Subject: Re: [PATCH net] xfrm: Avoid clang fortify warning in copy_to_user_tmpl()

From: Kees Cook <[email-masked]>
Date: Wed, 21 Feb 2024 14:32:01 -0800

On Wed, Feb 21, 2024 at 02:46:21PM -0700, Nathan Chancellor wrote:
> After a couple recent changes in LLVM, there is a warning (or error with
> CONFIG_WERROR=y or W=e) from the compile time fortify source routines,
> specifically the memset() in copy_to_user_tmpl().
> 
>   In file included from net/xfrm/xfrm_user.c:14:
>   ...
>   include/linux/fortify-string.h:438:4: error: call to '__write_overflow_field' declared with 'warning' attribute: detected write beyond size of field (1st parameter); maybe use struct_group()? [-Werror,-Wattribute-warning]
>     438 |                         __write_overflow_field(p_size_field, size);
>         |                         ^
>   1 error generated.
> 
> While ->xfrm_nr has been validated against XFRM_MAX_DEPTH when its value
> is first assigned in copy_templates() by calling validate_tmpl() first
> (so there should not be any issue in practice), LLVM/clang cannot really
> deduce that across the boundaries of these functions. Without that
> knowledge, it cannot assume that the loop stops before i is greater than
> XFRM_MAX_DEPTH, which would indeed result a stack buffer overflow in the
> memset().
> 
> To make the bounds of ->xfrm_nr clear to the compiler and add additional
> defense in case copy_to_user_tmpl() is ever used in a path where
> ->xfrm_nr has not been properly validated against XFRM_MAX_DEPTH first,
> add an explicit bound check and early return, which clears up the
> warning.
> 
> Cc: [email-masked]
> Link: https://github.com/ClangBuiltLinux/linux/issues/1985
> Signed-off-by: Nathan Chancellor <[email-masked]>

This seems reasonable to me. Thanks for chasing all this down!

Reviewed-by: Kees Cook <[email-masked]>

-Kees

> ---
>  net/xfrm/xfrm_user.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/net/xfrm/xfrm_user.c b/net/xfrm/xfrm_user.c
> index f037be190bae..912c1189ba41 100644
> --- a/net/xfrm/xfrm_user.c
> +++ b/net/xfrm/xfrm_user.c
> @@ -2017,6 +2017,9 @@ static int copy_to_user_tmpl(struct xfrm_policy *xp, struct sk_buff *skb)
>  	if (xp->xfrm_nr == 0)
>  		return 0;
>  
> +	if (xp->xfrm_nr > XFRM_MAX_DEPTH)
> +		return -ENOBUFS;
> +
>  	for (i = 0; i < xp->xfrm_nr; i++) {
>  		struct xfrm_user_tmpl *up = &vec[i];
>  		struct xfrm_tmpl *kp = &xp->xfrm_vec[i];
> 
> ---
> base-commit: 14dec56fdd4c70a0ebe40077368e367421ea6fef
> change-id: 20240221-xfrm-avoid-clang-fortify-warning-copy_to_user_tmpl-40cb10b003e3
> 
> Best regards,
> -- 
> Nathan Chancellor <[email-masked]>
> 

-- 
Kees Cook