[PATCH 5.15] ovl: fix null pointer dereference in ovl_get_acl_rcu()

[Zhihao Cheng]

[ Upstream commit f4e19e595cc2e76a8a58413eb19d3d9c51328b53 ]

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.

There are some changes from upstream commit:
1. Corrusponds to do_ovl_get_acl() in 5.15 is ovl_get_acl()
2. ovl_i_path_real is not imported in 5.15, we can get realinode by
   ovl_inode_real
3. CONFIG_FS_POSIX_ACL checking is dropped in commit
   ded536561a3674327dfa4bb389085705cae22b8a ("ovl: improve ovl_get_acl()
   if POSIX ACL support is off"), we still keep it in 5.15.

Link: https://bugzilla.kernel.org/show_bug.cgi?id=217404
Fixes: 332f606b32b6 ("ovl: enable RCU'd ->get_acl()")
Cc: <stable@xxxxxxxxxxxxxxx> # v5.15
Signed-off-by: Zhihao Cheng <chengzhihao1@xxxxxxxxxx>
Suggested-by: Christian Brauner <brauner@xxxxxxxxxx>
Suggested-by: Amir Goldstein <amir73il@xxxxxxxxx>
Signed-off-by: Amir Goldstein <amir73il@xxxxxxxxx>
Signed-off-by: Miklos Szeredi <mszeredi@xxxxxxxxxx>
---
 fs/overlayfs/inode.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/fs/overlayfs/inode.c b/fs/overlayfs/inode.c
index d41f0c8e0e2a..65e5e6eb761a 100644
--- a/fs/overlayfs/inode.c
+++ b/fs/overlayfs/inode.c
@@ -453,7 +453,15 @@ struct posix_acl *ovl_get_acl(struct inode *inode, int type, bool rcu)
 	const struct cred *old_cred;
 	struct posix_acl *acl;
 
-	if (!IS_ENABLED(CONFIG_FS_POSIX_ACL) || !IS_POSIXACL(realinode))
+	if (!IS_ENABLED(CONFIG_FS_POSIX_ACL))
+		return NULL;
+
+	if (!realinode) {
+		WARN_ON(!rcu);
+		return ERR_PTR(-ECHILD);
+	}
+
+	if (!IS_POSIXACL(realinode))
 		return NULL;
 
 	if (rcu)
-- 
2.39.2