Since per memcg cache destruction is scheduled when the last slab is freed, to avoid use-after-free in kmem_cache_free we should either rearrange code in kmem_cache_free so that it won't dereference the cache ptr after freeing the object, or wait for all kmem_cache_free's to complete before proceeding to cache destruction.
The former approach isn't a good option from the future development point of view, because every modifications to kmem_cache_free must be done with great care then. Hence we should provide a method to wait for all currently executing kmem_cache_free's to finish. This patch makes SLUB's implementation of kmem_cache_free non-preemptable. As a result, synchronize_sched() will work as a barrier against kmem_cache_free's in flight, so that issuing it before cache destruction will protect us against the use-after-free. This won't affect performance of kmem_cache_free, because we already disable preemption there, and this patch only moves preempt_enable to the end of the function. Neither should it affect the system latency, because kmem_cache_free is extremely short, even in its slow path. SLAB's version of kmem_cache_free already proceeds with irqs disabled, so we only add a comment explaining why it's necessary for kmemcg there. Signed-off-by: Vladimir Davydov <[email protected]> Acked-by: Christoph Lameter <[email protected]> --- mm/slab.c | 6 ++++++ mm/slub.c | 12 ++++++------ 2 files changed, 12 insertions(+), 6 deletions(-) diff --git a/mm/slab.c b/mm/slab.c index 9ca3b87edabc..b3af82419251 100644 --- a/mm/slab.c +++ b/mm/slab.c @@ -3450,6 +3450,12 @@ static inline void __cache_free(struct kmem_cache *cachep, void *objp, { struct array_cache *ac = cpu_cache_get(cachep); + /* + * Since we free objects with irqs and therefore preemption disabled, + * we can use synchronize_sched() to wait for all currently executing + * kfree's to finish. This is necessary to avoid use-after-free on + * per memcg cache destruction. + */ check_irq_off(); kmemleak_free_recursive(objp, cachep->flags); objp = cache_free_debugcheck(cachep, objp, caller); diff --git a/mm/slub.c b/mm/slub.c index 35741592be8c..52565a9426ef 100644 --- a/mm/slub.c +++ b/mm/slub.c @@ -2673,18 +2673,17 @@ static __always_inline void slab_free(struct kmem_cache *s, slab_free_hook(s, x); -redo: /* - * Determine the currently cpus per cpu slab. - * The cpu may change afterward. However that does not matter since - * data is retrieved via this pointer. If we are on the same cpu - * during the cmpxchg then the free will succedd. + * We could make this function fully preemptable, but then we wouldn't + * have a method to wait for all currently executing kfree's to finish, + * which is necessary to avoid use-after-free on per memcg cache + * destruction. */ preempt_disable(); +redo: c = this_cpu_ptr(s->cpu_slab); tid = c->tid; - preempt_enable(); if (likely(page == c->page)) { set_freepointer(s, object, c->freelist); @@ -2701,6 +2700,7 @@ redo: } else __slab_free(s, page, x, addr); + preempt_enable(); } void kmem_cache_free(struct kmem_cache *s, void *x) -- 1.7.10.4 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
