linux-kernel - Re: [syzbot] WARNING in mntput_no

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Date:   Mon, 5 Apr 2021 18:28:54 +0000
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Christian Brauner <christian.brauner@...ntu.com>
Cc:     Jens Axboe <axboe@...nel.dk>,
        syzbot <syzbot+c88a7030da47945a3cc3@...kaller.appspotmail.com>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        syzkaller-bugs@...glegroups.com, io-uring@...r.kernel.org
Subject: Re: [syzbot] WARNING in mntput_no_expire (2)

On Mon, Apr 05, 2021 at 06:23:49PM +0000, Al Viro wrote:
> On Mon, Apr 05, 2021 at 07:08:01PM +0200, Christian Brauner wrote:
> 
> > Ah dentry count of -127 looks... odd.
> 
> dead + 1...
> 
> void lockref_mark_dead(struct lockref *lockref)
> {
>         assert_spin_locked(&lockref->lock);
> 	lockref->count = -128;
> }
> 
> IOW, a leaked (uncounted) reference to dentry, that got dget() called on
> it after dentry had been freed.
> 
> 	IOW, current->fs->pwd.dentry happens to point to an already freed
> struct dentry here.  Joy...
> 
> 	Could you slap
> 
> spin_lock(&current->fs->lock);
> WARN_ON(d_count(current->fs->pwd.dentry) < 0);
> spin_unlock(&current->fs->lock);
> 
> before and after calls of io_issue_sqe() and see if it triggers?  We definitely
> are seeing buggered dentry refcounting here.

Check if this helps, please.

diff --git a/fs/namei.c b/fs/namei.c
index 216f16e74351..82344f1139ff 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -2289,6 +2289,9 @@ static const char *path_init(struct nameidata *nd, unsigned flags)
 	int error;
 	const char *s = nd->name->name;
 
+	nd->path.mnt = NULL;
+	nd->path.dentry = NULL;
+
 	/* LOOKUP_CACHED requires RCU, ask caller to retry */
 	if ((flags & (LOOKUP_RCU | LOOKUP_CACHED)) == LOOKUP_CACHED)
 		return ERR_PTR(-EAGAIN);
@@ -2322,8 +2325,6 @@ static const char *path_init(struct nameidata *nd, unsigned flags)
 	}
 
 	nd->root.mnt = NULL;
-	nd->path.mnt = NULL;
-	nd->path.dentry = NULL;
 
 	/* Absolute pathname -- fetch the root (LOOKUP_IN_ROOT uses nd->dfd). */
 	if (*s == '/' && !(flags & LOOKUP_IN_ROOT)) {