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

[Nathan Chancellor]

On Mon, Apr 08, 2024 at 09:06:21AM +0200, Arnd Bergmann wrote:
> On Fri, Feb 16, 2024, at 22:19, Kees Cook wrote:
> > 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;
> 
> I ran into this issue again and I see that Nathan's fix has
> made it into mainline and backports, but it's apparently
> not sufficient.
> 
> I don't see the warning with my patch from this thread, but
> there may still be a better fix.

Is it the exact same warning? clang-19 or older? What
architecture/configuration? If my change is not sufficient then maybe
there are two separate issues here? I have not seen this warning appear
in our CI since my change was applied.

Cheers,
Nathan