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    Subject[PATCH 4.9 35/66] ipv6: fix udpv6 sendmsg crash caused by too small MTU
    Date
    4.9-stable review patch.  If anyone has any objections, please let me know.

    ------------------

    From: Mike Maloney <maloney@google.com>


    [ Upstream commit 749439bfac6e1a2932c582e2699f91d329658196 ]

    The logic in __ip6_append_data() assumes that the MTU is at least large
    enough for the headers. A device's MTU may be adjusted after being
    added while sendmsg() is processing data, resulting in
    __ip6_append_data() seeing any MTU. For an mtu smaller than the size of
    the fragmentation header, the math results in a negative 'maxfraglen',
    which causes problems when refragmenting any previous skb in the
    skb_write_queue, leaving it possibly malformed.

    Instead sendmsg returns EINVAL when the mtu is calculated to be less
    than IPV6_MIN_MTU.

    Found by syzkaller:
    kernel BUG at ./include/linux/skbuff.h:2064!
    invalid opcode: 0000 [#1] SMP KASAN
    Dumping ftrace buffer:
    (ftrace buffer empty)
    Modules linked in:
    CPU: 1 PID: 14216 Comm: syz-executor5 Not tainted 4.13.0-rc4+ #2
    Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
    task: ffff8801d0b68580 task.stack: ffff8801ac6b8000
    RIP: 0010:__skb_pull include/linux/skbuff.h:2064 [inline]
    RIP: 0010:__ip6_make_skb+0x18cf/0x1f70 net/ipv6/ip6_output.c:1617
    RSP: 0018:ffff8801ac6bf570 EFLAGS: 00010216
    RAX: 0000000000010000 RBX: 0000000000000028 RCX: ffffc90003cce000
    RDX: 00000000000001b8 RSI: ffffffff839df06f RDI: ffff8801d9478ca0
    RBP: ffff8801ac6bf780 R08: ffff8801cc3f1dbc R09: 0000000000000000
    R10: ffff8801ac6bf7a0 R11: 43cb4b7b1948a9e7 R12: ffff8801cc3f1dc8
    R13: ffff8801cc3f1d40 R14: 0000000000001036 R15: dffffc0000000000
    FS: 00007f43d740c700(0000) GS:ffff8801dc100000(0000) knlGS:0000000000000000
    CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
    CR2: 00007f7834984000 CR3: 00000001d79b9000 CR4: 00000000001406e0
    DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
    DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
    Call Trace:
    ip6_finish_skb include/net/ipv6.h:911 [inline]
    udp_v6_push_pending_frames+0x255/0x390 net/ipv6/udp.c:1093
    udpv6_sendmsg+0x280d/0x31a0 net/ipv6/udp.c:1363
    inet_sendmsg+0x11f/0x5e0 net/ipv4/af_inet.c:762
    sock_sendmsg_nosec net/socket.c:633 [inline]
    sock_sendmsg+0xca/0x110 net/socket.c:643
    SYSC_sendto+0x352/0x5a0 net/socket.c:1750
    SyS_sendto+0x40/0x50 net/socket.c:1718
    entry_SYSCALL_64_fastpath+0x1f/0xbe
    RIP: 0033:0x4512e9
    RSP: 002b:00007f43d740bc08 EFLAGS: 00000216 ORIG_RAX: 000000000000002c
    RAX: ffffffffffffffda RBX: 00000000007180a8 RCX: 00000000004512e9
    RDX: 000000000000002e RSI: 0000000020d08000 RDI: 0000000000000005
    RBP: 0000000000000086 R08: 00000000209c1000 R09: 000000000000001c
    R10: 0000000000040800 R11: 0000000000000216 R12: 00000000004b9c69
    R13: 00000000ffffffff R14: 0000000000000005 R15: 00000000202c2000
    Code: 9e 01 fe e9 c5 e8 ff ff e8 7f 9e 01 fe e9 4a ea ff ff 48 89 f7 e8 52 9e 01 fe e9 aa eb ff ff e8 a8 b6 cf fd 0f 0b e8 a1 b6 cf fd <0f> 0b 49 8d 45 78 4d 8d 45 7c 48 89 85 78 fe ff ff 49 8d 85 ba
    RIP: __skb_pull include/linux/skbuff.h:2064 [inline] RSP: ffff8801ac6bf570
    RIP: __ip6_make_skb+0x18cf/0x1f70 net/ipv6/ip6_output.c:1617 RSP: ffff8801ac6bf570

    Reported-by: syzbot <syzkaller@googlegroups.com>
    Signed-off-by: Mike Maloney <maloney@google.com>
    Reviewed-by: Eric Dumazet <edumazet@google.com>
    Signed-off-by: David S. Miller <davem@davemloft.net>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    ---
    net/ipv6/ip6_output.c | 6 ++++--
    1 file changed, 4 insertions(+), 2 deletions(-)

    --- a/net/ipv6/ip6_output.c
    +++ b/net/ipv6/ip6_output.c
    @@ -1260,14 +1260,16 @@ static int ip6_setup_cork(struct sock *s
    v6_cork->tclass = ipc6->tclass;
    if (rt->dst.flags & DST_XFRM_TUNNEL)
    mtu = np->pmtudisc >= IPV6_PMTUDISC_PROBE ?
    - rt->dst.dev->mtu : dst_mtu(&rt->dst);
    + READ_ONCE(rt->dst.dev->mtu) : dst_mtu(&rt->dst);
    else
    mtu = np->pmtudisc >= IPV6_PMTUDISC_PROBE ?
    - rt->dst.dev->mtu : dst_mtu(rt->dst.path);
    + READ_ONCE(rt->dst.dev->mtu) : dst_mtu(rt->dst.path);
    if (np->frag_size < mtu) {
    if (np->frag_size)
    mtu = np->frag_size;
    }
    + if (mtu < IPV6_MIN_MTU)
    + return -EINVAL;
    cork->base.fragsize = mtu;
    if (dst_allfrag(rt->dst.path))
    cork->base.flags |= IPCORK_ALLFRAG;

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