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    Subject[PATCH 5.7 035/120] net/x25: Fix null-ptr-deref in x25_disconnect
    Date
    From: YueHaibing <yuehaibing@huawei.com>

    commit 8999dc89497ab1c80d0718828e838c7cd5f6bffe upstream.

    We should check null before do x25_neigh_put in x25_disconnect,
    otherwise may cause null-ptr-deref like this:

    #include <sys/socket.h>
    #include <linux/x25.h>

    int main() {
    int sck_x25;
    sck_x25 = socket(AF_X25, SOCK_SEQPACKET, 0);
    close(sck_x25);
    return 0;
    }

    BUG: kernel NULL pointer dereference, address: 00000000000000d8
    CPU: 0 PID: 4817 Comm: t2 Not tainted 5.7.0-rc3+ #159
    Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.9.3-
    RIP: 0010:x25_disconnect+0x91/0xe0
    Call Trace:
    x25_release+0x18a/0x1b0
    __sock_release+0x3d/0xc0
    sock_close+0x13/0x20
    __fput+0x107/0x270
    ____fput+0x9/0x10
    task_work_run+0x6d/0xb0
    exit_to_usermode_loop+0x102/0x110
    do_syscall_64+0x23c/0x260
    entry_SYSCALL_64_after_hwframe+0x49/0xb3

    Reported-by: syzbot+6db548b615e5aeefdce2@syzkaller.appspotmail.com
    Fixes: 4becb7ee5b3d ("net/x25: Fix x25_neigh refcnt leak when x25 disconnect")
    Signed-off-by: YueHaibing <yuehaibing@huawei.com>
    Signed-off-by: David S. Miller <davem@davemloft.net>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    ---
    net/x25/x25_subr.c | 10 ++++++----
    1 file changed, 6 insertions(+), 4 deletions(-)

    --- a/net/x25/x25_subr.c
    +++ b/net/x25/x25_subr.c
    @@ -363,10 +363,12 @@ void x25_disconnect(struct sock *sk, int
    x25->neighbour = NULL;
    read_unlock_bh(&x25_list_lock);
    }
    - read_lock_bh(&x25_list_lock);
    - x25_neigh_put(x25->neighbour);
    - x25->neighbour = NULL;
    - read_unlock_bh(&x25_list_lock);
    + if (x25->neighbour) {
    + read_lock_bh(&x25_list_lock);
    + x25_neigh_put(x25->neighbour);
    + x25->neighbour = NULL;
    + read_unlock_bh(&x25_list_lock);
    + }
    }

    /*

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