Re: BUG: User triggerable kernel panic in 4.8 (possibly 4.9)

[Anna Schumaker]

Hi Darren,

On 11/29/2016 03:40 AM, Jan Kara wrote:
> Hello,
> 
> Thanks for report. I suspect this bug got lost in the noise of
> linux-kernel. Adding more relevant lists and people to CC.
> 
> 								Honza
> 
> On Thu 20-10-16 15:24:37, Darren Austin wrote:
>> [ Please CC me with any replies as I will be leaving the list shortly ]
>>
>> Hi,
>>   I'm not sure if this is the best place to report an issue i've discovered 
>> with the kernel and the 'fsc' mount option - please let me know if there is 
>> some other mailing list or person I should be notifying about this.
>>
>> The bug appears (at least for me) when using an NFS server, and a client 
>> which mounts an export from that server with the 'fsc' option (whether or 
>> not the fscache daemon is running or not).  It also seems easiest to trigger 
>> using the 'nano' editor, but other commands will trigger it randomly.
>>
>> I've tested this bug on the Ubuntu 1610 kernel (4.8.0) and with the 4.8.2 
>> kernel from http://kernel.ubuntu.com/~kernel-ppa/mainline/.  The latter 
>> purports to be a kernel built from the unmodified kernel source, and simply 
>> packaged in a .deb, so this isn't a Ubuntu kernel problem.  Unless the 
>> problem has been corrected in the 4.9 series, I suspect the bug may also 
>> persist there - i've not had the opportunity to check 4.9.x kernels as yet.
>>
>> I can repeatidly reproduce this bug on my system, so it's definitely not a 
>> one off - it causes a kernel panic and complete lock-up every time.
>>
>> The NFS share I tested with is exported with options: 
>>   rw,async,insecure,insecure_locks,no_root_squash,anongid=99,anonuid=99,no_subtree_check
>> (but the export options don't seem to matter to trigger the  bug)
>> and mounted on the client with options:
>>   vers=4,hard,intr,acl,rw,fsc
>> via the Linux automounter (but the issue persists on mounts from fstab or 
>> when mounted manually).
>>
>> To reproduce the bug is quite simple...
>> 1) Set up the server export and client mount as detailed above.
>> 2) From the console (or in a terminal; but I only tested this once) in the 
>> directory where you've mounted the NFS share, run:
>>   nano testfile.txt
>> and add write some text to the file.
>> 2) Save the file and exit (Ctl+X is how I did it).
>> 3) When back at the prompt, immediately hit the up arrow on the keyboard (to
>> load the last typed command into the buffer) and hit enter.
>> 4) Watch as the pretty text of the panic scrolls by :)

I've got through these steps a couple of times, and I'm unable to reproduce this (using 4.8.11).  Can you send the stack trace of the panic so we can work from that?

Thanks,
Anna

>>
>> With the help of people on the nano-dev mailing list, I figured out that 
>> it's the 'fsc' option which causes the panic - repeated tests without that 
>> option active do not trigger it.  However, this is /not/ a nano specific bug 
>> - it can be triggered by any command used on the mount.  And besides, nano 
>> shouldn't be able to take down the system :)
>>
>> If any further information is required, please don't hesitate to reply.
>>
>> Darren.