On Apr 14, 2021, at 3:37 PM, Tom Parkin <[email protected]> wrote:
>
> On Tue, Apr 13, 2021 at 17:30:17 +0000, Gong, Sishuai wrote:
>> Hi,
>>
>> We found a concurrency bug in linux 5.12-rc3 and we are able to reproduce it
>> under x86. This bug happens when two l2tp functions l2tp_tunnel_register()
>> and l2tp_xmit_core() are running in parallel. In general,
>> l2tp_tunnel_register() registered a tunnel that hasn’t been fully
>> initialized and then l2tp_xmit_core() tries to access an uninitialized
>> attribute. The interleaving is shown below..
>>
>> ------------------------------------------
>> Execution interleaving
>>
>> Thread 1
>> Thread 2
>>
>> l2tp_tunnel_register()
>> spin_lock_bh(&pn->l2tp_tunnel_list_lock);
>> …
>> list_add_rcu(&tunnel->list, &pn->l2tp_tunnel_list);
>> // tunnel becomes visible
>> spin_unlock_bh(&pn->l2tp_tunnel_list_lock);
>>
>> pppol2tp_connect()
>>
>> …
>>
>> tunnel = l2tp_tunnel_get(sock_net(sk),
>> info.tunnel_id);
>>
>> // Successfully get the new tunnel
>>
>>
>> …
>>
>> l2tp_xmit_core()
>>
>> struct sock *sk = tunnel->sock;
>>
>> // uninitialized, sk=0
>>
>> …
>>
>> bh_lock_sock(sk);
>>
>> // Null-pointer exception happens
>> …
>> tunnel->sock = sk;
>>
>> ------------------------------------------
>> Impact & fix
>>
>> This bug causes a kernel NULL pointer deference error, as attached below.
>> Currently, we think a potential fix is to initialize tunnel->sock before
>> adding the tunnel into l2tp_tunnel_list.
>>
>> ------------------------------------------
>> Console output
>>
>> [ 806.566775][T10805] BUG: kernel NULL pointer dereference, address:
>> 00000070
>> [ 807.097222][T10805] #PF: supervisor read access in kernel mode
>> [ 807.647927][T10805] #PF: error_code(0x0000) - not-present page
>> [ 808.255377][T10805] *pde = 00000000
>> [ 808.757649][T10805] Oops: 0000 [#1] PREEMPT SMP
>> [ 809.367746][T10805] CPU: 1 PID: 10805 Comm: executor Not tainted
>> 5.12.0-rc3 #3
>> [ 810.590670][T10805] Hardware name: Bochs Bochs, BIOS Bochs 01/01/2007
>> [ 811.126044][T10805] EIP: _raw_spin_lock+0x16/0x50
>> [ 811.671747][T10805] Code: 00 00 00 00 55 89 d0 89 e5 e8 26 8c 20 fe 5d c3
>> 8d 74 26 00 55 89 c1 89 e5 53 64 ff 05 0c 97 fb c3 31 d2 bb 01 00 00 00 89
>> d0 <f0> 0f b1 19 75 0c 8b 5d fc c9 c3 8d b4 26
>> 00 00 00 00 8b 15 e8 7c
>> [ 813.375919][T10805] EAX: 00000000 EBX: 00000001 ECX: 00000070 EDX:
>> 00000000
>> [ 813.989487][T10805] ESI: cbb59300 EDI: cbac8c00 EBP: cf54fd68 ESP:
>> cf54fd64
>> [ 814.629205][T10805] DS: 007b ES: 007b FS: 00d8 GS: 00e0 SS: 0068 EFLAGS:
>> 00000246
>> [ 815.811079][T10805] CR0: 80050033 CR2: 00000070 CR3: 0efd3000 CR4:
>> 00000690
>> [ 816.526951][T10805] DR0: 00000000 DR1: 00000000 DR2: 00000000 DR3:
>> 00000000
>> [ 817.158214][T10805] DR6: 00000000 DR7: 00000000
>> [ 817.762257][T10805] Call Trace:
>> [ 818.322192][T10805] l2tp_xmit_skb+0x11a/0x530
>> [ 818.876097][T10805] pppol2tp_sendmsg+0x160/0x290
>> [ 819.438224][T10805] sock_sendmsg+0x2d/0x40
>> [ 820.077999][T10805] ____sys_sendmsg+0x1a2/0x1d0
>> [ 820.694928][T10805] ? import_iovec+0x13/0x20
>> [ 821.220194][T10805] ___sys_sendmsg+0x98/0xd0
>> [ 821.927886][T10805] ? file_update_time+0x4b/0x130
>> [ 822.458245][T10805] ? vfs_write+0x32c/0x3f0
>> [ 823.002593][T10805] __sys_sendmsg+0x39/0x80
>>
>>
>>
>> Thanks,
>> Sishuai
>>
>
> Hi Sishuai,
>
> Thanks for the report!
>
> Your analysis looks correct to me, and the suggested fix sounds
> reasonable too.
Thanks, I am glad I could be helpful:)
> Is this something you plan to submit a patch for?
We are not planning to submit a patch for now because we think experienced
developer have more comprehensive view than us, but we are very happy to test
any potential patches.
>
> Best regards,
> Tom
