Re: KASAN: use-after-free Read in link_path_walk

[Al Viro <viro@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?