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    Subject[PATCH 4.19 01/47] inet: protect against too small mtu values.
    Date
    From: Eric Dumazet <edumazet@google.com>

    [ Upstream commit 501a90c945103e8627406763dac418f20f3837b2 ]

    syzbot was once again able to crash a host by setting a very small mtu
    on loopback device.

    Let's make inetdev_valid_mtu() available in include/net/ip.h,
    and use it in ip_setup_cork(), so that we protect both ip_append_page()
    and __ip_append_data()

    Also add a READ_ONCE() when the device mtu is read.

    Pairs this lockless read with one WRITE_ONCE() in __dev_set_mtu(),
    even if other code paths might write over this field.

    Add a big comment in include/linux/netdevice.h about dev->mtu
    needing READ_ONCE()/WRITE_ONCE() annotations.

    Hopefully we will add the missing ones in followup patches.

    [1]

    refcount_t: saturated; leaking memory.
    WARNING: CPU: 0 PID: 9464 at lib/refcount.c:22 refcount_warn_saturate+0x138/0x1f0 lib/refcount.c:22
    Kernel panic - not syncing: panic_on_warn set ...
    CPU: 0 PID: 9464 Comm: syz-executor850 Not tainted 5.4.0-syzkaller #0
    Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
    Call Trace:
    __dump_stack lib/dump_stack.c:77 [inline]
    dump_stack+0x197/0x210 lib/dump_stack.c:118
    panic+0x2e3/0x75c kernel/panic.c:221
    __warn.cold+0x2f/0x3e kernel/panic.c:582
    report_bug+0x289/0x300 lib/bug.c:195
    fixup_bug arch/x86/kernel/traps.c:174 [inline]
    fixup_bug arch/x86/kernel/traps.c:169 [inline]
    do_error_trap+0x11b/0x200 arch/x86/kernel/traps.c:267
    do_invalid_op+0x37/0x50 arch/x86/kernel/traps.c:286
    invalid_op+0x23/0x30 arch/x86/entry/entry_64.S:1027
    RIP: 0010:refcount_warn_saturate+0x138/0x1f0 lib/refcount.c:22
    Code: 06 31 ff 89 de e8 c8 f5 e6 fd 84 db 0f 85 6f ff ff ff e8 7b f4 e6 fd 48 c7 c7 e0 71 4f 88 c6 05 56 a6 a4 06 01 e8 c7 a8 b7 fd <0f> 0b e9 50 ff ff ff e8 5c f4 e6 fd 0f b6 1d 3d a6 a4 06 31 ff 89
    RSP: 0018:ffff88809689f550 EFLAGS: 00010286
    RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000000000000
    RDX: 0000000000000000 RSI: ffffffff815e4336 RDI: ffffed1012d13e9c
    RBP: ffff88809689f560 R08: ffff88809c50a3c0 R09: fffffbfff15d31b1
    R10: fffffbfff15d31b0 R11: ffffffff8ae98d87 R12: 0000000000000001
    R13: 0000000000040100 R14: ffff888099041104 R15: ffff888218d96e40
    refcount_add include/linux/refcount.h:193 [inline]
    skb_set_owner_w+0x2b6/0x410 net/core/sock.c:1999
    sock_wmalloc+0xf1/0x120 net/core/sock.c:2096
    ip_append_page+0x7ef/0x1190 net/ipv4/ip_output.c:1383
    udp_sendpage+0x1c7/0x480 net/ipv4/udp.c:1276
    inet_sendpage+0xdb/0x150 net/ipv4/af_inet.c:821
    kernel_sendpage+0x92/0xf0 net/socket.c:3794
    sock_sendpage+0x8b/0xc0 net/socket.c:936
    pipe_to_sendpage+0x2da/0x3c0 fs/splice.c:458
    splice_from_pipe_feed fs/splice.c:512 [inline]
    __splice_from_pipe+0x3ee/0x7c0 fs/splice.c:636
    splice_from_pipe+0x108/0x170 fs/splice.c:671
    generic_splice_sendpage+0x3c/0x50 fs/splice.c:842
    do_splice_from fs/splice.c:861 [inline]
    direct_splice_actor+0x123/0x190 fs/splice.c:1035
    splice_direct_to_actor+0x3b4/0xa30 fs/splice.c:990
    do_splice_direct+0x1da/0x2a0 fs/splice.c:1078
    do_sendfile+0x597/0xd00 fs/read_write.c:1464
    __do_sys_sendfile64 fs/read_write.c:1525 [inline]
    __se_sys_sendfile64 fs/read_write.c:1511 [inline]
    __x64_sys_sendfile64+0x1dd/0x220 fs/read_write.c:1511
    do_syscall_64+0xfa/0x790 arch/x86/entry/common.c:294
    entry_SYSCALL_64_after_hwframe+0x49/0xbe
    RIP: 0033:0x441409
    Code: e8 ac e8 ff ff 48 83 c4 18 c3 0f 1f 80 00 00 00 00 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 0f 83 eb 08 fc ff c3 66 2e 0f 1f 84 00 00 00 00
    RSP: 002b:00007fffb64c4f78 EFLAGS: 00000246 ORIG_RAX: 0000000000000028
    RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 0000000000441409
    RDX: 0000000000000000 RSI: 0000000000000006 RDI: 0000000000000005
    RBP: 0000000000073b8a R08: 0000000000000010 R09: 0000000000000010
    R10: 0000000000010001 R11: 0000000000000246 R12: 0000000000402180
    R13: 0000000000402210 R14: 0000000000000000 R15: 0000000000000000
    Kernel Offset: disabled
    Rebooting in 86400 seconds..

    Fixes: 1470ddf7f8ce ("inet: Remove explicit write references to sk/inet in ip_append_data")
    Signed-off-by: Eric Dumazet <edumazet@google.com>
    Reported-by: syzbot <syzkaller@googlegroups.com>
    Signed-off-by: David S. Miller <davem@davemloft.net>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    ---
    include/linux/netdevice.h | 5 +++++
    include/net/ip.h | 5 +++++
    net/core/dev.c | 3 ++-
    net/ipv4/devinet.c | 5 -----
    net/ipv4/ip_output.c | 13 ++++++++-----
    5 files changed, 20 insertions(+), 11 deletions(-)

    --- a/include/linux/netdevice.h
    +++ b/include/linux/netdevice.h
    @@ -1834,6 +1834,11 @@ struct net_device {
    unsigned char if_port;
    unsigned char dma;

    + /* Note : dev->mtu is often read without holding a lock.
    + * Writers usually hold RTNL.
    + * It is recommended to use READ_ONCE() to annotate the reads,
    + * and to use WRITE_ONCE() to annotate the writes.
    + */
    unsigned int mtu;
    unsigned int min_mtu;
    unsigned int max_mtu;
    --- a/include/net/ip.h
    +++ b/include/net/ip.h
    @@ -692,4 +692,9 @@ int ip_misc_proc_init(void);
    int rtm_getroute_parse_ip_proto(struct nlattr *attr, u8 *ip_proto, u8 family,
    struct netlink_ext_ack *extack);

    +static inline bool inetdev_valid_mtu(unsigned int mtu)
    +{
    + return likely(mtu >= IPV4_MIN_MTU);
    +}
    +
    #endif /* _IP_H */
    --- a/net/core/dev.c
    +++ b/net/core/dev.c
    @@ -7595,7 +7595,8 @@ int __dev_set_mtu(struct net_device *dev
    if (ops->ndo_change_mtu)
    return ops->ndo_change_mtu(dev, new_mtu);

    - dev->mtu = new_mtu;
    + /* Pairs with all the lockless reads of dev->mtu in the stack */
    + WRITE_ONCE(dev->mtu, new_mtu);
    return 0;
    }
    EXPORT_SYMBOL(__dev_set_mtu);
    --- a/net/ipv4/devinet.c
    +++ b/net/ipv4/devinet.c
    @@ -1441,11 +1441,6 @@ skip:
    }
    }

    -static bool inetdev_valid_mtu(unsigned int mtu)
    -{
    - return mtu >= IPV4_MIN_MTU;
    -}
    -
    static void inetdev_send_gratuitous_arp(struct net_device *dev,
    struct in_device *in_dev)

    --- a/net/ipv4/ip_output.c
    +++ b/net/ipv4/ip_output.c
    @@ -1142,15 +1142,18 @@ static int ip_setup_cork(struct sock *sk
    cork->addr = ipc->addr;
    }

    - /*
    - * We steal reference to this route, caller should not release it
    - */
    - *rtp = NULL;
    cork->fragsize = ip_sk_use_pmtu(sk) ?
    - dst_mtu(&rt->dst) : rt->dst.dev->mtu;
    + dst_mtu(&rt->dst) : READ_ONCE(rt->dst.dev->mtu);
    +
    + if (!inetdev_valid_mtu(cork->fragsize))
    + return -ENETUNREACH;

    cork->gso_size = ipc->gso_size;
    +
    cork->dst = &rt->dst;
    + /* We stole this route, caller should not release it. */
    + *rtp = NULL;
    +
    cork->length = 0;
    cork->ttl = ipc->ttl;
    cork->tos = ipc->tos;

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