Re: [PATCH v4 12/35] kmsan: Support SLAB_POISON

Alexander Potapenko
2024-06-18T15:05:47Z

On Fri, Jun 14, 2024 at 1:44 AM Ilya Leoshkevich <iii@linux.ibm.com> wrote:
>
> On Thu, 2024-06-13 at 16:30 -0700, SeongJae Park wrote:
> > Hi Ilya,
> >
> > On Thu, 13 Jun 2024 17:34:14 +0200 Ilya Leoshkevich
> > <iii@linux.ibm.com> wrote:
> >
> > > Avoid false KMSAN negatives with SLUB_DEBUG by allowing
> > > kmsan_slab_free() to poison the freed memory, and by preventing
> > > init_object() from unpoisoning new allocations by using __memset().
> > >
> > > There are two alternatives to this approach. First, init_object()
> > > can be marked with __no_sanitize_memory. This annotation should be
> > > used
> > > with great care, because it drops all instrumentation from the
> > > function, and any shadow writes will be lost. Even though this is
> > > not a
> > > concern with the current init_object() implementation, this may
> > > change
> > > in the future.
> > >
> > > Second, kmsan_poison_memory() calls may be added after memset()
> > > calls.
> > > The downside is that init_object() is called from
> > > free_debug_processing(), in which case poisoning will erase the
> > > distinction between simply uninitialized memory and UAF.
> > >
> > > Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
> > > ---
> > > mm/kmsan/hooks.c | 2 +-
> > > mm/slub.c | 13 +++++++++----
> > > 2 files changed, 10 insertions(+), 5 deletions(-)
> > >
> > [...]
> > > --- a/mm/slub.c
> > > +++ b/mm/slub.c
> > > @@ -1139,7 +1139,12 @@ static void init_object(struct kmem_cache
> > > *s, void *object, u8 val)
> > > unsigned int poison_size = s->object_size;
> > >
> > > if (s->flags & SLAB_RED_ZONE) {
> > > - memset(p - s->red_left_pad, val, s->red_left_pad);
> > > + /*
> > > + * Use __memset() here and below in order to avoid
> > > overwriting
> > > + * the KMSAN shadow. Keeping the shadow makes it
> > > possible to
> > > + * distinguish uninit-value from use-after-free.
> > > + */
> > > + __memset(p - s->red_left_pad, val, s-
> > > >red_left_pad);
> >
> > I found my build test[1] fails with below error on latest mm-unstable
> > branch.
> > 'git bisect' points me this patch.
> >
> > CC mm/slub.o
> > /mm/slub.c: In function 'init_object':
> > /mm/slub.c:1147:17: error: implicit declaration of function
> > '__memset'; did you mean 'memset'? [-Werror=implicit-function-
> > declaration]
> > 1147 | __memset(p - s->red_left_pad, val, s-
> > >red_left_pad);
> > | ^~~~~~~~
> > | memset
> > cc1: some warnings being treated as errors
> >
> > I haven't looked in deep, but reporting first. Do you have any idea?
> >
> > [1]
> > https://github.com/awslabs/damon-tests/blob/next/corr/tests/build_m68k.sh
> >
> >
> > Thanks,
> > SJ
> >
> > [...]
>
> Thanks for the report.
>
> Apparently not all architectures have __memset(). We should probably go
> back to memset_no_sanitize_memory() [1], but this time mark it with
> noinline __maybe_unused __no_sanitize_memory, like it's done in, e.g.,
> 32/35.
>
> Alexander, what do you think?

We could probably go without __no_sanitize_memory assuming that
platforms supporting KMSAN always have __memset():

#if defined(CONFIG_KMSAN)
static inline void *memset_no_sanitize_memory(void *s, int c, size_t n)
{
return __memset(s, c, n);
}
#else
static inline void *memset_no_sanitize_memory(void *s, int c, size_t n)
{
return memset(s, c, n);
}
#endif