Re: [PATCH v2] ceph: Fix use-after-free in __ceph_remove_cap

[Luis Henriques <lhenriques@suse.com>]

On Sun, Oct 27, 2019 at 08:31:49AM -0400, Jeff Layton wrote:
> On Fri, 2019-10-25 at 14:05 +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 removing the cap from the inode list before being
> > removed from the session list, and thus eliminating the risk of an UAF.
> > 
> > Signed-off-by: Luis Henriques <lhenriques@suse.com>
> > ---
> > Hi!
> > 
> > So, after spending some time trying to find possible races throught code
> > review and testing, I modified the fix according to Jeff's suggestion.
> > 
> > Cheers,
> > Luis
> > 
> > fs/ceph/caps.c | 10 +++++-----
> >  1 file changed, 5 insertions(+), 5 deletions(-)
> > 
> > diff --git a/fs/ceph/caps.c b/fs/ceph/caps.c
> > index d3b9c9d5c1bd..a9ce858c37d0 100644
> > --- a/fs/ceph/caps.c
> > +++ b/fs/ceph/caps.c
> > @@ -1058,6 +1058,11 @@ void __ceph_remove_cap(struct ceph_cap *cap, bool queue_release)
> >  
> >  	dout("__ceph_remove_cap %p from %p\n", cap, &ci->vfs_inode);
> >  
> > +	/* remove from inode list */
> > +	rb_erase(&cap->ci_node, &ci->i_caps);
> > +	if (ci->i_auth_cap == cap)
> > +		ci->i_auth_cap = NULL;
> > +
> >  	/* remove from session list */
> >  	spin_lock(&session->s_cap_lock);
> >  	if (session->s_cap_iterator == cap) {
> > @@ -1091,11 +1096,6 @@ void __ceph_remove_cap(struct ceph_cap *cap, bool queue_release)
> >  
> >  	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;
> > -
> >  	if (removed)
> >  		ceph_put_cap(mdsc, cap);
> >  
> 
> Looks good. Merged with a slight modification to the comment:
> 
> +       /* remove from inode's cap rbtree, and clear auth cap */
> 

Awesome, thanks.  Regarding CC: stable@, feel free to add that too.  I
guess it makes sense, even if this issue has been there for a while
apparently without any reports.  (Although UAFs may have non-obvious
side effects, I guess...)

Cheers,
--
Luís

> Thanks!
> -- 
> Jeff Layton <jlayton@kernel.org>
>