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 qcDR+EYL 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When alloc_slab_obj_exts() is called later in time (instead of at slab > allocation & initialization step), slab->stride and slab->obj_exts are > set when the slab is already accessible by multiple CPUs. > > The current implementation does not enforce memory ordering between > slab->stride and slab->obj_exts. However, for correctness, slab->stride > must be visible before slab->obj_exts, otherwise concurrent readers > may observe slab->obj_exts as non-zero while stride is still stale, > leading to incorrect reference counting of object cgroups. > > There has been a bug report [1] that showed symptoms of incorrect > reference counting of object cgroups, which could be triggered by > this memory ordering issue. > > Fix this by unconditionally initializing slab->stride in > alloc_slab_obj_exts_early(), before the need_slab_obj_exts() check. > In case of SLAB_OBJ_EXT_IN_OBJ, it is overridden in the same function. > > This ensures stride is set before the slab becomes visible to > other CPUs via the per-node partial slab list (protected by spinlock > with acquire/release semantics), preventing them from observing > inconsistent stride value. > > Thanks to Shakeel Butt for pointing out this issue [2]. > > Fixes: 7a8e71bc619d ("mm/slab: use stride to access slabobj_ext") > Reported-by: Venkat Rao Bagalkote > Closes: https://lore.kernel.org/lkml/ca241daa-e7e7-4604-a48d-de91ec9184a5@linux.ibm.com [1] > Link: https://lore.kernel.org/linux-mm/aZu9G9mVIVzSm6Ft@hyeyoo [2] > Signed-off-by: Harry Yoo > --- Vlastimil, could you please update the changelog when applying this to the tree? I think this also explains [3] (thanks for raising it off-list, Vlastimil!): When alloc_slab_obj_exts() is called later (instead of during slab allocation and initialization), slab->stride and slab->obj_exts are updated after the slab is already accessible by multiple CPUs. The current implementation does not enforce memory ordering between slab->stride and slab->obj_exts. For correctness, slab->stride must be visible before slab->obj_exts. Otherwise, concurrent readers may observe slab->obj_exts as non-zero while stride is still stale. With stale slab->stride, slab_obj_ext() could return the wrong obj_ext. This could cause two problems: - obj_cgroup_put() is called on the wrong objcg, leading to a use-after-free due to incorrect reference counting [1] by decrementing the reference count more than it was incremented. - refill_obj_stock() is called on the wrong objcg, leading to a page_counter overflow [2] by uncharging more memory than charged. Fix this by unconditionally initializing slab->stride in alloc_slab_obj_exts_early(), before the need_slab_obj_exts() check. In the case of SLAB_OBJ_EXT_IN_OBJ, it is overridden in the function. This ensures updates to slab->stride become visible before the slab can be accessed by other CPUs via the per-node partial slab list (protected by spinlock with acquire/release semantics). Thanks to Shakeel Butt for pointing out this issue [3]. Fixes: 7a8e71bc619d ("mm/slab: use stride to access slabobj_ext") Reported-by: Venkat Rao Bagalkote Closes: https://lore.kernel.org/lkml/ca241daa-e7e7-4604-a48d-de91ec9184a5@linux.ibm.com [1] Closes: https://lore.kernel.org/all/ddff7c7d-c0c3-4780-808f-9a83268bbf0c@linux.ibm.com [2] Link: https://lore.kernel.org/linux-mm/aZu9G9mVIVzSm6Ft@hyeyoo [3] Signed-off-by: Harry Yoo -- Cheers, Harry / Hyeonggon