Re: [PATCH] ceph: Fix use-after-free in __ceph_remove_cap

[Luis Henriques]

On Tue, Oct 22, 2019 at 08:48:56PM +0800, Yan, Zheng wrote:
> On Mon, Oct 21, 2019 at 10:55 PM Luis Henriques <lhenriques@xxxxxxxx> wrote:
> 
> >
> > Jeff Layton <jlayton@xxxxxxxxxx> writes:
> >
> > > On Thu, 2019-10-17 at 15:46 +0100, Luis Henriques wrote:
> > >> KASAN reports a use-after-free when running xfstest generic/531, with
> > the
> > >> following trace:
> > >>
> > >> [  293.903362]  kasan_report+0xe/0x20
> > >> [  293.903365]  rb_erase+0x1f/0x790
> > >> [  293.903370]  __ceph_remove_cap+0x201/0x370
> > >> [  293.903375]  __ceph_remove_caps+0x4b/0x70
> > >> [  293.903380]  ceph_evict_inode+0x4e/0x360
> > >> [  293.903386]  evict+0x169/0x290
> > >> [  293.903390]  __dentry_kill+0x16f/0x250
> > >> [  293.903394]  dput+0x1c6/0x440
> > >> [  293.903398]  __fput+0x184/0x330
> > >> [  293.903404]  task_work_run+0xb9/0xe0
> > >> [  293.903410]  exit_to_usermode_loop+0xd3/0xe0
> > >> [  293.903413]  do_syscall_64+0x1a0/0x1c0
> > >> [  293.903417]  entry_SYSCALL_64_after_hwframe+0x44/0xa9
> > >>
> > >> This happens because __ceph_remove_cap() may queue a cap release
> > >> (__ceph_queue_cap_release) which can be scheduled before that cap is
> > >> removed from the inode list with
> > >>
> > >>      rb_erase(&cap->ci_node, &ci->i_caps);
> > >>
> > >> And, when this finally happens, the use-after-free will occur.
> > >>
> > >> This can be fixed by protecting the rb_erase with the s_cap_lock
> > spinlock,
> > >> which is used by ceph_send_cap_releases(), before the cap is freed.
> > >>
> > >> Signed-off-by: Luis Henriques <lhenriques@xxxxxxxx>
> > >> ---
> > >>  fs/ceph/caps.c | 4 ++--
> > >>  1 file changed, 2 insertions(+), 2 deletions(-)
> > >>
> > >> diff --git a/fs/ceph/caps.c b/fs/ceph/caps.c
> > >> index d3b9c9d5c1bd..21ee38cabe98 100644
> > >> --- a/fs/ceph/caps.c
> > >> +++ b/fs/ceph/caps.c
> > >> @@ -1089,13 +1089,13 @@ void __ceph_remove_cap(struct ceph_cap *cap,
> > bool queue_release)
> > >>      }
> > >>      cap->cap_ino = ci->i_vino.ino;
> > >>
> > >> -    spin_unlock(&session->s_cap_lock);
> > >> -
> > >>      /* remove from inode list */
> > >>      rb_erase(&cap->ci_node, &ci->i_caps);
> > >>      if (ci->i_auth_cap == cap)
> > >>              ci->i_auth_cap = NULL;
> > >>
> > >> +    spin_unlock(&session->s_cap_lock);
> > >> +
> > >>      if (removed)
> > >>              ceph_put_cap(mdsc, cap);
> > >>
> > >
> > > Is there any reason we need to wait until this point to remove it from
> > > the rbtree? ISTM that we ought to just do that at the beginning of the
> > > function, before we take the s_cap_lock.
> >
> > That sounds good to me, at least at a first glace.  I spent some time
> > looking for any possible issues in the code, and even run a few tests.
> >
> > However, looking at git log I found commit f818a73674c5 ("ceph: fix cap
> > removal races"), which moved that rb_erase from the beginning of the
> > function to it's current position.  So, unless the race mentioned in
> > this commit has disappeared in the meantime (which is possible, this
> > commit is from 2010!), this rbtree operation shouldn't be changed.
> >
> > And I now wonder if my patch isn't introducing a race too...
> > __ceph_remove_cap() is supposed to always be called with the session
> > mutex held, except for the ceph_evict_inode() path.  Which is where I'm
> > seeing the UAF.  So, maybe what's missing here is the s_mutex.  Hmm...
> >
> >
> we can't lock s_mutex here, because i_ceph_lock is locked

Well, my idea wasn't to get s_mutex here but earlier in the stack.
Maybe in ceph_evict_inode, protecting the call to __ceph_remove_caps.
But I didn't really looked into that yet, so I'm not really sure if
that's feasible (or even if that would fix this UAF).  I suspect that's
not possible anyway, due to the comment above __ceph_remove_cap:

  caller will not hold session s_mutex if called from destroy_inode.

Cheers,
--
Luís