fuse: fix control dir setup and teardown

syzbot is reporting NULL pointer dereference at fuse_ctl_remove_conn() [1].
Since fc->ctl_ndents is incremented by fuse_ctl_add_conn() when new_inode()
failed, fuse_ctl_remove_conn() reaches an inode-less dentry and tries to
clear d_inode(dentry)->i_private field.

Fix by only adding the dentry to the array after being fully set up.

When tearing down the control directory, do d_invalidate() on it to get rid
of any mounts that might have been added.

[1] https://syzkaller.appspot.com/bug?id=f396d863067238959c91c0b7cfc10b163638cac6
Reported-by: syzbot <syzbot+32c236387d66c4516827@syzkaller.appspotmail.com>
Fixes: bafa96541b25 ("[PATCH] fuse: add control filesystem")
Cc: <stable@vger.kernel.org> # v2.6.18
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

1 files changed, 10 insertions, 3 deletions

diff --git a/fs/fuse/control.c b/fs/fuse/control.c
index 78fb7a07c5ca..0b694655d988 100644
--- a/fs/fuse/control.c
+++ b/fs/fuse/control.c
@@ -211,10 +211,11 @@ static struct dentry *fuse_ctl_add_dentry(struct dentry *parent,
         if (!dentry)
                 return NULL;
 
-        fc->ctl_dentry[fc->ctl_ndents++] = dentry;
         inode = new_inode(fuse_control_sb);
-        if (!inode)
+        if (!inode) {
+                dput(dentry);
                 return NULL;
+        }
 
         inode->i_ino = get_next_ino();
         inode->i_mode = mode;
@@ -228,6 +229,9 @@ static struct dentry *fuse_ctl_add_dentry(struct dentry *parent,
         set_nlink(inode, nlink);
         inode->i_private = fc;
         d_add(dentry, inode);
+
+        fc->ctl_dentry[fc->ctl_ndents++] = dentry;
+
         return dentry;
 }
 
@@ -284,7 +288,10 @@ void fuse_ctl_remove_conn(struct fuse_conn *fc)
         for (i = fc->ctl_ndents - 1; i >= 0; i--) {
                 struct dentry *dentry = fc->ctl_dentry[i];
                 d_inode(dentry)->i_private = NULL;
-                d_drop(dentry);
+                if (!i) {
+                        /* Get rid of submounts: */
+                        d_invalidate(dentry);
+                }
                 dput(dentry);
         }
         drop_nlink(d_inode(fuse_control_sb->s_root));