Messages in this thread Patch in this message | | | Date | Tue, 28 Jun 2011 23:01:25 -0700 | From | Ben Greear <> | Subject | Re: Question on debugging use-after-free memory issues. |
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On 06/28/2011 10:41 PM, Eric Dumazet wrote: > Le mercredi 29 juin 2011 à 00:00 +0200, Jiri Kosina a écrit : >> On Mon, 27 Jun 2011, Ben Greear wrote:
>> Anyway, I'd propose to start with kmemcheck (see >> Documentation/kmemcheck.txt). It could pin-point the problemtic spot >> immediately (or it might not). >> > > kmemcheck is fine if problem is not coming from an SMP bug only. Also > kmemcheck is so slow it makes a rare bug becoming very hard to trigger.
I think I've pretty much verified that deleted memory is passed down a certain call path with the slub patches I posted. What I can't figure out is how that came to be.
> Ben, given that you know that RPC might have a problem on a given small > object (struct rpcbind_args ), you could afford changing the > kmalloc()/kfree() used to allocate/free such objects by calls to page > allocator, and dont free the page but unmap it from kernel mapping so > that any further read/write access triggers a fault. You can then have a > more precise idea of what's happening, without slowing down whole > kernel. Of course there is a mem leak for each "struct rpcbind_args" > allocated, so this is a debugging aid only. > > DEBUG_PAGEALLOC might be too expensive, so try this patch (untested, you > might need to complete it) > > diff --git a/net/sunrpc/rpcb_clnt.c b/net/sunrpc/rpcb_clnt.c > index 9a80a92..9b4dbaf 100644 > --- a/net/sunrpc/rpcb_clnt.c > +++ b/net/sunrpc/rpcb_clnt.c > @@ -158,7 +158,7 @@ static void rpcb_map_release(void *data) > rpcb_wake_rpcbind_waiters(map->r_xprt, map->r_status); > xprt_put(map->r_xprt); > kfree(map->r_addr); > - kfree(map); > + kernel_map_pages(virt_to_page(map), 1, 0); > } > > /* > @@ -668,7 +668,7 @@ void rpcb_getport_async(struct rpc_task *task) > goto bailout_nofree; > } > > - map = kzalloc(sizeof(struct rpcbind_args), GFP_ATOMIC); > + map = (struct rpcbind_args *)__get_free_page(GFP_ATOMIC | __GFP_ZERO); > if (!map) { > status = -ENOMEM; > dprintk("RPC: %5u %s: no memory available\n", >
It takes possibly hours of heavy load to hit the problem, so I think I cannot afford to leak that much memory.
Interestingly, I added this code below, and haven't hit the problem since. I'm not sure if it just changed the timing, or what...or maybe I'll hit it overnight...
I tried setting this (below) to 0x6b instead of 0x0 (mempool doesn't really kmalloc/free too often, so the slub poisoning doesn't help), but never did hit the bug again.
I suspect that somehow the task object is still on the work-queue, when it is deleted, but since the 0x6b and 0x0 poisoning didn't cause any funny crashes, I could easily be wrong about that.
diff --git a/net/sunrpc/sched.c b/net/sunrpc/sched.c index 17c3e3a..d94f009 100644 --- a/net/sunrpc/sched.c +++ b/net/sunrpc/sched.c @@ -859,6 +859,12 @@ static void rpc_free_task(struct rpc_task *task)
if (task->tk_flags & RPC_TASK_DYNAMIC) { dprintk("RPC: %5u freeing task\n", task->tk_pid); + /* HACK: Have been seeing use-after-free of calldata. Zero this memory + * so that it cannot happen here. Seems to have fixed the problem + * in 3.0 kernel, but maybe it just adjusted timing..either way, + * it's not a real fix. --Ben + */ + memset(task, 0, sizeof(*task)); mempool_free(task, rpc_task_mempool); } rpc_release_calldata(tk_ops, calldata);
Thanks, Ben
-- Ben Greear <greearb@candelatech.com> Candela Technologies Inc http://www.candelatech.com -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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