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

[David Miller]

From: Will Deacon <will@xxxxxxxxxx>
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@xxxxxxxxxxxxx>
> Cc: Alexei Starovoitov <ast@xxxxxxxxxx>
> Cc: Daniel Borkmann <daniel@xxxxxxxxxxxxx>
> CC: Eric Dumazet <edumazet@xxxxxxxxxx>
> Acked-by: Jason Wang <jasowang@xxxxxxxxxx>
> Fixes: 8ae1aff0b331 ("tuntap: split out XDP logic")
> Signed-off-by: Will Deacon <will@xxxxxxxxxx>

Applied and queued up for -stable, thank you.