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ax25: Fix refcount imbalance on inbound connections

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When releasing a socket in ax25_release(), we call netdev_put() to
decrease the refcount on the associated ax.25 device. However, the
execution path for accepting an incoming connection never calls
netdev_hold(). This imbalance leads to refcount errors, and ultimately
to kernel crashes.

A typical call trace for the above situation will start with one of the
following errors:

    refcount_t: decrement hit 0; leaking memory.
    refcount_t: underflow; use-after-free.

And will then have a trace like:

    Call Trace:
    <TASK>
    ? show_regs+0x64/0x70
    ? __warn+0x83/0x120
    ? refcount_warn_saturate+0xb2/0x100
    ? report_bug+0x158/0x190
    ? prb_read_valid+0x20/0x30
    ? handle_bug+0x3e/0x70
    ? exc_invalid_op+0x1c/0x70
    ? asm_exc_invalid_op+0x1f/0x30
    ? refcount_warn_saturate+0xb2/0x100
    ? refcount_warn_saturate+0xb2/0x100
    ax25_release+0x2ad/0x360
    __sock_release+0x35/0xa0
    sock_close+0x19/0x20
    [...]

On reboot (or any attempt to remove the interface), the kernel gets
stuck in an infinite loop:

    unregister_netdevice: waiting for ax0 to become free. Usage count = 0

This patch corrects these issues by ensuring that we call netdev_hold()
and ax25_dev_hold() for new connections in ax25_accept(). This makes the
logic leading to ax25_accept() match the logic for ax25_bind(): in both
cases we increment the refcount, which is ultimately decremented in
ax25_release().

Fixes: 9fd75b66b8 ("ax25: Fix refcount leaks caused by ax25_cb_del()")
Signed-off-by: Lars Kellogg-Stedman <lars@oddbit.com>
Tested-by: Duoming Zhou <duoming@zju.edu.cn>
Tested-by: Dan Cross <crossd@gmail.com>
Tested-by: Chris Maness <christopher.maness@gmail.com>
Reviewed-by: Dan Carpenter <dan.carpenter@linaro.org>
Link: https://lore.kernel.org/r/20240529210242.3346844-2-lars@oddbit.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
This commit is contained in:
Lars Kellogg-Stedman 2024-05-29 17:02:43 -04:00 committed by Jakub Kicinski
parent 45c0a209dc
commit 3c34fb0bd4
1 changed files with 6 additions and 0 deletions
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6
net/ax25/af_ax25.c
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@ -1378,8 +1378,10 @@ static int ax25_accept(struct socket *sock, struct socket *newsock,
{
struct sk_buff *skb;
struct sock *newsk;
ax25_dev *ax25_dev;
DEFINE_WAIT(wait);
struct sock *sk;
ax25_cb *ax25;
int err = 0;
if (sock->state != SS_UNCONNECTED)
@ -1434,6 +1436,10 @@ static int ax25_accept(struct socket *sock, struct socket *newsock,
kfree_skb(skb);
sk_acceptq_removed(sk);
newsock->state = SS_CONNECTED;
ax25 = sk_to_ax25(newsk);
ax25_dev = ax25->ax25_dev;
netdev_hold(ax25_dev->dev, &ax25->dev_tracker, GFP_ATOMIC);
ax25_dev_hold(ax25_dev);
out:
release_sock(sk);