Re: [PATCH] [RFC] xfrm: work around a clang-19 fortifiy-string false-positive

[Kees Cook]

On Fri, Feb 16, 2024 at 09:26:40PM +0100, Arnd Bergmann wrote:
> From: Arnd Bergmann <arnd@xxxxxxxx>
> 
> clang-19 recently got branched from clang-18 and is not yet released.
> The current version introduces exactly one new warning that I came
> across in randconfig testing, in the copy_to_user_tmpl() function:
> 
> include/linux/fortify-string.h:420:4: 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]
>   420 |                         __write_overflow_field(p_size_field, size);
> 
> I have not yet produced a minimized test case for it, but I have a
> local workaround, which avoids the memset() here by replacing it with
> an initializer.
> 
> The memset is required to avoid leaking stack data to user space
> and was added in commit 1f86840f8977 ("xfrm_user: fix info leak in
> copy_to_user_tmpl()"). Simply changing the initializer to set all fields
> still risks leaking data in the padding between them, which the compiler
> is free to do here. To work around that problem, explicit padding fields
> have to get added as well.

Per C11, padding bits are zero initialized if there is an initializer,
so "= { }" here should be sufficient -- no need to add the struct
members.

> Since this is a false positive, a better fix would likely be to
> fix the compiler.

As Nathan has found, this appears to be a loop unrolling bug in Clang.
https://github.com/ClangBuiltLinux/linux/issues/1985

The shorter fix (in the issue) is to explicitly range-check before
the loop:

       if (xp->xfrm_nr > XFRM_MAX_DEPTH)
               return -ENOBUFS;

-- 
Kees Cook