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Subject[RFC PATCH] tun: Don't put_page() for all negative return values from XDP program
Date
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@davemloft.net>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Cc: Jason Wang <jasowang@redhat.com>
CC: Eric Dumazet <edumazet@google.com>
Signed-off-by: Will Deacon <will@kernel.org>
---

Sending as RFC because I've not been able to confirm that this fixes anything.

drivers/net/tun.c | 10 ++++++----
1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/drivers/net/tun.c b/drivers/net/tun.c
index 650c937ed56b..9de9b7d8aedd 100644
--- a/drivers/net/tun.c
+++ b/drivers/net/tun.c
@@ -1715,8 +1715,12 @@ static struct sk_buff *tun_build_skb(struct tun_struct *tun,
alloc_frag->offset += buflen;
}
err = tun_xdp_act(tun, xdp_prog, &xdp, act);
- if (err < 0)
- goto err_xdp;
+ if (err < 0) {
+ if (act == XDP_REDIRECT || act == XDP_TX)
+ put_page(alloc_frag->page);
+ goto out;
+ }
+
if (err == XDP_REDIRECT)
xdp_do_flush();
if (err != XDP_PASS)
@@ -1730,8 +1734,6 @@ static struct sk_buff *tun_build_skb(struct tun_struct *tun,

return __tun_build_skb(tfile, alloc_frag, buf, buflen, len, pad);

-err_xdp:
- put_page(alloc_frag->page);
out:
rcu_read_unlock();
local_bh_enable();
--
2.26.0.rc2.310.g2932bb562d-goog
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