netdev - Re: [PATCH net] tun: fix tun_napi_alloc

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Message-Id: <20180216.162117.1787932940366763800.davem@davemloft.net>
Date:   Fri, 16 Feb 2018 16:21:17 -0500 (EST)
From:   David Miller <davem@...emloft.net>
To:     eric.dumazet@...il.com
Cc:     mark.rutland@....com, netdev@...r.kernel.org, willemb@...gle.com,
        peterpenkov96@...il.com
Subject: Re: [PATCH net] tun: fix tun_napi_alloc_frags() frag allocator

From: Eric Dumazet <eric.dumazet@...il.com>
Date: Thu, 15 Feb 2018 14:47:15 -0800

> From: Eric Dumazet <edumazet@...gle.com>
> 
> <Mark Rutland reported>
>     While fuzzing arm64 v4.16-rc1 with Syzkaller, I've been hitting a
>     misaligned atomic in __skb_clone:
> 
>         atomic_inc(&(skb_shinfo(skb)->dataref));
> 
>    where dataref doesn't have the required natural alignment, and the
>    atomic operation faults. e.g. i often see it aligned to a single
>    byte boundary rather than a four byte boundary.
> 
>    AFAICT, the skb_shared_info is misaligned at the instant it's
>    allocated in __napi_alloc_skb()  __napi_alloc_skb()
> </end of report>
> 
> Problem is caused by tun_napi_alloc_frags() using
> napi_alloc_frag() with user provided seg sizes,
> leading to other users of this API getting unaligned
> page fragments.
> 
> Since we would like to not necessarily add paddings or alignments to
> the frags that tun_napi_alloc_frags() attaches to the skb, switch to
> another page frag allocator.
> 
> As a bonus skb_page_frag_refill() can use GFP_KERNEL allocations,
> meaning that we can not deplete memory reserves as easily.
> 
> Fixes: 90e33d459407 ("tun: enable napi_gro_frags() for TUN/TAP driver")
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> Reported-by: Mark Rutland <mark.rutland@....com>
> Tested-by: Mark Rutland <mark.rutland@....com>

Applied and queued up for -stable, thanks Eric.