On Tue, Mar 31, 2020 at 10:59:01AM +0800, Jason Wang wrote:

On 2020/3/31 上午12:12, Will Deacon wrote:

quoted

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

I think the patch fixes the issue reported. Since I can see the warn of bad
page state in put_page().

[...]

Acked-by: Jason Wang <jasowang@redhat.com>

Thanks, Jason. In which case, I'll add this tag along with:

Fixes: 8ae1aff0b331 ("tuntap: split out XDP logic")

Will