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Subject: Re: [PATCH] net: xfrm: use preempt-safe this_cpu_read() in
 ipcomp_alloc_tfms()
To:     Eric Dumazet <eric.dumazet@gmail.com>,
        Steffen Klassert <steffen.klassert@secunet.com>,
        Herbert Xu <herbert@gondor.apana.org.au>,
        "David S. Miller" <davem@davemloft.net>
Cc:     netdev@vger.kernel.org, linux-kernel@vger.kernel.org
References: <20180307192416.233202-1-ghackmann@google.com>
 <1520454081.109662.54.camel@gmail.com>
From:   Greg Hackmann <ghackmann@google.com>
Message-ID: <b33b7912-1cc1-142e-5e86-0014fc7d437e@google.com>
Date:   Wed, 7 Mar 2018 12:50:00 -0800
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On 03/07/2018 12:21 PM, Eric Dumazet wrote:
> On Wed, 2018-03-07 at 11:24 -0800, Greg Hackmann wrote:
>> f7c83bcbfaf5 ("net: xfrm: use __this_cpu_read per-cpu helper") added
>> a
>> __this_cpu_read() call inside ipcomp_alloc_tfms().  Since this call
>> was
>> introduced, the rules around per-cpu accessors have been tightened
>> and
>> __this_cpu_read() cannot be used in a preemptible context.
>>
>> syzkaller reported this leading to the following kernel BUG while
>> fuzzing sendmsg:
>>
>>
>> Cc: stable@vger.kernel.org
> 
> Hi Greg, thanks for this patch.
> 
> 1) Please read Documentation/networking/netdev-FAQ.txt
> around line 133
> 
> 2) If you believe this needs to be backported, you must provide
> a Fixes: tag to clearly identify commit that changed the behavior.
> 
> Otherwise dozens of stable teams will have to figure out by themselves,
> wasting precious time.
> 
> Thanks.
> 

Apologies, I forgot that netdev has its own stable rules.  I'll
double-check which kernels are impacted and resend with an updated
commit log.