linux-kernel - Re: BUG: unable to handle kernel paging request in n_tty

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Message-ID: <323c1b8d-c778-3175-79a0-5adcc0b470b9@kernel.org>
Date:   Mon, 13 Feb 2023 07:18:29 +0100
From:   Jiri Slaby <jirislaby@...nel.org>
To:     Dipanjan Das <mail.dipanjan.das@...il.com>,
        Greg KH <gregkh@...uxfoundation.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Cc:     syzkaller <syzkaller@...glegroups.com>,
        Marius Fleischer <fleischermarius@...glemail.com>,
        Priyanka Bose <its.priyanka.bose@...il.com>
Subject: Re: BUG: unable to handle kernel paging request in n_tty_open

On 12. 02. 23, 18:00, Dipanjan Das wrote:
> Hi,
> 
> We would like to report the following bug which has been found by our
> modified version of syzkaller.
> 
> ======================================================
> description: BUG: unable to handle kernel paging request in n_tty_open
> affected file: drivers/tty/tty_ldisc.c
> kernel version: 6.2.0-rc5
> kernel commit: 41c66f47061608dc1fd493eebce198f0e74cc2d7
> git tree: kmsan
> kernel config: https://syzkaller.appspot.com/text?tag=KernelConfig&x=a9a22da1efde3af6
> crash reproducer: attached
> ======================================================
> Crash log:
> ======================================================
> RBP: 00007f7117670c30 R08: 0000000000000000 R09: 0000000000000000
> R10: 0000000000000000 R11: 0000000000000246 R12: 000000000000000d
> R13: 00007f71165abf6c R14: 00007f71165abf60 R15: 00007f7117670d80
>   </TASK>
> BUG: unable to handle page fault for address: ffffd10016bfa050
> #PF: supervisor write access in kernel mode
> #PF: error_code(0x0002) - not-present page
> PGD 11410067 P4D 11410067 PUD 1157d067 PMD 31910067 PTE 0
> Oops: 0002 [#1] PREEMPT SMP
> CPU: 1 PID: 14496 Comm: syz-executor.4 Not tainted
> 6.2.0-rc5-00010-g41c66f470616 #8
> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS
> 1.13.0-1ubuntu1.1 04/01/2014
> RIP: 0010:n_tty_open+0xa5/0x240 drivers/tty/n_tty.c:1883

         ldata = vzalloc(sizeof(*ldata));
         if (!ldata)
                 return -ENOMEM;

         ldata->overrun_time = jiffies;

COuld you explain how this assignment can ever fail? ldata is either 
allocated correctly (non-NULL) and the assignment cannot fail or not 
(NULL) and the assignment cannot fail (as it doesn't happen).

thanks,
-- 
js
suse labs