Re: [PATCH v3] slab: Avoid race on slab->obj_exts in alloc_slab_obj_exts

[Suren Baghdasaryan <surenb@google.com>]

On Tue, Oct 21, 2025 at 12:04 AM Vlastimil Babka <vbabka@suse.cz> wrote:
>
> On 10/21/25 03:03, Hao Ge wrote:
> > From: Hao Ge <gehao@kylinos.cn>
> >
> > If two competing threads enter alloc_slab_obj_exts() and one of them
> > fails to allocate the object extension vector, it might override the
> > valid slab->obj_exts allocated by the other thread with
> > OBJEXTS_ALLOC_FAIL. This will cause the thread that lost this race and
> > expects a valid pointer to dereference a NULL pointer later on.
> >
> > Update slab->obj_exts atomically using cmpxchg() to avoid
> > slab->obj_exts overrides by racing threads.
> >
> > Thanks for Vlastimil and Suren's help with debugging.
> >
> > Fixes: f7381b911640 ("slab: mark slab->obj_exts allocation failures unconditionally")
> > Cc: <stable@vger.kernel.org>
> > Suggested-by: Suren Baghdasaryan <surenb@google.com>
> > Signed-off-by: Hao Ge <gehao@kylinos.cn>

Reviewed-by: Suren Baghdasaryan <surenb@google.com>

Thanks for the fix, Hao!

>
> Added to slab/for-next-fixes, thanks!
>
> > ---
> > v3: According to Suren's suggestion, simplify the commit message and the code comments.
> >     Thanks for Suren.
> >
> > v2: Incorporate handling for the scenario where, if mark_failed_objexts_alloc wins the race,
> >     the other process (that previously succeeded in allocation) will lose the race, based on Suren's suggestion.
> >     Add Suggested-by: Suren Baghdasaryan <surenb@google.com>
> > ---
> >  mm/slub.c | 9 ++++++---
> >  1 file changed, 6 insertions(+), 3 deletions(-)
> >
> > diff --git a/mm/slub.c b/mm/slub.c
> > index 2e4340c75be2..d4403341c9df 100644
> > --- a/mm/slub.c
> > +++ b/mm/slub.c
> > @@ -2054,7 +2054,7 @@ static inline void mark_objexts_empty(struct slabobj_ext *obj_exts)
> >
> >  static inline void mark_failed_objexts_alloc(struct slab *slab)
> >  {
> > -     slab->obj_exts = OBJEXTS_ALLOC_FAIL;
> > +     cmpxchg(&slab->obj_exts, 0, OBJEXTS_ALLOC_FAIL);
> >  }
> >
> >  static inline void handle_failed_objexts_alloc(unsigned long obj_exts,
> > @@ -2136,6 +2136,7 @@ int alloc_slab_obj_exts(struct slab *slab, struct kmem_cache *s,
> >  #ifdef CONFIG_MEMCG
> >       new_exts |= MEMCG_DATA_OBJEXTS;
> >  #endif
> > +retry:
> >       old_exts = READ_ONCE(slab->obj_exts);
> >       handle_failed_objexts_alloc(old_exts, vec, objects);
> >       if (new_slab) {
> > @@ -2145,8 +2146,7 @@ int alloc_slab_obj_exts(struct slab *slab, struct kmem_cache *s,
> >                * be simply assigned.
> >                */
> >               slab->obj_exts = new_exts;
> > -     } else if ((old_exts & ~OBJEXTS_FLAGS_MASK) ||
> > -                cmpxchg(&slab->obj_exts, old_exts, new_exts) != old_exts) {
> > +     } else if (old_exts & ~OBJEXTS_FLAGS_MASK) {
> >               /*
> >                * If the slab is already in use, somebody can allocate and
> >                * assign slabobj_exts in parallel. In this case the existing
> > @@ -2158,6 +2158,9 @@ int alloc_slab_obj_exts(struct slab *slab, struct kmem_cache *s,
> >               else
> >                       kfree(vec);
> >               return 0;
> > +     } else if (cmpxchg(&slab->obj_exts, old_exts, new_exts) != old_exts) {
> > +             /* Retry if a racing thread changed slab->obj_exts from under us. */
> > +             goto retry;
> >       }
> >
> >       if (allow_spin)
>