[patch 31/47] slub: Fix use-after-preempt of per-CPU data structure

From: Greg KH
Date: Tue Jul 22 2008 - 19:28:52 EST

2.6.25-stable review patch. If anyone has any objections, please let us

From: Dmitry Adamushko <dmitry.adamushko@xxxxxxxxx>

commit bdb21928512a860a60e6a24a849dc5b63cbaf96a upstream

Vegard Nossum reported a crash in kmem_cache_alloc():

BUG: unable to handle kernel paging request at da87d000
IP: [<c01991c7>] kmem_cache_alloc+0xc7/0xe0
*pde = 28180163 *pte = 1a87d160
Pid: 3850, comm: grep Not tainted (2.6.26-rc9-00059-gb190333 #5)
EIP: 0060:[<c01991c7>] 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 <dmitry.adamushko@xxxxxxxxx>
Reported-by: Vegard Nossum <vegard.nossum@xxxxxxxxx>
Acked-by: Ingo Molnar <mingo@xxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxx>

mm/slub.c | 4 +++-
1 file changed, 3 insertions(+), 1 deletion(-)

--- a/mm/slub.c
+++ b/mm/slub.c
@@ -1575,9 +1575,11 @@ static __always_inline void *slab_alloc(
void **object;
struct kmem_cache_cpu *c;
unsigned long flags;
+ unsigned int objsize;

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);
@@ -1590,7 +1592,7 @@ static __always_inline void *slab_alloc(

if (unlikely((gfpflags & __GFP_ZERO) && object))
- memset(object, 0, c->objsize);
+ memset(object, 0, objsize);

return object;

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