fs: add kern_path_locked_negative()

The audit code relies on the fact that kern_path_locked() returned a
path even for a negative dentry. If it doesn't find a valid dentry it
immediately calls:

    audit_find_parent(d_backing_inode(parent_path.dentry));

which assumes that parent_path.dentry is still valid. But it isn't since
kern_path_locked() has been changed to path_put() also for a negative
dentry.

Fix this by adding a helper that implements the required audit semantics
and allows us to fix the immediate bleeding. We can find a unified
solution for this afterwards.

Link: https://lore.kernel.org/20250414-rennt-wimmeln-f186c3a780f1@brauner
Fixes: 1c3cb50b58 ("VFS: change kern_path_locked() and user_path_locked_at() to never return negative dentry")
Reported-and-tested-by: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Christian Brauner <brauner@kernel.org>

fs/namei.c

@@ -1665,27 +1665,20 @@ static struct dentry *lookup_dcache(const struct qstr *name,
 	return dentry;
 }
 
-/*
- * Parent directory has inode locked exclusive.  This is one
- * and only case when ->lookup() gets called on non in-lookup
- * dentries - as the matter of fact, this only gets called
- * when directory is guaranteed to have no in-lookup children
- * at all.
- * Will return -ENOENT if name isn't found and LOOKUP_CREATE wasn't passed.
- * Will return -EEXIST if name is found and LOOKUP_EXCL was passed.
- */
-struct dentry *lookup_one_qstr_excl(const struct qstr *name,
-				    struct dentry *base,
-				    unsigned int flags)
+static struct dentry *lookup_one_qstr_excl_raw(const struct qstr *name,
+					       struct dentry *base,
+					       unsigned int flags)
 {
-	struct dentry *dentry = lookup_dcache(name, base, flags);
+	struct dentry *dentry;
 	struct dentry *old;
-	struct inode *dir = base->d_inode;
+	struct inode *dir;
 
+	dentry = lookup_dcache(name, base, flags);
 	if (dentry)
-		goto found;
+		return dentry;
 
 	/* Don't create child dentry for a dead directory. */
+	dir = base->d_inode;
 	if (unlikely(IS_DEADDIR(dir)))
 		return ERR_PTR(-ENOENT);
 
@@ -1698,7 +1691,24 @@ struct dentry *lookup_one_qstr_excl(const struct qstr *name,
 		dput(dentry);
 		dentry = old;
 	}
-found:
+	return dentry;
+}
+
+/*
+ * Parent directory has inode locked exclusive.  This is one
+ * and only case when ->lookup() gets called on non in-lookup
+ * dentries - as the matter of fact, this only gets called
+ * when directory is guaranteed to have no in-lookup children
+ * at all.
+ * Will return -ENOENT if name isn't found and LOOKUP_CREATE wasn't passed.
+ * Will return -EEXIST if name is found and LOOKUP_EXCL was passed.
+ */
+struct dentry *lookup_one_qstr_excl(const struct qstr *name,
+				    struct dentry *base, unsigned int flags)
+{
+	struct dentry *dentry;
+
+	dentry = lookup_one_qstr_excl_raw(name, base, flags);
 	if (IS_ERR(dentry))
 		return dentry;
 	if (d_is_negative(dentry) && !(flags & LOOKUP_CREATE)) {
@@ -2762,6 +2772,29 @@ static struct dentry *__kern_path_locked(int dfd, struct filename *name, struct
 	return d;
 }
 
+struct dentry *kern_path_locked_negative(const char *name, struct path *path)
+{
+	struct filename *filename __free(putname) = getname_kernel(name);
+	struct dentry *d;
+	struct qstr last;
+	int type, error;
+
+	error = filename_parentat(AT_FDCWD, filename, 0, path, &last, &type);
+	if (error)
+		return ERR_PTR(error);
+	if (unlikely(type != LAST_NORM)) {
+		path_put(path);
+		return ERR_PTR(-EINVAL);
+	}
+	inode_lock_nested(path->dentry->d_inode, I_MUTEX_PARENT);
+	d = lookup_one_qstr_excl_raw(&last, path->dentry, 0);
+	if (IS_ERR(d)) {
+		inode_unlock(path->dentry->d_inode);
+		path_put(path);
+	}
+	return d;
+}
+
 struct dentry *kern_path_locked(const char *name, struct path *path)
 {
 	struct filename *filename = getname_kernel(name);

include/linux/namei.h

@@ -62,6 +62,7 @@ extern struct dentry *kern_path_create(int, const char *, struct path *, unsigne
 extern struct dentry *user_path_create(int, const char __user *, struct path *, unsigned int);
 extern void done_path_create(struct path *, struct dentry *);
 extern struct dentry *kern_path_locked(const char *, struct path *);
+extern struct dentry *kern_path_locked_negative(const char *, struct path *);
 extern struct dentry *user_path_locked_at(int , const char __user *, struct path *);
 int vfs_path_parent_lookup(struct filename *filename, unsigned int flags,
 			   struct path *parent, struct qstr *last, int *type,

kernel/audit_watch.c

@@ -347,12 +347,17 @@ static void audit_remove_parent_watches(struct audit_parent *parent)
 /* Get path information necessary for adding watches. */
 static int audit_get_nd(struct audit_watch *watch, struct path *parent)
 {
-	struct dentry *d = kern_path_locked(watch->path, parent);
+	struct dentry *d;
+
+	d = kern_path_locked_negative(watch->path, parent);
 	if (IS_ERR(d))
 		return PTR_ERR(d);
-	/* update watch filter fields */
-	watch->dev = d->d_sb->s_dev;
-	watch->ino = d_backing_inode(d)->i_ino;
+
+	if (d_is_positive(d)) {
+		/* update watch filter fields */
+		watch->dev = d->d_sb->s_dev;
+		watch->ino = d_backing_inode(d)->i_ino;
+	}
 
 	inode_unlock(d_backing_inode(parent->dentry));
 	dput(d);
@@ -418,11 +423,10 @@ int audit_add_watch(struct audit_krule *krule, struct list_head **list)
 	/* caller expects mutex locked */
 	mutex_lock(&audit_filter_mutex);
 
-	if (ret && ret != -ENOENT) {
+	if (ret) {
 		audit_put_watch(watch);
 		return ret;
 	}
-	ret = 0;
 
 	/* either find an old parent or attach a new one */
 	parent = audit_find_parent(d_backing_inode(parent_path.dentry));