linux-kernel - Re: [PATCH] net: hso: fix null-ptr-deref during tty device unregistration

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Message-Id: <20210406.163921.1678926610292877597.davem@davemloft.net>
Date:   Tue, 06 Apr 2021 16:39:21 -0700 (PDT)
From:   David Miller <davem@...emloft.net>
To:     mail@...rudhrb.com
Cc:     kuba@...nel.org, oneukum@...e.com, kernel@...il.dk,
        geert@...ux-m68k.org, zhengyongjun3@...wei.com, rkovhaev@...il.com,
        gregkh@...uxfoundation.org,
        syzbot+c49fe6089f295a05e6f8@...kaller.appspotmail.com,
        linux-usb@...r.kernel.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] net: hso: fix null-ptr-deref during tty device
 unregistration

From: Anirudh Rayabharam <mail@...rudhrb.com>
Date: Tue,  6 Apr 2021 18:13:59 +0530

> Multiple ttys try to claim the same the minor number causing a double
> unregistration of the same device. The first unregistration succeeds
> but the next one results in a null-ptr-deref.
> 
> The get_free_serial_index() function returns an available minor number
> but doesn't assign it immediately. The assignment is done by the caller
> later. But before this assignment, calls to get_free_serial_index()
> would return the same minor number.
> 
> Fix this by modifying get_free_serial_index to assign the minor number
> immediately after one is found to be and rename it to obtain_minor()
> to better reflect what it does. Similary, rename set_serial_by_index()
> to release_minor() and modify it to free up the minor number of the
> given hso_serial. Every obtain_minor() should have corresponding
> release_minor() call.
> 
> Reported-by: syzbot+c49fe6089f295a05e6f8@...kaller.appspotmail.com
> Tested-by: syzbot+c49fe6089f295a05e6f8@...kaller.appspotmail.com
> 
> Signed-off-by: Anirudh Rayabharam <mail@...rudhrb.com>

This adds a new build warning:

  CC [M]  drivers/net/usb/hso.o
drivers/net/usb/hso.c: In function ‘hso_serial_common_create’:
drivers/net/usb/hso.c:2256:6: warning: unused variable ‘minor’ [-Wunused-variable]

Please fix this and add an appropriate Fixes: tag, thank you.