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From: Greg Kurz <groug@kaod.org>
To: Miklos Szeredi <miklos@szeredi.hu>
Cc: linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org,
	Vivek Goyal <vgoyal@redhat.com>,
	virtio-fs@redhat.com, Greg Kurz <groug@kaod.org>,
	mreitz@redhat.com, stable@vger.kernel.org
Subject: [PATCH 1/4] fuse: Fix crash in fuse_dentry_automount() error path
Date: Tue, 25 May 2021 17:02:27 +0200	[thread overview]
Message-ID: <20210525150230.157586-2-groug@kaod.org> (raw)
In-Reply-To: <20210525150230.157586-1-groug@kaod.org>

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: mreitz@redhat.com
Cc: stable@vger.kernel.org # v5.10+
Signed-off-by: Greg Kurz <groug@kaod.org>
---
 fs/fuse/dir.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/fs/fuse/dir.c b/fs/fuse/dir.c
index 1b6c001a7dd1..01559061cbfb 100644
--- a/fs/fuse/dir.c
+++ b/fs/fuse/dir.c
@@ -339,8 +339,12 @@ static struct vfsmount *fuse_dentry_automount(struct path *path)
 
 	/* 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);
-- 
2.31.1


  reply	other threads:[~2021-05-25 15:02 UTC|newest]

Thread overview: 11+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-05-25 15:02 [PATCH 0/4] fuse: Some fixes for submounts Greg Kurz
2021-05-25 15:02 ` Greg Kurz [this message]
2021-05-27  9:51   ` [Virtio-fs] [PATCH 1/4] fuse: Fix crash in fuse_dentry_automount() error path Max Reitz
2021-05-25 15:02 ` [PATCH 2/4] fuse: Fix infinite loop in sget_fc() Greg Kurz
2021-05-27 10:08   ` [Virtio-fs] " Max Reitz
2021-05-27 12:31     ` Greg Kurz
2021-05-25 15:02 ` [PATCH 3/4] fuse: Call vfs_get_tree() for submounts Greg Kurz
2021-05-27 13:24   ` [Virtio-fs] " Max Reitz
2021-06-03  7:34     ` Greg Kurz
2021-05-25 15:02 ` [PATCH 4/4] fuse: Make fuse_fill_super_submount() static Greg Kurz
2021-05-27 13:28   ` [Virtio-fs] " Max Reitz

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