg Thelen wrote:
> The arm64 post-syscall code (below) checks the syscall retval (x0) and
> conditionally sets errno:
> 
> __syscall_common:
>         cmp     x0, #0x0
>         b.ge    2f
>         neg     x0, x0
>         ldr     x8, 1f
>         str     x0, [x8]
>         mov     x0, #-1
> 2:
>         ret
> 1:
>         .dword  errno
> 
> There is a bug. When the syscall returns a negative value "str x0, [x8]"
> stores 8 bytes in the 4 byte errno. The 4 bytes that follow errno are
> clobbered, which depending on linker data placement can corrupt
> important process memory.
> 
> Only store 4 bytes in errno to avoid corruption.

Good catch.  I've applied this.

Ben.

> Fixes: e4a2c914446b ("[klibc] arm64: Add arm64 support")
> Signed-off-by: Greg Thelen <gthelen at google.com>
> ---
>  usr/klibc/arch/arm64/syscall.S | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/usr/klibc/arch/arm64/syscall.S b/usr/klibc/arch/arm64/syscall.S
> index 3ce91fb77aa5..e1004122577d 100644
> --- a/usr/klibc/arch/arm64/syscall.S
> +++ b/usr/klibc/arch/arm64/syscall.S
> @@ -17,7 +17,7 @@ __syscall_common:
>  	b.ge	2f
>  	neg	x0, x0
>  	ldr	x8, 1f
> -	str	x0, [x8]
> +	str	w0, [x8]
>  	mov	x0, #-1
>  2:
>  	ret

-- 
Ben Hutchings
It is a miracle that curiosity survives formal education.
                                                      - Albert Einstein
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