From bdb21928512a860a60e6a24a849dc5b63cbaf96a Mon Sep 17 00:00:00 2001 From: Dmitry Adamushko Date: Thu, 10 Jul 2008 22:21:58 +0200 Subject: [PATCH] slub: Fix use-after-preempt of per-CPU data structure Vegard Nossum reported a crash in kmem_cache_alloc(): BUG: unable to handle kernel paging request at da87d000 IP: [] kmem_cache_alloc+0xc7/0xe0 *pde = 28180163 *pte = 1a87d160 Oops: 0002 [#1] PREEMPT SMP DEBUG_PAGEALLOC Pid: 3850, comm: grep Not tainted (2.6.26-rc9-00059-gb190333 #5) EIP: 0060:[] EFLAGS: 00210203 CPU: 0 EIP is at kmem_cache_alloc+0xc7/0xe0 EAX: 00000000 EBX: da87c100 ECX: 1adad71a EDX: 6b6b6b6b ESI: 00200282 EDI: da87d000 EBP: f60bfe74 ESP: f60bfe54 DS: 007b ES: 007b FS: 00d8 GS: 0033 SS: 0068 and analyzed it: "The register %ecx looks innocent but is very important here. The disassembly: mov %edx,%ecx shr $0x2,%ecx rep stos %eax,%es:(%edi) <-- the fault So %ecx has been loaded from %edx... which is 0x6b6b6b6b/POISON_FREE. (0x6b6b6b6b >> 2 == 0x1adadada.) %ecx is the counter for the memset, from here: memset(object, 0, c->objsize); i.e. %ecx was loaded from c->objsize, so "c" must have been freed. Where did "c" come from? Uh-oh... c = get_cpu_slab(s, smp_processor_id()); This looks like it has very much to do with CPU hotplug/unplug. Is there a race between SLUB/hotplug since the CPU slab is used after it has been freed?" Good analysis. Yeah, it's possible that a caller of kmem_cache_alloc() -> slab_alloc() can be migrated on another CPU right after local_irq_restore() and before memset(). The inital cpu can become offline in the mean time (or a migration is a consequence of the CPU going offline) so its 'kmem_cache_cpu' structure gets freed ( slab_cpuup_callback). At some point of time the caller continues on another CPU having an obsolete pointer... Signed-off-by: Dmitry Adamushko Reported-by: Vegard Nossum Acked-by: Ingo Molnar Cc: stable@kernel.org Signed-off-by: Linus Torvalds --- mm/slub.c | 4 +++- 1 file changed, 3 insertions(+), 1 deletion(-) diff --git a/mm/slub.c b/mm/slub.c index 1a427c0ae83b..315c392253c7 100644 --- a/mm/slub.c +++ b/mm/slub.c @@ -1628,9 +1628,11 @@ static __always_inline void *slab_alloc(struct kmem_cache *s, void **object; struct kmem_cache_cpu *c; unsigned long flags; + unsigned int objsize; local_irq_save(flags); c = get_cpu_slab(s, smp_processor_id()); + objsize = c->objsize; if (unlikely(!c->freelist || !node_match(c, node))) object = __slab_alloc(s, gfpflags, node, addr, c); @@ -1643,7 +1645,7 @@ static __always_inline void *slab_alloc(struct kmem_cache *s, local_irq_restore(flags); if (unlikely((gfpflags & __GFP_ZERO) && object)) - memset(object, 0, c->objsize); + memset(object, 0, objsize); return object; } -- 2.20.1