linux-kernel - [PATCH 5.12 112/700] fuse: Fix crash in fuse_dentry

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Message-Id: <20210712060940.666974643@linuxfoundation.org>
Date:   Mon, 12 Jul 2021 08:03:15 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Greg Kurz <groug@...d.org>,
        Max Reitz <mreitz@...hat.com>,
        Miklos Szeredi <mszeredi@...hat.com>
Subject: [PATCH 5.12 112/700] fuse: Fix crash in fuse_dentry_automount() error path

From: Greg Kurz <groug@...d.org>

commit d92d88f0568e97c437eeb79d9c9609bd8277406f upstream.

If fuse_fill_super_submount() returns an error, the error path
triggers a crash:

[   26.206673] BUG: kernel NULL pointer dereference, address: 0000000000000000
[...]
[   26.226362] RIP: 0010:__list_del_entry_valid+0x25/0x90
[...]
[   26.247938] Call Trace:
[   26.248300]  fuse_mount_remove+0x2c/0x70 [fuse]
[   26.248892]  virtio_kill_sb+0x22/0x160 [virtiofs]
[   26.249487]  deactivate_locked_super+0x36/0xa0
[   26.250077]  fuse_dentry_automount+0x178/0x1a0 [fuse]

The crash happens because fuse_mount_remove() assumes that the FUSE
mount was already added to list under the FUSE connection, but this
only done after fuse_fill_super_submount() has returned success.

This means that until fuse_fill_super_submount() has returned success,
the FUSE mount isn't actually owned by the superblock. We should thus
reclaim ownership by clearing sb->s_fs_info, which will skip the call
to fuse_mount_remove(), and perform rollback, like virtio_fs_get_tree()
already does for the root sb.

Fixes: bf109c64040f ("fuse: implement crossmounts")
Cc: stable@...r.kernel.org # v5.10+
Signed-off-by: Greg Kurz <groug@...d.org>
Reviewed-by: Max Reitz <mreitz@...hat.com>
Signed-off-by: Miklos Szeredi <mszeredi@...hat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

---
 fs/fuse/dir.c |    6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

--- a/fs/fuse/dir.c
+++ b/fs/fuse/dir.c
@@ -339,8 +339,12 @@ static struct vfsmount *fuse_dentry_auto
 
 	/* Initialize superblock, making @mp_fi its root */
 	err = fuse_fill_super_submount(sb, mp_fi);
-	if (err)
+	if (err) {
+		fuse_conn_put(fc);
+		kfree(fm);
+		sb->s_fs_info = NULL;
 		goto out_put_sb;
+	}
 
 	sb->s_flags |= SB_ACTIVE;
 	fsc->root = dget(sb->s_root);