linux-kernel - Re: [PATCH] tun: Don't put_page() for all negative return values from XDP program

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Message-Id: <20200406.100123.2069151116269746360.davem@davemloft.net>
Date:   Mon, 06 Apr 2020 10:01:23 -0700 (PDT)
From:   David Miller <davem@...emloft.net>
To:     will@...nel.org
Cc:     netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        bpf@...r.kernel.org, kernel-team@...roid.com, ast@...nel.org,
        daniel@...earbox.net, edumazet@...gle.com, jasowang@...hat.com
Subject: Re: [PATCH] tun: Don't put_page() for all negative return values
 from XDP program

From: Will Deacon <will@...nel.org>
Date: Fri,  3 Apr 2020 16:13:21 +0100

> When an XDP program is installed, tun_build_skb() grabs a reference to
> the current page fragment page if the program returns XDP_REDIRECT or
> XDP_TX. However, since tun_xdp_act() passes through negative return
> values from the XDP program, it is possible to trigger the error path by
> mistake and accidentally drop a reference to the fragments page without
> taking one, leading to a spurious free. This is believed to be the cause
> of some KASAN use-after-free reports from syzbot [1], although without a
> reproducer it is not possible to confirm whether this patch fixes the
> problem.
> 
> Ensure that we only drop a reference to the fragments page if the XDP
> transmit or redirect operations actually fail.
> 
> [1] https://syzkaller.appspot.com/bug?id=e76a6af1be4acd727ff6bbca669833f98cbf5d95
> 
> Cc: "David S. Miller" <davem@...emloft.net>
> Cc: Alexei Starovoitov <ast@...nel.org>
> Cc: Daniel Borkmann <daniel@...earbox.net>
> CC: Eric Dumazet <edumazet@...gle.com>
> Acked-by: Jason Wang <jasowang@...hat.com>
> Fixes: 8ae1aff0b331 ("tuntap: split out XDP logic")
> Signed-off-by: Will Deacon <will@...nel.org>

Applied and queued up for -stable, thank you.