linux-kernel - Re: WARNING in ovl_real_fdget

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Date:   Fri, 26 Jul 2019 11:11:54 +0300
From:   Amir Goldstein <amir73il@...il.com>
To:     syzbot <syzbot+032bc63605089a199d30@...kaller.appspotmail.com>
Cc:     "J. Bruce Fields" <bfields@...ldses.org>,
        Jeff Layton <jlayton@...nel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        overlayfs <linux-unionfs@...r.kernel.org>,
        Miklos Szeredi <miklos@...redi.hu>,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>,
        Al Viro <viro@...iv.linux.org.uk>
Subject: Re: WARNING in ovl_real_fdget_meta

On Thu, Jul 25, 2019 at 7:24 AM syzbot
<syzbot+032bc63605089a199d30@...kaller.appspotmail.com> wrote:
>
> syzbot has bisected this bug to:
>
> commit 387e3746d01c34457d6a73688acd90428725070b
> Author: Amir Goldstein <amir73il@...il.com>
> Date:   Fri Jun 7 14:24:38 2019 +0000
>
>      locks: eliminate false positive conflicts for write lease
>
> bisection log:  https://syzkaller.appspot.com/x/bisect.txt?x=15a79594600000
> start commit:   c6dd78fc Merge branch 'x86-urgent-for-linus' of git://git...
> git tree:       upstream
> final crash:    https://syzkaller.appspot.com/x/report.txt?x=17a79594600000
> console output: https://syzkaller.appspot.com/x/log.txt?x=13a79594600000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=3c8985c08e1f9727
> dashboard link: https://syzkaller.appspot.com/bug?extid=032bc63605089a199d30
> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=15855334600000
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=17fcc4c8600000
>
> Reported-by: syzbot+032bc63605089a199d30@...kaller.appspotmail.com
> Fixes: 387e3746d01c ("locks: eliminate false positive conflicts for write
> lease")
>
> For information about bisection process see: https://goo.gl/tpsmEJ#bisection

The repro:
#{"repeat":true,"procs":1,"sandbox":"none","fault_call":-1,"cgroups":true,"close_fds":true,"tmpdir":true}
mkdir(&(0x7f0000000100)='./file0\x00', 0x0)
mkdirat$cgroup_root(0xffffffffffffff9c,
&(0x7f0000000000)='./cgroup.net/syz1\x00', 0x1ff)
mount$fuse(0x20000000, &(0x7f0000000140)='./file0\x00', 0x0, 0x1004, 0x0)
mount$overlay(0x400000, &(0x7f0000000100)='./file0\x00',
&(0x7f00000001c0)='overlay\x00', 0x0,
&(0x7f0000000040)=ANY=[@ANYBLOB=',lowerdir=.:file0'])
r0 = open(&(0x7f0000000500)='./file0\x00', 0x0, 0x0)
r1 = openat$cgroup_procs(r0, &(0x7f00000004c0)='cgroup.procs\x00', 0x48, 0x0)
dup3(r1, r0, 0x0)
fcntl$setlease(r0, 0x400, 0x1)
lseek(r0, 0x4, 0x0)

I though we would stop these family of overlapping layers fuzzers with:
146d62e5a586 ("ovl: detect overlapping layers")

But syzbot got the upper hand, because we do not check for overlapping layers
that cross fs boundary. Not sure if we should (?).

./ is a tmpfs dir and ./file0/ is some kind of fuse mount (?)
then after one cycle, ./file0/ itself is an overlapping overlay mount
(lowerdir=./:./file0/)
and after another cycle, ./file0/ is a nested overlapping overlayfs mount.
Fine. Whatever.

What I don't understand is if dup3 succeeds r0 should not be an overlayfs fd
and even if dup3 fails r0 should be an overlayfs directory fd (./file0/), so how
the hell did we get to ovl_llseek => ... ovl_change_flags() with this repro??

There is not a single regular file in this test.

Thanks,
Amir.