netdev - Re: [PATCH v4] ax25: Fix refcount imbalance on inbound connections

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Date: Thu, 23 May 2024 18:05:34 +0300
From: Dan Carpenter <dan.carpenter@...aro.org>
To: lars@...bit.com, Duoming Zhou <duoming@....edu.cn>
Cc: linux-hams@...r.kernel.org, netdev@...r.kernel.org
Subject: Re: [PATCH v4] ax25: Fix refcount imbalance on inbound connections

On Wed, May 22, 2024 at 02:31:34PM -0400, lars@...bit.com wrote:
> From: Lars Kellogg-Stedman <lars@...bit.com>
> 
> 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 looks like this:
> 
>     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().
> 
> Fixes: 7d8a3a477b ("ax25: Fix ax25 session cleanup problems")

I thought the fixes tag was:

Fixes: 9fd75b66b8f6 ("ax25: Fix refcount leaks caused by ax25_cb_del()")

I've already said that I don't think the patch is correct and offered
an alternative which takes a reference in accept() but also adds a
matching put()...  But I can't really test my patch so if we're going to
do something that we know is wrong, I'd prefer to just revert Duoming's
patch.

regards,
dan carpenter