ovl: fix null pointer dereference in ovl_get_acl_rcu()

Following process:
         P1                     P2
 path_openat
  link_path_walk
   may_lookup
    inode_permission(rcu)
     ovl_permission
      acl_permission_check
       check_acl
        get_cached_acl_rcu
	 ovl_get_inode_acl
	  realinode = ovl_inode_real(ovl_inode)
	                      drop_cache
		               __dentry_kill(ovl_dentry)
				iput(ovl_inode)
		                 ovl_destroy_inode(ovl_inode)
		                  dput(oi->__upperdentry)
		                   dentry_kill(upperdentry)
		                    dentry_unlink_inode
				     upperdentry->d_inode = NULL
	    ovl_inode_upper
	     upperdentry = ovl_i_dentry_upper(ovl_inode)
	     d_inode(upperdentry) // returns NULL
	  IS_POSIXACL(realinode) // NULL pointer dereference
, will trigger an null pointer dereference at realinode:
  [  205.472797] BUG: kernel NULL pointer dereference, address:
                 0000000000000028
  [  205.476701] CPU: 2 PID: 2713 Comm: ls Not tainted
                 6.3.0-12064-g2edfa098e750-dirty #1216
  [  205.478754] RIP: 0010:do_ovl_get_acl+0x5d/0x300
  [  205.489584] Call Trace:
  [  205.489812]  <TASK>
  [  205.490014]  ovl_get_inode_acl+0x26/0x30
  [  205.490466]  get_cached_acl_rcu+0x61/0xa0
  [  205.490908]  generic_permission+0x1bf/0x4e0
  [  205.491447]  ovl_permission+0x79/0x1b0
  [  205.491917]  inode_permission+0x15e/0x2c0
  [  205.492425]  link_path_walk+0x115/0x550
  [  205.493311]  path_lookupat.isra.0+0xb2/0x200
  [  205.493803]  filename_lookup+0xda/0x240
  [  205.495747]  vfs_fstatat+0x7b/0xb0

Fetch a reproducer in [Link].

Use the helper ovl_i_path_realinode() to get realinode and then do
non-nullptr checking.

Link: https://bugzilla.kernel.org/show_bug.cgi?id=217404
Fixes: 332f606 ("ovl: enable RCU'd ->get_acl()")
Cc: <[email protected]> # v5.15
Signed-off-by: Zhihao Cheng <[email protected]>
Suggested-by: Christian Brauner <[email protected]>
Suggested-by: Amir Goldstein <[email protected]>
Signed-off-by: Amir Goldstein <[email protected]>
Signed-off-by: Miklos Szeredi <[email protected]>

Author: Zhihao Cheng

fs/overlayfs/inode.c

@@ -558,20 +558,20 @@ struct posix_acl *do_ovl_get_acl(struct mnt_idmap *idmap,
 				 struct inode *inode, int type,
 				 bool rcu, bool noperm)
 {
-	struct inode *realinode = ovl_inode_real(inode);
+	struct inode *realinode;
 	struct posix_acl *acl;
 	struct path realpath;
 
-	if (!IS_POSIXACL(realinode))
-		return NULL;
-
 	/* Careful in RCU walk mode */
-	ovl_i_path_real(inode, &realpath);
-	if (!realpath.dentry) {
+	realinode = ovl_i_path_real(inode, &realpath);
+	if (!realinode) {
 		WARN_ON(!rcu);
 		return ERR_PTR(-ECHILD);
 	}
 
+	if (!IS_POSIXACL(realinode))
+		return NULL;
+
 	if (rcu) {
 		/*
 		 * If the layer is idmapped drop out of RCU path walk