Re: [PATCH 1/4] ipv4: ipv6: netfilter: Adjust the frag mem limit when truesize changes

[Stefan Bader]

On 29.05.19 12:37, Greg KH wrote:
> On Wed, May 29, 2019 at 12:25:39PM +0200, Stefan Bader wrote:
>> From: Jiri Wiesner <jwiesner@xxxxxxxx>
>>
>> The *_frag_reasm() functions are susceptible to miscalculating the byte
>> count of packet fragments in case the truesize of a head buffer changes.
>> The truesize member may be changed by the call to skb_unclone(), leaving
>> the fragment memory limit counter unbalanced even if all fragments are
>> processed. This miscalculation goes unnoticed as long as the network
>> namespace which holds the counter is not destroyed.
>>
>> Should an attempt be made to destroy a network namespace that holds an
>> unbalanced fragment memory limit counter the cleanup of the namespace
>> never finishes. The thread handling the cleanup gets stuck in
>> inet_frags_exit_net() waiting for the percpu counter to reach zero. The
>> thread is usually in running state with a stacktrace similar to:
>>
>>  PID: 1073   TASK: ffff880626711440  CPU: 1   COMMAND: "kworker/u48:4"
>>   #5 [ffff880621563d48] _raw_spin_lock at ffffffff815f5480
>>   #6 [ffff880621563d48] inet_evict_bucket at ffffffff8158020b
>>   #7 [ffff880621563d80] inet_frags_exit_net at ffffffff8158051c
>>   #8 [ffff880621563db0] ops_exit_list at ffffffff814f5856
>>   #9 [ffff880621563dd8] cleanup_net at ffffffff814f67c0
>>  #10 [ffff880621563e38] process_one_work at ffffffff81096f14
>>
>> It is not possible to create new network namespaces, and processes
>> that call unshare() end up being stuck in uninterruptible sleep state
>> waiting to acquire the net_mutex.
>>
>> The bug was observed in the IPv6 netfilter code by Per Sundstrom.
>> I thank him for his analysis of the problem. The parts of this patch
>> that apply to IPv4 and IPv6 fragment reassembly are preemptive measures.
>>
>> Signed-off-by: Jiri Wiesner <jwiesner@xxxxxxxx>
>> Reported-by: Per Sundstrom <per.sundstrom@xxxxxxxxxx>
>> Acked-by: Peter Oskolkov <posk@xxxxxxxxxx>
>> Signed-off-by: David S. Miller <davem@xxxxxxxxxxxxx>
>>
>> (backported from commit ebaf39e6032faf77218220707fc3fa22487784e0)
>> [smb: context adjustments in net/ipv6/netfilter/nf_conntrack_reasm.c]
>> Signed-off-by: Stefan Bader <stefan.bader@xxxxxxxxxxxxx>
> 
> I can't take a patch for 4.4.y that is not in 4.9.y as anyone upgrading
> kernel versions would have a regression :(
> 
> Can you also provide a backport of the needed patches for 4.9.y for this
> issue so I can take these?

I will, once it is clear that a) the backport looks alright and b) is ok to be
done. Alternatively it might be decided that only the parts necessary for
pulling out a frag head should be picked.

Or the net-devs might decide they want to send things out. The problem
potentially exists in anything that has some stable support up to v5.1
and I have no complete overview where this was backported to.

So this is more a start of discussion that a request to apply it. stable was
just included to make stable maintainers aware.

-Stefan
> 
> thanks,
> 
> greg k-h
>

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