FAILED: patch "[PATCH] ovl: fix null pointer dereference in ovl_get_acl_rcu()" failed to apply to 6.1-stable tree

[<gregkh@linuxfoundation.org>]

The patch below does not apply to the 6.1-stable tree.
If someone wants it applied there, or to any other stable or longterm
tree, then please email the backport, including the original git commit
id to <stable@vger.kernel.org>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-6.1.y
git checkout FETCH_HEAD
git cherry-pick -x f4e19e595cc2e76a8a58413eb19d3d9c51328b53
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<stable@vger.kernel.org>' --in-reply-to '2023071621-charcoal-prewashed-932b@gregkh' --subject-prefix 'PATCH 6.1.y' HEAD^..

Possible dependencies:

f4e19e595cc2 ("ovl: fix null pointer dereference in ovl_get_acl_rcu()")
6c0a8bfb84af ("ovl: implement get acl method")
cac2f8b8d8b5 ("fs: rename current get acl method")

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From f4e19e595cc2e76a8a58413eb19d3d9c51328b53 Mon Sep 17 00:00:00 2001
From: Zhihao Cheng <chengzhihao1@huawei.com>
Date: Tue, 16 May 2023 22:16:19 +0800
Subject: [PATCH] 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: 332f606b32b6 ("ovl: enable RCU'd ->get_acl()")
Cc: <stable@vger.kernel.org> # v5.15
Signed-off-by: Zhihao Cheng <chengzhihao1@huawei.com>
Suggested-by: Christian Brauner <brauner@kernel.org>
Suggested-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

diff --git a/fs/overlayfs/inode.c b/fs/overlayfs/inode.c
index ca56b1328a2c..e7e888dea634 100644
--- a/fs/overlayfs/inode.c
+++ b/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