Subject: Re: [PATCH net] net: sctp: sctp_seq_dump_local_addrs: fix NULL pointer dereference

From: Daniel Borkmann <[email-masked]>
Date: Wed, 12 Jun 2013 15:19:49 +0200

On 06/12/2013 03:04 PM, Daniel Borkmann wrote:
> On 06/12/2013 02:56 PM, Neil Horman wrote:
>> On Tue, Jun 11, 2013 at 06:47:46PM +0200, Daniel Borkmann wrote:
>>> The following NULL pointer dereference has occured on a 2.6.32-358 kernel,
>>> but upstream is affected as well since there are not many differences:
>>>
>>> sctp protocol violation state 4 chunkid 8
>>> BUG: unable to handle kernel NULL pointer dereference at 0000000000000078
>>> IP: [<ffffffffa0354cac>] sctp_v4_cmp_addr+0xc/0x30 [sctp]
>>> PGD c2758c067 PUD 87ecf1067 PMD 0
>>> Oops: 0000 [#1] SMP
>>> last sysfs file: /sys/devices/pci0000:00/0000:00:0a.0/0000:02:00.1/local_cpus
>>> CPU 7
>>> Modules linked in: [...]
>>> Pid: 15475, comm: netstat Not tainted 2.6.32-358.el6.x86_64 #1 KONTRON AT8050/FYA/AT8050/FYA
>>> RIP: 0010:[<ffffffffa0354cac>]  [<ffffffffa0354cac>] sctp_v4_cmp_addr+0xc/0x30 [sctp]
>>> RSP: 0018:ffff880c280f3d08  EFLAGS: 00010206
>>> RAX: 0000000000000002 RBX: ffff880827a25c00 RCX: 0000000000000001
>>> RDX: 000000000000027f RSI: 0000000000000078 RDI: ffff880827a25c20
>>> RBP: ffff880c280f3d08 R08: 00000000fffffffd R09: 0000000000000001
>>> R10: 0000000000000003 R11: 0000000000000000 R12: ffff880c27d9d870
>>> R13: ffffffffa03751c0 R14: ffff880827a25c20 R15: 0000000000000078
>>> FS:  00007fb7b7d0f7a0(0000) GS:ffff8800282e0000(0000) knlGS:0000000000000000
>>> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>>>   -- MORE --  forward: <SPACE>, <ENTER> or j  backward: b or k  quit: q
>>> CR2: 0000000000000078 CR3: 000000087c36d000 CR4: 00000000000007e0
>>> DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
>>> DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
>>> Process netstat (pid: 15475, threadinfo ffff880c280f2000, task ffff88091e361540)
>>> Stack:
>>>   ffff880c280f3d58 ffffffffa036dbac ffff880c280f3d28 ffff88098fefebc0
>>> <d> ffff880c280f3d38 ffff880c27d9d800 ffff88098fefebc0 0000000000000f83
>>> <d> ffff8808d12a7b80 ffff880c27d9d948 ffff880c280f3e18 ffffffffa036de3d
>>> Call Trace:
>>>   [<ffffffffa036dbac>] sctp_seq_dump_local_addrs+0x6c/0xc0 [sctp]
>>>   [<ffffffffa036de3d>] sctp_assocs_seq_show+0x12d/0x250 [sctp]
>>>   [<ffffffff811a4ee0>] ? seq_read+0x0/0x400
>>>   [<ffffffff811a5169>] seq_read+0x289/0x400
>>>   [<ffffffff811e966e>] proc_reg_read+0x7e/0xc0
>>>   [<ffffffff811816c5>] vfs_read+0xb5/0x1a0
>>>   [<ffffffff81181801>] sys_read+0x51/0x90
>>>   [<ffffffff810dc565>] ? __audit_syscall_exit+0x265/0x290
>>>   [<ffffffff8100b072>] system_call_fastpath+0x16/0x1b
>>> Code: 02 00 08 8b 47 04 89 46 04 b8 08 00 00 00 c9 c3 66 66 66 66 66 66
>>>        2e 0f 1f 84 00 00 00 00 00 55 48 89 e5 0f 1f 44 00 00 0f b7 07 <66>
>>>        3b 06 74 07 31 c0 c9 c3 0f 1f 00 0f b7 47 02 66 3b 46 02 75
>>> RIP  [<ffffffffa0354cac>] sctp_v4_cmp_addr+0xc/0x30 [sctp]
>>>   RSP <ffff880c280f3d08>
>>> CR2: 0000000000000078
>>>
>>> ./decodecode < oops.file:
>>>    ...
>>>    1f:    55                       push   %rbp
>>>    20:    48 89 e5                 mov    %rsp,%rbp
>>>    23:    0f 1f 44 00 00           nopl   0x0(%rax,%rax,1)
>>>    28:    0f b7 07                 movzwl (%rdi),%eax
>>>    2b:*    66 3b 06                 cmp    (%rsi),%ax  <-- trapping instruction
>>>    2e:    74 07                    je     0x37       (1st 'if' in sctp_v4_cmp_addr)
>>>    30:    31 c0                    xor    %eax,%eax
>>>    32:    c9                       leaveq
>>>    33:    c3                       retq
>>>    34:    0f 1f 00                 nopl   (%rax)
>>>    37:    0f b7 47 02              movzwl 0x2(%rdi),%eax
>>>    ...
>>>
>>> There have been some approaches to fix corruptions with the same or
>>> very similar stack trace such as 2eebc1e ("sctp: Fix list corruption
>>> resulting from freeing an association on a list"), 0b0fe913 ("sctp:
>>> proc: protect bind_addr->address_list accesses with rcu_read_lock()"),
>>> 45122ca ("sctp: Add RCU protection to assoc->transport_addr_list")
>>> that are all important fixes, but the panic still can occur in some
>>> cases with such a stack trace above.
>>>
>>> When entering into sctp_seq_dump_local_addrs(), the sctp_ep_common
>>> structure is correctly of type SCTP_EP_TYPE_ASSOCIATION, and has a
>>> refcnt of 1 and dead is 0. Also from kdump, the ebp's pointer members
>>> do not look corrupted. When entering sctp_v4_cmp_addr(), the first
>>> sctp_addr argument looks good/valid, but the second sctp_addr argument
>>> ('primary') is 0000000000000078 and results suddenly in a NULL pointer
>>> _dereference_ in sctp_v4_cmp_addr() although in the test before it
>>> seems to have been "primary != NULL".
>>>
>>> Now, how can this happen? Let's assume asoc->peer.primary_path is NULL
>>> which is possible to happen. Here, we do not check for it, but take the
>>> address (note we do not actually dereference it!) of its saddr member.
>>> This is then exactly 0000000000000078 as reported in the trace, since
>>> that is equal to the offset of saddr from the primary_path container
>>> (in the reported kernel). Thus, the check for NULL (== (void *) 0) will
>>> pass, eventually leading to the NULL pointer dereference in cmp_addr()
>>> since this address is within the NULL page.
>>>
>>> Lets fix it by being paranoid and checking first if our primary transport
>>> is != NULL and in case of != NULL if it is alive. Then, let us hold a
>>> copy of the sctp_addr on the stack instead of using a pointer just in
>>> case the transport could be destroyed at a later point in time (we're not
>>> in a hurry anyway). By that, we can avoid this very scenario, only just
>>> for the sake of printing this asterisk and can fix the NULL pointer
>>> dereference eventually. Introduced by bca735bd ("Extend the info exported
>>> via /proc/net/sctp to support netstat for SCTP.").
>>>
>> First off, nice analysis Daniel, that very clear and consice.  The patch below
>> makes sense to me, but I'm getting the impression that, if we need to use the
>> dead flag here, that rcu_locking isn't going to be safe. If we aren't guaranteed
>> that saddr is going to be a valid pointer or NULL, and as a consequence are
>> going to need to rely on the ->dead flag, then we have a potential race
>> condition to worry about.  I think the transport pointer is safe here, as its
>> rcu protected in the free path, but its parent association is not.  By getting
>> the assocation that a given trasport points to, we risk a race with the
>> destruction of that association (I think).  We probably need to convert the
>> association create/free paths (as well as the endpoint, etc, create/free paths),
>> to be rcu sensitive, but for the time being, it may be sufficient to simply hold
>> the association in sctp_seq_dump_local_addrs.
>
> Well, checking for ->dead might be unnecessary and can probably be deleted (if wished
> I can do that and resubmit).
>
> At the time of the crash, the association was in SCTP_STATE_CLOSED and the pointer to
> primary_path in fact NULL.
>
> The association looks as follows:
>
> crash> sctp_association ffff880c27d9d800 |grep primary_path
>      primary_path = 0x0,

Probably this could be the case of a SCTP TCP-style socket that is still around, but
eventually removed in sctp_close():

	if (sctp_style(sk, TCP)) {
		/* A closed association can still be in the list if
		 * it belongs to a TCP-style listening socket that is
		 * not yet accepted. If so, free it. If not, send an
		 * ABORT or SHUTDOWN based on the linger options.
		 */
		if (sctp_state(asoc, CLOSED)) {
			sctp_unhash_established(asoc);
			sctp_association_free(asoc);
			continue;
		}
	}

This would explain the SCTP_STATE_CLOSED of the association.