linux-kernel - Re: [PATCH v2 net] net: xfrm: use preempt-safe this_cpu_read() in ipcomp_alloc

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Message-ID: <20180308082312.GA8403@gondor.apana.org.au>
Date:   Thu, 8 Mar 2018 16:23:12 +0800
From:   Herbert Xu <herbert@...dor.apana.org.au>
To:     Greg Hackmann <ghackmann@...gle.com>
Cc:     Steffen Klassert <steffen.klassert@...unet.com>,
        "David S. Miller" <davem@...emloft.net>, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 net] net: xfrm: use preempt-safe this_cpu_read() in
 ipcomp_alloc_tfms()

On Wed, Mar 07, 2018 at 02:42:53PM -0800, Greg Hackmann wrote:
> f7c83bcbfaf5 ("net: xfrm: use __this_cpu_read per-cpu helper") added a
> __this_cpu_read() call inside ipcomp_alloc_tfms().
> 
> At the time, __this_cpu_read() required the caller to either not care
> about races or to handle preemption/interrupt issues.  3.15 tightened
> the rules around some per-cpu operations, and now __this_cpu_read()
> should never be used in a preemptible context.  On 3.15 and later, we
> need to use this_cpu_read() instead.
> 
> syzkaller reported this leading to the following kernel BUG while
> fuzzing sendmsg:

Please explain why we can't revert f7c83bcbfaf5 instead.

Your patch contradicts the comment above the line that you're
changing.

Thanks,
-- 
Email: Herbert Xu <herbert@...dor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt