Messages in this thread Patch in this message | | | From | "Toshio" <> | Subject | unable to handle kernel NULL pointer dereference in skb_dequeue | Date | Fri, 3 Dec 2010 13:42:07 +0100 |
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I have also hit on Bug 20292 (https://bugzilla.kernel.org/show_bug.cgi?id=20292) in final 2.6.36. After investigating changes made between 2.6.35.4, which worked, and 2.6.36 which started oopsing, I think the problem was in double freeing of skb caused by change of return value for __pppoe_xmit in case of errors.
As it turned out this might be the cause of random BUG reports throught the kernel, whenever something touched skb. Most common BUG with my use case happened at skb_dequeue:
00000060 <skb_dequeue>: 60: 53 push %ebx 61: 89 c2 mov %eax,%edx 63: 9c pushf 64: 59 pop %ecx 65: fa cli 66: 8b 00 mov (%eax),%eax 68: 39 c2 cmp %eax,%edx 6a: 74 24 je 90 <skb_dequeue+0x30> 6c: 85 c0 test %eax,%eax 6e: 74 1a je 8a <skb_dequeue+0x2a> 70: ff 4a 08 decl 0x8(%edx) 73: 8b 18 mov (%eax),%ebx 75: c7 00 00 00 00 00 movl $0x0,(%eax) 7b: 8b 50 04 mov 0x4(%eax),%edx 7e: c7 40 04 00 00 00 00 movl $0x0,0x4(%eax) 85: 89 53 04 mov %edx,0x4(%ebx) 88:* 89 1a mov %ebx,(%edx) 8a: 51 push %ecx 8b: 9d popf 8c: 5b pop %ebx 8d: c3 ret 8e: 66 90 xchg %ax,%ax 90: b8 00 00 00 00 mov $0x0,%eax 95: eb f3 jmp 8a <skb_dequeue+0x2a> 97: 89 f6 mov %esi,%esi 99: 8d bc 27 00 00 00 00 lea 0x0(%edi,%eiz,1),%edi
This location corresponds to line "next = next->next" from inlined __skb_dequeue(). The only reason BUG could happen here is something overwriting or otherwise corrupting skb list.
Patch that works for me is below. Now I only hope I haven't (re)introduced a memory leak...
I am not subscribed to LKML, so please reply-to-all if you need to contact me.
----------------------------------------------------------------------------- --- linux-2.6.36/drivers/net/pppoe.c 2010-10-20 22:30:22.000000000 +0200 +++ linux-2.6.36.toshio/drivers/net/pppoe.c 2010-12-03 13:11:56.000000000 +0100 @@ -924,8 +924,10 @@ /* Copy the data if there is no space for the header or if it's * read-only. */ - if (skb_cow_head(skb, sizeof(*ph) + dev->hard_header_len)) + if (skb_cow_head(skb, sizeof(*ph) + dev->hard_header_len)) { + kfree_skb(skb); goto abort; + } __skb_push(skb, sizeof(*ph)); skb_reset_network_header(skb); @@ -947,7 +949,6 @@ return 1; abort: - kfree_skb(skb); return 0; }
----------------------------------------------------------------------------- Andrej Ota.
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