[PATCH 5.15.y] pstore: inode: Only d_invalidate() is needed

[Robert Garcia]

From: Kees Cook <keescook@xxxxxxxxxxxx>

[ Upstream commit a43e0fc5e9134a46515de2f2f8d4100b74e50de3 ]

Unloading a modular pstore backend with records in pstorefs would
trigger the dput() double-drop warning:

  WARNING: CPU: 0 PID: 2569 at fs/dcache.c:762 dput.part.0+0x3f3/0x410

Using the combo of d_drop()/dput() (as mentioned in
Documentation/filesystems/vfs.rst) isn't the right approach here, and
leads to the reference counting problem seen above. Use d_invalidate()
and update the code to not bother checking for error codes that can
never happen.

Suggested-by: Alexander Viro <viro@xxxxxxxxxxxxxxxxxx>
Fixes: 609e28bb139e ("pstore: Remove filesystem records when backend is unregistered")
Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
---
Cc: "Guilherme G. Piccoli" <gpiccoli@xxxxxxxxxx>
Cc: Tony Luck <tony.luck@xxxxxxxxx>
Cc: linux-hardening@xxxxxxxxxxxxxxx
[ Minor context change fixed. ]
Signed-off-by: Robert Garcia <rob_garcia@xxxxxxx>
---
 fs/pstore/inode.c | 10 +++-------
 1 file changed, 3 insertions(+), 7 deletions(-)

diff --git a/fs/pstore/inode.c b/fs/pstore/inode.c
index 14658b009f1b..f56e0b105be7 100644
--- a/fs/pstore/inode.c
+++ b/fs/pstore/inode.c
@@ -312,7 +312,6 @@ int pstore_put_backend_records(struct pstore_info *psi)
 {
 	struct pstore_private *pos, *tmp;
 	struct dentry *root;
-	int rc = 0;
 
 	root = psinfo_lock_root();
 	if (!root)
@@ -322,11 +321,8 @@ int pstore_put_backend_records(struct pstore_info *psi)
 	list_for_each_entry_safe(pos, tmp, &records_list, list) {
 		if (pos->record->psi == psi) {
 			list_del_init(&pos->list);
-			rc = simple_unlink(d_inode(root), pos->dentry);
-			if (WARN_ON(rc))
-				break;
-			d_drop(pos->dentry);
-			dput(pos->dentry);
+			d_invalidate(pos->dentry);
+			simple_unlink(d_inode(root), pos->dentry);
 			pos->dentry = NULL;
 		}
 	}
@@ -334,7 +330,7 @@ int pstore_put_backend_records(struct pstore_info *psi)
 
 	inode_unlock(d_inode(root));
 
-	return rc;
+	return 0;
 }
 
 /*
-- 
2.34.1