ax25: Fix refcount imbalance on inbound connections

[larsks]

This is a backport of 3c34fb0bd4a4237592c5ecb5b2e2531900c55774, which recently merged in the mainline kernel.

Without this patch, making ax.25 connections to a Pi will cause it to crash and possibly fail to reboot (unless you have the hardware watchdog enabled).

The original text of the commit is below.

---

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: 9fd75b6 ("ax25: Fix refcount leaks caused by ax25_cb_del()")
Signed-off-by: Lars Kellogg-Stedman [email protected]
Tested-by: Duoming Zhou [email protected]
Tested-by: Dan Cross [email protected]
Tested-by: Chris Maness [email protected]
Reviewed-by: Dan Carpenter [email protected]
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Jakub Kicinski [email protected]
(cherry picked from commit 3c34fb0)

[pelwell]

Applied as 333b34f.