linux-kernel - Re: [PATCH virt] virt: fix uninit-value in vhost_vsock_dev

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Message-ID: <20240422100010-mutt-send-email-mst@kernel.org>
Date: Mon, 22 Apr 2024 10:20:43 -0400
From: "Michael S. Tsirkin" <mst@...hat.com>
To: Stefan Hajnoczi <stefanha@...hat.com>
Cc: Jeongjun Park <aha310510@...il.com>, jasowang@...hat.com,
	kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
	sgarzare@...hat.com,
	syzbot+6c21aeb59d0e82eb2782@...kaller.appspotmail.com,
	syzkaller-bugs@...glegroups.com, virtualization@...ts.linux.dev,
	Arseny Krasnov <arseny.krasnov@...persky.com>
Subject: Re: [PATCH virt] virt: fix uninit-value in vhost_vsock_dev_open

On Mon, Apr 22, 2024 at 09:00:31AM -0400, Stefan Hajnoczi wrote:
> On Sun, Apr 21, 2024 at 12:06:06PM +0900, Jeongjun Park wrote:
> > static bool vhost_transport_seqpacket_allow(u32 remote_cid)
> > {
> > ....
> > 	vsock = vhost_vsock_get(remote_cid);
> > 
> > 	if (vsock)
> > 		seqpacket_allow = vsock->seqpacket_allow;
> > ....
> > }
> > 
> > I think this is due to reading a previously created uninitialized 
> > vsock->seqpacket_allow inside vhost_transport_seqpacket_allow(), 
> > which is executed by the function pointer present in the if statement.
> 
> CCing Arseny, author of commit ced7b713711f ("vhost/vsock: support
> SEQPACKET for transport").
> 
> Looks like a genuine bug in the commit. vhost_vsock_set_features() sets
> seqpacket_allow to true when the feature is negotiated. The assumption
> is that the field defaults to false.
> 
> The rest of the vhost_vsock.ko code is written to initialize the
> vhost_vsock fields, so you could argue seqpacket_allow should just be
> explicitly initialized to false.
> 
> However, eliminating this class of errors by zeroing seems reasonable in
> this code path. vhost_vsock_dev_open() is not performance-critical.
> 
> Acked-by: Stefan Hajnoczi <stefanha@...hat.com>



But now that it's explained, the bugfix as proposed is incomplete:
userspace can set features twice and the second time will leak
old VIRTIO_VSOCK_F_SEQPACKET bit value.

And I am pretty sure the Fixes tag is wrong.

So I wrote this, but I actually don't have a set for
seqpacket to test this. Arseny could you help test maybe?
Thanks!


commit bcc17a060d93b198d8a17a9b87b593f41337ee28
Author: Michael S. Tsirkin <mst@...hat.com>
Date:   Mon Apr 22 10:03:13 2024 -0400

vhost/vsock: always initialize seqpacket_allow

There are two issues around seqpacket_allow:
1. seqpacket_allow is not initialized when socket is
created. Thus if features are never set, it will be
read uninitialized.
2. if VIRTIO_VSOCK_F_SEQPACKET is set and then cleared,
then seqpacket_allow will not be cleared appropriately
(existing apps I know about don't usually do this but
it's legal and there's no way to be sure no one relies
on this).

To fix:
    - initialize seqpacket_allow after allocation
    - set it unconditionally in set_features

Reported-by: syzbot+6c21aeb59d0e82eb2782@...kaller.appspotmail.com
Reported-by: Jeongjun Park <aha310510@...il.com>
Fixes: ced7b713711f ("vhost/vsock: support SEQPACKET for transport").
Cc: Arseny Krasnov <arseny.krasnov@...persky.com>
Cc: David S. Miller <davem@...emloft.net>
Cc: Stefan Hajnoczi <stefanha@...hat.com>
Signed-off-by: Michael S. Tsirkin <mst@...hat.com>

diff --git a/drivers/vhost/vsock.c b/drivers/vhost/vsock.c
index ec20ecff85c7..bf664ec9341b 100644
--- a/drivers/vhost/vsock.c
+++ b/drivers/vhost/vsock.c
@@ -667,6 +667,7 @@ static int vhost_vsock_dev_open(struct inode *inode, struct file *file)
 	}
 
 	vsock->guest_cid = 0; /* no CID assigned yet */
+	vsock->seqpacket_allow = false;
 
 	atomic_set(&vsock->queued_replies, 0);
 
@@ -810,8 +811,7 @@ static int vhost_vsock_set_features(struct vhost_vsock *vsock, u64 features)
 			goto err;
 	}
 
-	if (features & (1ULL << VIRTIO_VSOCK_F_SEQPACKET))
-		vsock->seqpacket_allow = true;
+	vsock->seqpacket_allow = features & (1ULL << VIRTIO_VSOCK_F_SEQPACKET);
 
 	for (i = 0; i < ARRAY_SIZE(vsock->vqs); i++) {
 		vq = &vsock->vqs[i];