linux-kernel - Re: WARNING in ovl_real_fdget

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Date:   Fri, 26 Jul 2019 13:06:48 +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 Fri, Jul 26, 2019 at 11:11 AM Amir Goldstein <amir73il@...il.com> wrote:
>
> 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 (?).

No, we shouldn't care about that.
overlayfs doesn't follow cross-fs in underlying layers.

>
> ./ 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.

But damage can still be created if a lower overlayfs layer
overlaps with the another nested overlay lower underlying layer.
It actually shouldn't be too hard to add a guard also on the
nested overlay lower underlying layer inode.

>
> 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.
>

I was wrong here of course.
./file0/cgroup.procs is a regular overlayfs file (I was confused by the name)
which is later also exposed at ./file0/file0/cgroup.procs in the nested
overlay mount.

Still not sure about the rest of the way to ovl_change_flags() failure,
but I think I'll try to block this new syzbot overlap attack.

Thanks,
Amir.