Subject: Re: ipv4: crash at leaf_walk_rcu

From: "Paul E. McKenney" <[email-masked]>
Date: Wed, 31 Jul 2013 05:55:13 -0700

On Wed, Jul 31, 2013 at 04:40:47PM +0530, vinayak menon wrote:
> Hi,
> 
> A crash was seen on 3.4.5 kernel during some random wlan operations.
> 
> CPU: Single core ARM Cortex A9.
> 
> fib_route_seq_next was called with second argument (void *v) as 0xd6e3e360
> which is a "freed" object of the "ip_fib_trie" cache. I confirmed that the
> object was freed with crash utility.
> 
> Sequence: fib_route_seq_next->trie_nextleaf->leaf_walk_rcu
> 
> As "v" was a freed object, inside trie_nextleaf(), node_parent_rcu()
> returned an invalid tnode. But as I had enabled slab poisoning and the
> object was already freed, the tnode was 0x6b6b6b6b. And this was passed to
> leaf_walk_rcu and resulted in the crash.
> 
> fib_route_seq_start, takes rcu_read_lock(), but free_leaf
> calls call_rcu_bh. Can this be the problem ?
> Should rcu_read_lock() in fib_route_seq_start be changed to rcu_read_lock_bh()
> ?

One way or the other, the RCU read-side primitives need to match the RCU
update-side primitives.  Adding netdev...

							Thanx, Paul

> ----------------------------------------------------------------------------
> PC is at leaf_walk_rcu+0x10/0xa0
> LR is at fib_route_seq_next+0x58/0x74
> pc : [<c0500e5c>]    lr : [<c050108c>]    psr: a0000013
> sp : c150bee0  ip : 00000000  fp : 00000000
> r10: 00000400  r9 : 53701020  r8 : c32345c0
> r7 : 00000000  r6 : 00000001  r5 : 00000000  r4 : 00000002
> r3 : 6b6b6b6b  r2 : 00000001  r1 : d6e3e360  r0 : 6b6b6b6a
> Flags: NzCv  IRQs on  FIQs on  Mode SVC_32  ISA ARM  Segment user
> Control: 10c53c7d  Table: 835dc059  DAC: 00000015
> 
> Backtrace:
> [<c0500e5c>] (leaf_walk_rcu+0x10/0xa0) from [<c050108c>]
> (fib_route_seq_next+0x58/0x74)
> [<c050108c>] (fib_route_seq_next+0x58/0x74) from [<c011c06c>]
> (seq_read+0x2cc/0x438)
> [<c011c06c>] (seq_read+0x2cc/0x438) from [<c0145734>]
> (proc_reg_read+0xb0/0xcc)
> [<c0145734>] (proc_reg_read+0xb0/0xcc) from [<c0100798>]
> (vfs_read+0xac/0x124)
> [<c0100798>] (vfs_read+0xac/0x124) from [<c0100848>] (sys_read+0x38/0x64)
> [<c0100848>] (sys_read+0x38/0x64) from [<c000e100>]
> (ret_fast_syscall+0x0/0x48)
> 
> Thanks,
> Vinayak