Re: [syzbot] [fs?] KASAN: slab-use-after-free Read in lockref_get_not_dead (2)

[Hillf Danton]

On Fri, 06 Sep 2024 01:40:23 -0700
> syzbot found the following issue on:
> 
> HEAD commit:    9b0874286768 Merge branch 'mctp-serial-tx-escapes'
> git tree:       net
> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=1627ab2b980000

#syz test

--- x/fs/libfs.c
+++ y/fs/libfs.c
@@ -2234,6 +2234,7 @@ int path_from_stashed(struct dentry **st
 	path->dentry = stash_dentry(stashed, dentry);
 	if (path->dentry != dentry)
 		dput(dentry);
+	dget(dentry);
 
 out_path:
 	WARN_ON_ONCE(path->dentry->d_fsdata != stashed);
@@ -2258,5 +2259,6 @@ void stashed_dentry_prune(struct dentry
 	 * already cleared out @dentry and stashed their own
 	 * dentry in there.
 	 */
-	cmpxchg(stashed, dentry, NULL);
+	if (dentry == cmpxchg(stashed, dentry, NULL))
+		dput(dentry);
 }
--