Re: [PATCH] 9p: add missing locking around taking dentry fid list

[Dominique Martinet]

Christian Schoenebeck wrote on Wed, May 22, 2024 at 04:35:19PM +0200:

Thanks for the review!

> On Tuesday, May 21, 2024 2:29:46 PM CEST Dominique Martinet wrote:
> > Fix a use-after-free on dentry's d_fsdata fid list when a thread
> > lookups a fid through dentry while another thread unlinks it:
> 
> I guess that's "looks up". :)

Err, I guess.

> > UAF thread:
> > refcount_t: addition on 0; use-after-free.
> >  p9_fid_get linux/./include/net/9p/client.h:262
> >  v9fs_fid_find+0x236/0x280 linux/fs/9p/fid.c:129
> >  v9fs_fid_lookup_with_uid linux/fs/9p/fid.c:181
> >  v9fs_fid_lookup+0xbf/0xc20 linux/fs/9p/fid.c:314
> >  v9fs_vfs_getattr_dotl+0xf9/0x360 linux/fs/9p/vfs_inode_dotl.c:400
> >  vfs_statx+0xdd/0x4d0 linux/fs/stat.c:248
> > 
> > Freed by:
> >  p9_client_clunk+0xb0/0xe0 linux/net/9p/client.c:1456
> 
> That line number looks weird.

I have a p9_fid_destroy there (as of a v6.9-rc5 tree); might have moved
a bit though.
Unfortunately it's inlined so the stack trace only has kfree() next
which is why I cut the trace there; I don't think it really matters?

> >  p9_fid_put linux/./include/net/9p/client.h:278
> >  v9fs_dentry_release+0xb5/0x140 linux/fs/9p/vfs_dentry.c:55
> >  v9fs_remove+0x38f/0x620 linux/fs/9p/vfs_inode.c:518
> >  vfs_unlink+0x29a/0x810 linux/fs/namei.c:4335
> > 
> > The problem is that d_fsdata was not accessed under d_lock, because
> > d_release() normally is only called once the dentry is otherwise no
> > longer accessible but since we also call it explicitly in v9fs_remove
> > that lock is required:
> > move the hlist out of the dentry under lock then unref its fids once
> > they are no longer accessible.
> > 
> > Fixes: 154372e67d40 ("fs/9p: fix create-unlink-getattr idiom")
> > Cc: stable@xxxxxxxxxxxxxxx
> > Reported-by: Meysam Firouzi
> > Reported-by: Amirmohammad Eftekhar
> > Signed-off-by: Dominique Martinet <asmadeus@xxxxxxxxxxxxx>
> > ---
> >  fs/9p/vfs_dentry.c | 9 +++++++--
> >  1 file changed, 7 insertions(+), 2 deletions(-)
> > 
> > diff --git a/fs/9p/vfs_dentry.c b/fs/9p/vfs_dentry.c
> > index f16f73581634..01338d4c2d9e 100644
> > --- a/fs/9p/vfs_dentry.c
> > +++ b/fs/9p/vfs_dentry.c
> > @@ -48,12 +48,17 @@ static int v9fs_cached_dentry_delete(const struct dentry *dentry)
> >  static void v9fs_dentry_release(struct dentry *dentry)
> >  {
> >  	struct hlist_node *p, *n;
> > +	struct hlist_head head;
> >  
> >  	p9_debug(P9_DEBUG_VFS, " dentry: %pd (%p)\n",
> >  		 dentry, dentry);
> > -	hlist_for_each_safe(p, n, (struct hlist_head *)&dentry->d_fsdata)
> > +
> > +	spin_lock(&dentry->d_lock);
> > +	hlist_move_list((struct hlist_head *)&dentry->d_fsdata, &head);
> > +	spin_unlock(&dentry->d_lock);
> > +
> > +	hlist_for_each_safe(p, n, &head)
> >  		p9_fid_put(hlist_entry(p, struct p9_fid, dlist));
> > -	dentry->d_fsdata = NULL;
> >  }
> 
> I'm not sure if that works out. So you are moving the list from dentry to a
> local variable. But if you look at v9fs_fid_find() [fs/9p/fid.c#123] it reads
> dentry->d_fsdata (twice) and holds it as local variable before taking a
> lock. So the lock in v9fs_fid_find() should happen earlier, no?

The comment still works -- if detry->d_fsdata is NULL then
hlist_for_each_entry will stop short and not iterate over anything (it
won't bug out), so that part is fine in my opinion.

What should be improved though is that if dentry->d_inode we can still
look by inode even if there was a d_fsdata as log as fid wasn't found,
e.g.:
-----
diff --git a/fs/9p/fid.c b/fs/9p/fid.c
index de009a33e0e2..c72825fb0ece 100644
--- a/fs/9p/fid.c
+++ b/fs/9p/fid.c
@@ -131,9 +131,9 @@ static struct p9_fid *v9fs_fid_find(struct dentry *dentry, kuid_t uid, int any)
 			}
 		}
 		spin_unlock(&dentry->d_lock);
-	} else {
-		if (dentry->d_inode)
-			ret = v9fs_fid_find_inode(dentry->d_inode, false, uid, any);
+	}
+	if (!ret && dentry->d_inode)
+		ret = v9fs_fid_find_inode(dentry->d_inode, false, uid, any);
 	}
 
 	return ret;
----

I don't think that has to be part of this commit though, the worst that
can happen here is an extra lookup to server instead of a use after
free; I'll send a separate patch for this.

-- 
Dominique Martinet | Asmadeus