From: Al Viro <viro@ZenIV.linux.org.uk>
To: "Dae R. Jeong" <threeearcat@gmail.com>
Cc: linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
byoungyoung@purdue.edu, kt0755@gmail.com, bammanag@purdue.edu
Subject: Re: KASAN: use-after-free Read in link_path_walk
Date: Tue, 24 Jul 2018 06:29:29 +0100 [thread overview]
Message-ID: <20180724052929.GI30522@ZenIV.linux.org.uk> (raw)
In-Reply-To: <20180724051726.GH30522@ZenIV.linux.org.uk>
On Tue, Jul 24, 2018 at 06:17:26AM +0100, Al Viro wrote:
> On Tue, Jul 24, 2018 at 12:45:42PM +0900, Dae R. Jeong wrote:
> > Diagnosis:
> > We think that it is possible that link_path_walk() dereferences a
> > freed pointer when cleanup_mnt() is executed between path_init() and
> > link_path_walk().
> >
> > Since I'm not an expert on a file system and don't fully understand
> > the crash, please see a executed program and a crash log below in
> > case that my understanding is wrong.
> >
> >
> > Executed Program:
> > Thread0 Thread1
> > mkdir("./file0")
> > |--------------------------|
> > | mount("./file0", "./file0", "devpts", 0x0, "")
> > | |
> > openat(AT_FDCWD, chroot("./file0")
> > "/dev/vcs", 0x200, 0x0) umount("./file0", 0x2)
> >
> > openat(), chroot(), umount() syscalls are executed after mount() syscall.
> > We think a race occurs between openat() and chroot() because RaceFuzzer
> > executed openat() and chroot() concurrently.
> >
> >
> > (Possible) Thread interleaving:
> > CPU0 (path_openat) CPU1 (cleanup_mnt)
Wait a bloody minute. Where does cleanup_mnt() come from in that thing?
You are doing lazy-umount of the thing you've chrooted into; if it ends
up with zero refcount on that mount, we are already in deep, deep trouble,
races with open() on not. Simply following that with stat / (in thread 1,
without thread0 at all) would end up accessing the same vfsmount. And
if it's been freed, we are well and truly fucked, race or no race.
I really want details. *Is* cleanup_mnt() called by thread 1 in your
reproducer before the use-after-free hits? And what's the root of
thread 0 at that point?
next prev parent reply other threads:[~2018-07-24 6:34 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-07-24 3:45 KASAN: use-after-free Read in link_path_walk Dae R. Jeong
2018-07-24 4:08 ` DaeRyong Jeong
2018-07-24 5:17 ` Al Viro
2018-07-24 5:29 ` Al Viro [this message]
2018-07-24 5:54 ` DaeRyong Jeong
2018-08-06 13:01 ` Al Viro
2018-11-28 17:40 syzbot
2019-03-25 4:52 ` syzbot
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