[PATCH] fs: make sure to call fsnotify_inoderemove() only once when inode is dead

[Jan Kara <jack@suse.cz>]

From: Amir Goldstein <amir73il@gmail.com>

Daehyeon reported a race wherein fsnotify_removeinode() can be called
when inode still has a live alias.  This can lead to several undesired
outcomes such as crashes in fsnotify code or avoidance of inode mark
notifications.

Check that inode is really "dead", meaning no nlink and no more aliases
before calling fsnotify_inoderemove() for final cleanup of inode marks.
Also do not allow open_by_handle() to add new aliases to a "dead" inode.

Ideally, we'd call fsnotify_inoderemove() once last inode reference is
dropped to make things simpler. Alas fsnotify inode marks currently hold
inode references and thus this doesn't work. Once we teach fsnotify to
avoid holding inode references, these games with dead inodes can go as
well.

Reported-and-tested-by: Daehyeon Ko <4ncienth@gmail.com>
Closes: https://lore.kernel.org/linux-fsdevel/20260827045007.3831259-1-4ncienth@gmail.com/
CC: stable@vger.kernel.org
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Jan Kara <jack@suse.cz>
---
 fs/dcache.c | 27 ++++++++++++++++++++++++---
 1 file changed, 24 insertions(+), 3 deletions(-)

diff --git a/fs/dcache.c b/fs/dcache.c
index 1b1a81f10da6..8f17db2724b3 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -450,6 +450,17 @@ static void dentry_free(struct dentry *dentry)
 		call_rcu(&dentry->d_rcu, __d_free);
 }
 
+/*
+ * If inode is unlinked and doesn't have any aliases (i.e., all fds pointing to
+ * it are closed), it is pretty much dead. Except that file handle lookup could
+ * still revive it which causes issues to fsnotify. So once inode reaches this
+ * state we make sure to block creating any new aliases.
+ */
+static bool inode_notify_dead(struct inode *inode)
+{
+	return !inode->i_nlink && hlist_empty(&inode->i_dentry);
+}
+
 /*
  * Release the dentry's inode, using the filesystem
  * d_iput() operation if defined.
@@ -459,6 +470,7 @@ static void dentry_unlink_inode(struct dentry * dentry)
 	__releases(dentry->d_inode->i_lock)
 {
 	struct inode *inode = dentry->d_inode;
+	bool notify_dead;
 
 	raw_write_seqcount_begin(&dentry->d_seq);
 	__d_clear_type_and_inode(dentry);
@@ -469,9 +481,10 @@ static void dentry_unlink_inode(struct dentry * dentry)
 	 */
 	dentry->waiters = NULL;
 	raw_write_seqcount_end(&dentry->d_seq);
+	notify_dead = inode_notify_dead(inode);
 	spin_unlock(&dentry->d_lock);
 	spin_unlock(&inode->i_lock);
-	if (!inode->i_nlink)
+	if (notify_dead)
 		fsnotify_inoderemove(inode);
 	if (dentry->d_op && dentry->d_op->d_iput)
 		dentry->d_op->d_iput(dentry, inode);
@@ -2237,7 +2250,12 @@ static struct dentry *__d_obtain_alias(struct inode *inode, bool disconnected)
 
 	sb = inode->i_sb;
 
-	res = d_find_any_alias(inode); /* existing alias? */
+	spin_lock(&inode->i_lock);
+	if (!inode_notify_dead(inode))
+		res = __d_find_any_alias(inode);	/* existing alias? */
+	else
+		res = ERR_PTR(-ESTALE);
+	spin_unlock(&inode->i_lock);
 	if (res)
 		goto out;
 
@@ -2249,7 +2267,10 @@ static struct dentry *__d_obtain_alias(struct inode *inode, bool disconnected)
 
 	security_d_instantiate(new, inode);
 	spin_lock(&inode->i_lock);
-	res = __d_find_any_alias(inode); /* recheck under lock */
+	if (!inode_notify_dead(inode))
+		res = __d_find_any_alias(inode);	/* recheck under lock */
+	else
+		res = ERR_PTR(-ESTALE);
 	if (likely(!res)) { /* still no alias, attach a disconnected dentry */
 		unsigned add_flags = d_flags_for_inode(inode);
 
-- 
2.51.0