Merge tag 'v6.6-rc7.vfs.fixes' of git://git.kernel.org/pub/scm/linux/kernel/git/vfs/vfs

Pull vfs fix from Christian Brauner:
 "An openat() call from io_uring triggering an audit call can apparently
  cause the refcount of struct filename to be incremented from multiple
  threads concurrently during async execution, triggering a refcount
  underflow and hitting a BUG_ON(). That bug has been lurking around
  since at least v5.16 apparently.

  Switch to an atomic counter to fix that. The underflow check is
  downgraded from a BUG_ON() to a WARN_ON_ONCE() but we could easily
  remove that check altogether tbh"

* tag 'v6.6-rc7.vfs.fixes' of git://git.kernel.org/pub/scm/linux/kernel/git/vfs/vfs:
  audit,io_uring: io_uring openat triggers audit reference count underflow

fs/namei.c

@@ -188,7 +188,7 @@ getname_flags(const char __user *filename, int flags, int *empty)
 		}
 	}
 
-	result->refcnt = 1;
+	atomic_set(&result->refcnt, 1);
 	/* The empty path is special. */
 	if (unlikely(!len)) {
 		if (empty)
@@ -249,7 +249,7 @@ getname_kernel(const char * filename)
 	memcpy((char *)result->name, filename, len);
 	result->uptr = NULL;
 	result->aname = NULL;
-	result->refcnt = 1;
+	atomic_set(&result->refcnt, 1);
 	audit_getname(result);
 
 	return result;
@@ -261,9 +261,10 @@ void putname(struct filename *name)
 	if (IS_ERR(name))
 		return;
 
-	BUG_ON(name->refcnt <= 0);
+	if (WARN_ON_ONCE(!atomic_read(&name->refcnt)))
+		return;
 
-	if (--name->refcnt > 0)
+	if (!atomic_dec_and_test(&name->refcnt))
 		return;
 
 	if (name->name != name->iname) {

include/linux/fs.h

@@ -2403,7 +2403,7 @@ struct audit_names;
 struct filename {
 	const char		*name;	/* pointer to actual string */
 	const __user char	*uptr;	/* original userland pointer */
-	int			refcnt;
+	atomic_t		refcnt;
 	struct audit_names	*aname;
 	const char		iname[];
 };

kernel/auditsc.c

@@ -2212,7 +2212,7 @@ __audit_reusename(const __user char *uptr)
 		if (!n->name)
 			continue;
 		if (n->name->uptr == uptr) {
-			n->name->refcnt++;
+			atomic_inc(&n->name->refcnt);
 			return n->name;
 		}
 	}
@@ -2241,7 +2241,7 @@ void __audit_getname(struct filename *name)
 	n->name = name;
 	n->name_len = AUDIT_NAME_FULL;
 	name->aname = n;
-	name->refcnt++;
+	atomic_inc(&name->refcnt);
 }
 
 static inline int audit_copy_fcaps(struct audit_names *name,
@@ -2373,7 +2373,7 @@ void __audit_inode(struct filename *name, const struct dentry *dentry,
 		return;
 	if (name) {
 		n->name = name;
-		name->refcnt++;
+		atomic_inc(&name->refcnt);
 	}
 
 out:
@@ -2500,7 +2500,7 @@ void __audit_inode_child(struct inode *parent,
 		if (found_parent) {
 			found_child->name = found_parent->name;
 			found_child->name_len = AUDIT_NAME_FULL;
-			found_child->name->refcnt++;
+			atomic_inc(&found_child->name->refcnt);
 		}
 	}