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

[Steffen Klassert <steffen.klassert@secunet.com>]

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: stable@vger.kernel.org
> Link: https://github.com/ClangBuiltLinux/linux/issues/1985
> Signed-off-by: Nathan Chancellor <nathan@kernel.org>

Applied to the ipsec tree, thanks a lot!