On Thu, 2016-11-10 at 09:47 -0500, Vladis Dronov wrote:
> Hello,
>
> It was discovered by Marco Grassi <marco.gra@xxxxxxxxx> (many thanks) that the
> latest stable Linux kernel v4.8.6 is crashing in tcp_collapse() after making
> certain syscalls:
>
> [ 9.622886] kernel BUG at net/ipv4/tcp_input.c:4813!
> [ 9.623299] invalid opcode: 0000 [#1] SMP
> [ 9.623642] Modules linked in: iptable_nat nf_nat_ipv4 nf_nat
> [ 9.624287] CPU: 2 PID: 2871 Comm: poc Not tainted 4.8.6 #2
> [ 9.624730] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.8.2-20150714_191134- 04/01/2014
> [ 9.625459] task: ffff8801387b9a00 task.stack: ffff8801380e4000
> [ 9.625929] RIP: 0010:[<ffffffff8178d4ec>] [<ffffffff8178d4ec>] tcp_collapse+0x3ac/0x3b0
> [ 9.626609] RSP: 0018:ffff8801380e7b78 EFLAGS: 00010282
> [ 9.627028] RAX: 00000000fffffff2 RBX: 0000000000000ec0 RCX: 0000000000000ec0
> [ 9.627587] RDX: ffff8801365cd000 RSI: 0000000000000000 RDI: ffff8801364106e0
> [ 9.628142] RBP: ffff8801380e7bc8 R08: 0000000000000000 R09: ffff88013b003300
> [ 9.628704] R10: ffff8801365cd000 R11: 0000000000000000 R12: 0000000000000ec0
> [ 9.629259] R13: ffff88013663ae00 R14: 00000000cdf0ca26 R15: ffff8801364106e0
> [ 9.629819] FS: 00007f2cef695800(0000) GS:ffff88013fc80000(0000) knlGS:0000000000000000
> [ 9.630945] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> [ 9.631655] CR2: 000000002002a000 CR3: 0000000139d46000 CR4: 00000000001406e0
> [ 9.632462] Stack:
> [ 9.632900] 0000000000000000 cdf0da2600000001 ffff880138050000 ffff8801380500a8
> [ 9.634138] ffff880100000000 ffff880138050688 0000000000000900 ffff8801364136e0
> [ 9.635379] ffff880138050000 ffff880138050688 ffff8801380e7c00 ffffffff8178d630
> [ 9.636622] Call Trace:
> [ 9.637087] [<ffffffff8178d630>] tcp_try_rmem_schedule+0x140/0x380
> [ 9.637834] [<ffffffff81791aa8>] tcp_data_queue+0x898/0xcf0
> [ 9.638538] [<ffffffff8179210b>] tcp_rcv_established+0x20b/0x6c0
> [ 9.639268] [<ffffffff81710143>] ? sk_reset_timer+0x13/0x30
> [ 9.639968] [<ffffffff81813009>] tcp_v6_do_rcv+0x1b9/0x420
> [ 9.640666] [<ffffffff81710b02>] __release_sock+0x82/0xf0
> [ 9.641353] [<ffffffff81710b9b>] release_sock+0x2b/0x90
> [ 9.642029] [<ffffffff817890ca>] tcp_sendmsg+0x55a/0xb60
> [ 9.642714] [<ffffffff817b29d0>] inet_sendmsg+0x60/0x90
> [ 9.643389] [<ffffffff8170c7b3>] sock_sendmsg+0x33/0x40
> [ 9.644064] [<ffffffff8170ccee>] SYSC_sendto+0xee/0x160
> [ 9.645530] [<ffffffff8170d6f9>] SyS_sendto+0x9/0x10
> [ 9.646190] [<ffffffff81909df2>] entry_SYSCALL_64_fastpath+0x1a/0xa4
> [ 9.646947] Code: 48 c7 07 00 00 00 00 48 89 42 08 48 89 10 e8 cc 7e f8 ff 49 8b 47 30 48 8b 80 80 01 00 00 65 48 ff 80 b0 01 00 00 e9 72 fd ff ff <0f> 0b 66 90 55 48 89 e5 41 57 41 56 41 55 41 54 49 89 fe 53 8b
> [ 9.651794] RIP [<ffffffff8178d4ec>] tcp_collapse+0x3ac/0x3b0
> [ 9.652554] RSP <ffff8801380e7b78>
>
> The reproducer is generated by the syzkaller, please, see attached. The
> following BUG() is hit:
>
> [net/ipv4/tcp_input.c]
> static void
> tcp_collapse(struct sock *sk, struct sk_buff_head *list,
> struct sk_buff *head, struct sk_buff *tail,
> u32 start, u32 end)
> {
> ...
> /* Copy data, releasing collapsed skbs. */
> while (copy > 0) {
> int offset = start - TCP_SKB_CB(skb)->seq;
> int size = TCP_SKB_CB(skb)->end_seq - start;
>
> BUG_ON(offset < 0);
> if (size > 0) {
> size = min(copy, size);
> 4812: if (skb_copy_bits(skb, offset, skb_put(nskb, size), size))
> 4813: BUG();
>
> /usr/src/linux-4.8.6/net/ipv4/tcp_input.c: 4812
> 0xffffffff8178d390 <tcp_collapse+0x250>: mov %r12d,%esi
> 0xffffffff8178d393 <tcp_collapse+0x253>: callq 0xffffffff81713ce0 <skb_put>
> 0xffffffff8178d398 <tcp_collapse+0x258>: mov -0x30(%rbp),%r8d
> 0xffffffff8178d39c <tcp_collapse+0x25c>: mov %r12d,%ecx
> 0xffffffff8178d39f <tcp_collapse+0x25f>: mov %rax,%rdx
> 0xffffffff8178d3a2 <tcp_collapse+0x262>: mov %r15,%rdi
> 0xffffffff8178d3a5 <tcp_collapse+0x265>: mov %r8d,%esi
> 0xffffffff8178d3a8 <tcp_collapse+0x268>: callq 0xffffffff81714b90 <skb_copy_bits>
> 0xffffffff8178d3ad <tcp_collapse+0x26d>: test %eax,%eax
> 0xffffffff8178d3af <tcp_collapse+0x26f>: jne 0xffffffff8178d4ec <tcp_collapse+0x3ac>
> ...
> /usr/src/linux-4.8.6/net/ipv4/tcp_input.c: 4813
> 0xffffffff8178d4ec <tcp_collapse+0x3ac>: ud2
>
> I have checked that the reproducer can cause hitting this BUG() in the kernels
> since, at least v4.0. I was not checking the earlier kernels except RHEL-7 ones
> (3.10.0-xxx) which are not vulnerable.
>
> The upstream kernels since v4.9-rc1 are not vulnerable too and I have bisected
> the repo to the commit c9c3321257 which fixes the issue.
>
> $ git tag --contain c9c3321257e1b95be9b375f811fb250162af8d39
> v4.9-rc1
>
> Stable v4.8.6 kernel with the c9c3321257 commit applied does not hit the BUG(),
> so I believe this commit should be backported to the stable branch. This commit
> applies cleanly to the v4.8.6 tree with just line offsets.
>
> Meanwhile, I see that commit c9c3321257 just increases(?) an skb buffer(?)
> (which fixes hitting the BUG() for this exact reproducer), but does not fix the
> real reason (so another set of syscalls still may cause hitting the BUG()). This
> is why I'm emailing not only to stable@, but also to netdev@, asking to review the
> data above and probably develop a fix.
I do not believe c9c3321257 is a proper fix for this issue.
It only works around the bug for this specific use case, probably
because of :
+ /* In case all data was pulled from skb frags (in __pskb_pull_tail()),
+ * we can fix skb->truesize to its real value to avoid future drops.
+ * This is valid because skb is not yet charged to the socket.
+ * It has been noticed pure SACK packets were sometimes dropped
+ * (if cooked by drivers without copybreak feature).
+ */
+ if (!skb->data_len)
+ skb->truesize = SKB_TRUESIZE(skb_end_offset(skb));
Meaning that (tcp_win_from_space(skb->truesize) > skb->len) expression
result differs in tcp_collapse() later.
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