Follow @Openwall on Twitter for new release announcements and other news
[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Date: Sun, 12 Dec 2021 14:35:02 -0500
From: Rich Felker <dalias@...c.org>
To: jvoisin <julien.voisin@...tri.org>
Cc: musl@...ts.openwall.com
Subject: Re: [PATCH] Zero the leading stack canary byte

On Sun, Dec 12, 2021 at 07:34:40PM +0100, jvoisin wrote:
> This reduces entropy of the canary from 64-bit to 56-bit in exchange for
> mitigating non-terminated C string overflows.
> 
> Checking the byte order should be "good enough", since I think that the stacks
> on all architectures supported by musl are growing downwards. Worse case, this
> can always be improved later if needed.
> 
> This is taken from https://github.com/GrapheneOS/platform_bionic/commit/7024d880b51f03a796ff8832f1298f2f1531fd7b
> ---
>  src/env/__stack_chk_fail.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/src/env/__stack_chk_fail.c b/src/env/__stack_chk_fail.c
> index bf5a280a..45f948fe 100644
> --- a/src/env/__stack_chk_fail.c
> +++ b/src/env/__stack_chk_fail.c
> @@ -2,6 +2,11 @@
>  #include <stdint.h>
>  #include "pthread_impl.h"
>  
> +// Sacrifice 8 bits of entropy to mitigate non-terminated C string overflows
> +static const uintptr_t canary_mask = __BYTE_ORDER == __BIG_ENDIAN ?
> +  0x00ffffffffffffffUL :
> +  0xffffffffffffff00UL ;
> +
>  uintptr_t __stack_chk_guard;
>  
>  void __init_ssp(void *entropy)
> @@ -9,7 +14,7 @@ void __init_ssp(void *entropy)
>  	if (entropy) memcpy(&__stack_chk_guard, entropy, sizeof(uintptr_t));
>  	else __stack_chk_guard = (uintptr_t)&__stack_chk_guard * 1103515245;
>  
> -	__pthread_self()->canary = __stack_chk_guard;
> +	__pthread_self()->canary = __stack_chk_guard & canary_mask;
>  }
>  
>  void __stack_chk_fail(void)
> -- 
> 2.30.2

As written this patch assumes a 64-bit uintptr_t, which isn't ok.
Indeed 56 bits should be fine on a 64-bit arch, but dropping from 32
to 24 on a 32-bit arch severely weakens the protection. So it probably
needs to be conditional on 64-bit.

Also, zeroing the first byte means we can no longer catch buffer
overflows of the form "off-by-one string length". This seems
unfortunate. Putting the 0 byte at the end would solve that at the
expense of allowing the canary value to be leaked via missing
termination bugs, and overall I would lean towards catching actual
buffer overflow bugs vs stopping canary leaks.

Rich

Powered by blists - more mailing lists

Confused about mailing lists and their use? Read about mailing lists on Wikipedia and check out these guidelines on proper formatting of your messages.