linux-kernel - [PATCH v2 2/7] fuse: Fix crash if superblock of submount gets killed early

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Message-Id: <20210604161156.408496-3-groug@kaod.org>
Date:   Fri,  4 Jun 2021 18:11:51 +0200
From:   Greg Kurz <groug@...d.org>
To:     Miklos Szeredi <miklos@...redi.hu>
Cc:     linux-kernel@...r.kernel.org, Max Reitz <mreitz@...hat.com>,
        linux-fsdevel@...r.kernel.org, virtio-fs@...hat.com,
        Vivek Goyal <vgoyal@...hat.com>, Greg Kurz <groug@...d.org>
Subject: [PATCH v2 2/7] fuse: Fix crash if superblock of submount gets killed early

As soon as fuse_dentry_automount() does up_write(&sb->s_umount), the
superblock can theoretically be killed. If this happens before the
submount was added to the &fc->mounts list, fuse_mount_remove() later
crashes in list_del_init() because it assumes the submount to be
already there.

Add the submount before dropping sb->s_umount to fix the inconsistency.
It is okay to nest fc->killsb under sb->s_umount, we already do this
on the ->kill_sb() path.

Signed-off-by: Greg Kurz <groug@...d.org>
---
 fs/fuse/dir.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/fs/fuse/dir.c b/fs/fuse/dir.c
index 01559061cbfb..3fd1b71e546b 100644
--- a/fs/fuse/dir.c
+++ b/fs/fuse/dir.c
@@ -346,15 +346,15 @@ static struct vfsmount *fuse_dentry_automount(struct path *path)
 		goto out_put_sb;
 	}
 
+	down_write(&fc->killsb);
+	list_add_tail(&fm->fc_entry, &fc->mounts);
+	up_write(&fc->killsb);
+
 	sb->s_flags |= SB_ACTIVE;
 	fsc->root = dget(sb->s_root);
 	/* We are done configuring the superblock, so unlock it */
 	up_write(&sb->s_umount);
 
-	down_write(&fc->killsb);
-	list_add_tail(&fm->fc_entry, &fc->mounts);
-	up_write(&fc->killsb);
-
 	/* Create the submount */
 	mnt = vfs_create_mount(fsc);
 	if (IS_ERR(mnt)) {
-- 
2.31.1