netdev - A concurrency bug between l2tp_tunnel_register() and l2tp_xmit

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Message-ID: <400E2FE1-A1E7-43EE-9ABA-41C65601C6EB@purdue.edu>
Date:   Tue, 13 Apr 2021 17:30:17 +0000
From:   "Gong, Sishuai" <sishuai@...due.edu>
To:     "jchapman@...alix.com" <jchapman@...alix.com>,
        "tparkin@...alix.com" <tparkin@...alix.com>
CC:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: A concurrency bug between l2tp_tunnel_register() and l2tp_xmit_core() 

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