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
