Re: [PATCH v3 34/35] codetag: debug: introduce OBJEXTS_ALLOC_FAIL to mark failed slab_ext allocations

Suren Baghdasaryan
2024-02-13T00:09:37Z

On Mon, Feb 12, 2024 at 2:49 PM Kees Cook <keescook@chromium.org> wrote:
>
> On Mon, Feb 12, 2024 at 01:39:20PM -0800, Suren Baghdasaryan wrote:
> > If slabobj_ext vector allocation for a slab object fails and later on it
> > succeeds for another object in the same slab, the slabobj_ext for the
> > original object will be NULL and will be flagged in case when
> > CONFIG_MEM_ALLOC_PROFILING_DEBUG is enabled.
> > Mark failed slabobj_ext vector allocations using a new objext_flags flag
> > stored in the lower bits of slab->obj_exts. When new allocation succeeds
> > it marks all tag references in the same slabobj_ext vector as empty to
> > avoid warnings implemented by CONFIG_MEM_ALLOC_PROFILING_DEBUG checks.
> >
> > Signed-off-by: Suren Baghdasaryan <surenb@google.com>
> > ---
> > include/linux/memcontrol.h | 4 +++-
> > mm/slab.h | 25 +++++++++++++++++++++++++
> > mm/slab_common.c | 22 +++++++++++++++-------
> > 3 files changed, 43 insertions(+), 8 deletions(-)
> >
> > diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> > index 2b010316016c..f95241ca9052 100644
> > --- a/include/linux/memcontrol.h
> > +++ b/include/linux/memcontrol.h
> > @@ -365,8 +365,10 @@ enum page_memcg_data_flags {
> > #endif /* CONFIG_MEMCG */
> >
> > enum objext_flags {
> > + /* slabobj_ext vector failed to allocate */
> > + OBJEXTS_ALLOC_FAIL = __FIRST_OBJEXT_FLAG,
> > /* the next bit after the last actual flag */
> > - __NR_OBJEXTS_FLAGS = __FIRST_OBJEXT_FLAG,
> > + __NR_OBJEXTS_FLAGS = (__FIRST_OBJEXT_FLAG << 1),
> > };
> >
> > #define OBJEXTS_FLAGS_MASK (__NR_OBJEXTS_FLAGS - 1)
> > diff --git a/mm/slab.h b/mm/slab.h
> > index cf332a839bf4..7bb3900f83ef 100644
> > --- a/mm/slab.h
> > +++ b/mm/slab.h
> > @@ -586,9 +586,34 @@ 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;
>
> Uh, does this mean slab->obj_exts is suddenly non-NULL? Is everything
> that accesses obj_exts expecting this?

Hi Kees,
Thank you for the reviews!
Yes, I believe everything that accesses slab->obj_exts directly
(currently alloc_slab_obj_exts() and free_slab_obj_exts()) handle this
special non-NULL case. kfence_init_pool() initialized slab->obj_exts
directly, but since it's setting it and not accessing it, it does not
need to handle OBJEXTS_ALLOC_FAIL. All other slab->obj_exts users use
slab_obj_exts() which applies OBJEXTS_FLAGS_MASK and masks out any
special bits.
Thanks,
Suren.

>
> -Kees
>
> > +}
> > +
> > +static inline void handle_failed_objexts_alloc(unsigned long obj_exts,
> > + struct slabobj_ext *vec, unsigned int objects)
> > +{
> > + /*
> > + * If vector previously failed to allocate then we have live
> > + * objects with no tag reference. Mark all references in this
> > + * vector as empty to avoid warnings later on.
> > + */
> > + if (obj_exts & OBJEXTS_ALLOC_FAIL) {
> > + unsigned int i;
> > +
> > + for (i = 0; i < objects; i++)
> > + set_codetag_empty(&vec[i].ref);
> > + }
> > +}
> > +
> > +
> > #else /* CONFIG_MEM_ALLOC_PROFILING_DEBUG */
> >
> > static inline void mark_objexts_empty(struct slabobj_ext *obj_exts) {}
> > +static inline void mark_failed_objexts_alloc(struct slab *slab) {}
> > +static inline void handle_failed_objexts_alloc(unsigned long obj_exts,
> > + struct slabobj_ext *vec, unsigned int objects) {}
> >
> > #endif /* CONFIG_MEM_ALLOC_PROFILING_DEBUG */
> >
> > diff --git a/mm/slab_common.c b/mm/slab_common.c
> > index d5f75d04ced2..489c7a8ba8f1 100644
> > --- a/mm/slab_common.c
> > +++ b/mm/slab_common.c
> > @@ -214,29 +214,37 @@ int alloc_slab_obj_exts(struct slab *slab, struct kmem_cache *s,
> > gfp_t gfp, bool new_slab)
> > {
> > unsigned int objects = objs_per_slab(s, slab);
> > - unsigned long obj_exts;
> > - void *vec;
> > + unsigned long new_exts;
> > + unsigned long old_exts;
> > + struct slabobj_ext *vec;
> >
> > gfp &= ~OBJCGS_CLEAR_MASK;
> > /* Prevent recursive extension vector allocation */
> > gfp |= __GFP_NO_OBJ_EXT;
> > vec = kcalloc_node(objects, sizeof(struct slabobj_ext), gfp,
> > slab_nid(slab));
> > - if (!vec)
> > + if (!vec) {
> > + /* Mark vectors which failed to allocate */
> > + if (new_slab)
> > + mark_failed_objexts_alloc(slab);
> > +
> > return -ENOMEM;
> > + }
> >
> > - obj_exts = (unsigned long)vec;
> > + new_exts = (unsigned long)vec;
> > #ifdef CONFIG_MEMCG
> > - obj_exts |= MEMCG_DATA_OBJEXTS;
> > + new_exts |= MEMCG_DATA_OBJEXTS;
> > #endif
> > + old_exts = slab->obj_exts;
> > + handle_failed_objexts_alloc(old_exts, vec, objects);
> > if (new_slab) {
> > /*
> > * If the slab is brand new and nobody can yet access its
> > * obj_exts, no synchronization is required and obj_exts can
> > * be simply assigned.
> > */
> > - slab->obj_exts = obj_exts;
> > - } else if (cmpxchg(&slab->obj_exts, 0, obj_exts)) {
> > + slab->obj_exts = new_exts;
> > + } else if (cmpxchg(&slab->obj_exts, old_exts, new_exts) != old_exts) {
> > /*
> > * If the slab is already in use, somebody can allocate and
> > * assign slabobj_exts in parallel. In this case the existing
> > --
> > 2.43.0.687.g38aa6559b0-goog
> >
>
> --
> Kees Cook