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Message-ID: <CAM_iQpWQogVishsmVWckbcJpKKFfjKVDoDMH1MT1a24_M_5u-Q@mail.gmail.com> Date: Tue, 7 Mar 2017 14:04:38 -0800 From: Cong Wang <xiyou.wangcong@...il.com> To: Dmitry Vyukov <dvyukov@...gle.com> Cc: David Miller <davem@...emloft.net>, Hannes Frederic Sowa <hannes@...essinduktion.org>, Willy Tarreau <w@....eu>, netdev <netdev@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>, Eric Dumazet <edumazet@...gle.com>, Al Viro <viro@...iv.linux.org.uk>, syzkaller <syzkaller@...glegroups.com> Subject: Re: net: BUG in unix_notinflight On Tue, Mar 7, 2017 at 12:37 AM, Dmitry Vyukov <dvyukov@...gle.com> wrote: > On Mon, Mar 6, 2017 at 11:34 PM, Cong Wang <xiyou.wangcong@...il.com> wrote: >> The problem here is there is no lock protecting concurrent unix_detach_fds() >> even though unix_notinflight() is already serialized, if we call >> unix_notinflight() >> twice on the same file pointer, we trigger this bug... >> >> I don't know what is the right lock here to serialize it. > > > What exactly here needs to be protected? > > 1484 static void unix_detach_fds(struct scm_cookie *scm, struct sk_buff *skb) > 1485 { > 1486 int i; > 1487 > 1488 scm->fp = UNIXCB(skb).fp; > 1489 UNIXCB(skb).fp = NULL; > 1490 > 1491 for (i = scm->fp->count-1; i >= 0; i--) > 1492 unix_notinflight(scm->fp->user, scm->fp->fp[i]); > 1493 } > > Whole unix_notinflight happens under global unix_gc_lock. > > Is it that 2 threads call unix_detach_fds for the same skb, and then > call unix_notinflight for the same fd twice? Not the same skb, but their UNIXCB(skb).fp points to the same place, therefore we call unix_notinflight() twice on the same fp->user and fp->fp[i], although we have refcounting but still able to trigger this warning.
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