Linux Virtualization - Jan 2017 - [BUG/RFC] vhost: net: big endian viring access despite virtio 1

Hi!

Recently I have been investigating some strange migration problems on
s390x.

It turned out under certain circumstances vhost_net corrupts avail.idx by
using wrong endianness.

I managed to track the problem down (I'm pretty sure). It boils down to
the following.

When stopping vhost userspace (QEMU) calls vhost_net_set_backend with
the fd argument set to -1, this leads to is_le being reset in
vhost_vq_init_access.  On a BE system resetting to legacy means resetting
to BE. Usually this is not a problem, but in the case when oldubufs is not
zero (which is not likely if no network stress applied) it is a problem.
That code path needs to write avail.idx, and ends up using wrong
endianness when doing that (but only on a BE system).

That is the story in prose, now let's see the corresponding code annotated
with some comments.

from drivers/vhost/net.c:
static long vhost_net_set_backend(struct vhost_net *n, unsigned index, int fd)
{
/* [..] some not too interesting stuff  */
        sock = get_socket(fd);
/* fd == -1 --> sock == NULL */
        if (IS_ERR(sock)) {
                r = PTR_ERR(sock);
                goto err_vq;
        }

        /* start polling new socket */
        oldsock = vq->private_data;
        if (sock != oldsock) {
                ubufs = vhost_net_ubuf_alloc(vq,
                                             sock &&
vhost_sock_zcopy(sock));
                if (IS_ERR(ubufs)) {
                        r = PTR_ERR(ubufs);
                        goto err_ubufs;
                }

                vhost_net_disable_vq(n, vq);
==>             vq->private_data = sock; 
/* now vq->private_data is NULL */
==>             r = vhost_vq_init_access(vq);
                if (r)
                        goto err_used;
/* vq endianness has been reset to BE on s390 */
                r = vhost_net_enable_vq(n, vq);
                if (r)
                        goto err_used;

==>             oldubufs = nvq->ubufs;
/* here oldubufs might become != 0 */               
                nvq->ubufs = ubufs;

                n->tx_packets = 0;
                n->tx_zcopy_err = 0;
                n->tx_flush = false;
        }
        mutex_unlock(&vq->mutex);

        if (oldubufs) {
                vhost_net_ubuf_put_wait_and_free(oldubufs);
                mutex_lock(&vq->mutex);
==>             vhost_zerocopy_signal_used(n, vq);
/* tries to update virtqueue structures; endianness is BE on s390
 * now (but should be LE for virtio-1) */
                mutex_unlock(&vq->mutex);
        }
/*[..] rest of the function */
}


from drivers/vhost/vhost.c:

int vhost_vq_init_access(struct vhost_virtqueue *vq)
{
        __virtio16 last_used_idx;
        int r;
        bool is_le = vq->is_le;

        if (!vq->private_data) {
==>             vhost_reset_is_le(vq);
/* resets to native endianness and returns */
                return 0;
        }

==>      vhost_init_is_le(vq);
/* here we init is_le */

        r = vhost_update_used_flags(vq);
        if (r)
                goto err;
        vq->signalled_used_valid = false;
        if (!vq->iotlb &&
            !access_ok(VERIFY_READ, &vq->used->idx, sizeof
vq->used->idx)) {
                r = -EFAULT;
                goto err;
        }
        r = vhost_get_user(vq, last_used_idx, &vq->used->idx);
        if (r) {
                vq_err(vq, "Can't access used idx at %p\n",
                       &vq->used->idx);
                goto err;
        }
        vq->last_used_idx = vhost16_to_cpu(vq, last_used_idx);
        return 0;

err:
        vq->is_le = is_le;
        return r;
}

AFAIU this can be fixed very simply by omitting the reset. Below the
patch. I'm not sure though, the endianness handling ain't simple in
vhost. Am I going in the right direction?

We have a test (on s390x only) running for a couple of hours now and so
far so good but it's a bit early to announce it is tested  for s390x.
If the patch is reasonable, I'm intend to do a version with proper
attribution and stuff.

By the way the reset was first introduced by 
https://lkml.org/lkml/2015/4/10/226 (dug it up in the hope that reasons
for the reset were discussed -- but no enlightenment for me).

Finally I would like to credit Dave Gilbert for hinting that the
unreasonable avail.idx may be due to an endianness problem and Michael A.
Tebolt for reporting the bug and testing.

-------------------------8<--------------
>From b26e2bbdc03832a0204ee2b42967a1b49a277dc8 Mon Sep 17 00:00:00 2001
From: Halil Pasic <pasic at linux.vnet.ibm.com>
Date: Thu, 26 Jan 2017 00:06:15 +0100
Subject: [PATCH] vhost: remove useless/dangerous reset of is_le

The reset of is_le does no good, but it contributes its fair share to a
bug in vhost_net, which occurs if we have some oldubufs when stopping and
setting a fd = -1 as a backend. Instead of doing something convoluted in
vhost_net, let's just get rid of the reset.

Signed-off-by: Halil Pasic <pasic at linux.vnet.ibm.com>
Fixes: commit 2751c9882b94 
---
 drivers/vhost/vhost.c | 4 +---
 1 file changed, 1 insertion(+), 3 deletions(-)

diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
index d643260..08072a2 100644
--- a/drivers/vhost/vhost.c
+++ b/drivers/vhost/vhost.c
@@ -1714,10 +1714,8 @@ int vhost_vq_init_access(struct vhost_virtqueue *vq)
        int r;
        bool is_le = vq->is_le;

-       if (!vq->private_data) {
-               vhost_reset_is_le(vq);
+       if (!vq->private_data)
                return 0;
-       }

        vhost_init_is_le(vq);

-- 
2.8.4

On Thu, Jan 26, 2017 at 06:39:14PM +0100, Halil Pasic
wrote:
> 
> Hi!
> 
> Recently I have been investigating some strange migration problems on
> s390x.
> 
> It turned out under certain circumstances vhost_net corrupts avail.idx by
> using wrong endianness.
> 
> I managed to track the problem down (I'm pretty sure). It boils down to
> the following.
> 
> When stopping vhost userspace (QEMU) calls vhost_net_set_backend with
> the fd argument set to -1, this leads to is_le being reset in
> vhost_vq_init_access.  On a BE system resetting to legacy means resetting
> to BE. Usually this is not a problem, but in the case when oldubufs is not
> zero (which is not likely if no network stress applied) it is a problem.
> That code path needs to write avail.idx, and ends up using wrong
> endianness when doing that (but only on a BE system).
> 
> That is the story in prose, now let's see the corresponding code
annotated
> with some comments.
> 
> from drivers/vhost/net.c:
> static long vhost_net_set_backend(struct vhost_net *n, unsigned index, int
fd)
> {
> /* [..] some not too interesting stuff  */
>         sock = get_socket(fd);
> /* fd == -1 --> sock == NULL */
>         if (IS_ERR(sock)) {
>                 r = PTR_ERR(sock);
>                 goto err_vq;
>         }
> 
>         /* start polling new socket */
>         oldsock = vq->private_data;
>         if (sock != oldsock) {
>                 ubufs = vhost_net_ubuf_alloc(vq,
>                                              sock &&
vhost_sock_zcopy(sock));
>                 if (IS_ERR(ubufs)) {
>                         r = PTR_ERR(ubufs);
>                         goto err_ubufs;
>                 }
> 
>                 vhost_net_disable_vq(n, vq);
> ==>             vq->private_data = sock; 
> /* now vq->private_data is NULL */
> ==>             r = vhost_vq_init_access(vq);
>                 if (r)
>                         goto err_used;
> /* vq endianness has been reset to BE on s390 */
>                 r = vhost_net_enable_vq(n, vq);
>                 if (r)
>                         goto err_used;
> 
> ==>             oldubufs = nvq->ubufs;
> /* here oldubufs might become != 0 */               
>                 nvq->ubufs = ubufs;
> 
>                 n->tx_packets = 0;
>                 n->tx_zcopy_err = 0;
>                 n->tx_flush = false;
>         }
>         mutex_unlock(&vq->mutex);
> 
>         if (oldubufs) {
>                 vhost_net_ubuf_put_wait_and_free(oldubufs);
>                 mutex_lock(&vq->mutex);
> ==>             vhost_zerocopy_signal_used(n, vq);
> /* tries to update virtqueue structures; endianness is BE on s390
>  * now (but should be LE for virtio-1) */
>                 mutex_unlock(&vq->mutex);
>         }
> /*[..] rest of the function */
> }
> 
> from drivers/vhost/vhost.c:
> 
> int vhost_vq_init_access(struct vhost_virtqueue *vq)
> {
>         __virtio16 last_used_idx;
>         int r;
>         bool is_le = vq->is_le;
> 
>         if (!vq->private_data) {
> ==>             vhost_reset_is_le(vq);
> /* resets to native endianness and returns */
>                 return 0;
>         }
> 
> ==>      vhost_init_is_le(vq);
> /* here we init is_le */
> 
>         r = vhost_update_used_flags(vq);
>         if (r)
>                 goto err;
>         vq->signalled_used_valid = false;
>         if (!vq->iotlb &&
>             !access_ok(VERIFY_READ, &vq->used->idx, sizeof
vq->used->idx)) {
>                 r = -EFAULT;
>                 goto err;
>         }
>         r = vhost_get_user(vq, last_used_idx, &vq->used->idx);
>         if (r) {
>                 vq_err(vq, "Can't access used idx at %p\n",
>                        &vq->used->idx);
>                 goto err;
>         }
>         vq->last_used_idx = vhost16_to_cpu(vq, last_used_idx);
>         return 0;
> 
> err:
>         vq->is_le = is_le;
>         return r;
> }
> 
> AFAIU this can be fixed very simply by omitting the reset. Below the
> patch. I'm not sure though, the endianness handling ain't simple in
> vhost. Am I going in the right direction?
> 
> We have a test (on s390x only) running for a couple of hours now and so
> far so good but it's a bit early to announce it is tested  for s390x.
> If the patch is reasonable, I'm intend to do a version with proper
> attribution and stuff.
> 
> By the way the reset was first introduced by 
> https://lkml.org/lkml/2015/4/10/226 (dug it up in the hope that reasons
> for the reset were discussed -- but no enlightenment for me).
> 
> Finally I would like to credit Dave Gilbert for hinting that the
> unreasonable avail.idx may be due to an endianness problem and Michael A.
> Tebolt for reporting the bug and testing.
> 
> -------------------------8<--------------
> >From b26e2bbdc03832a0204ee2b42967a1b49a277dc8 Mon Sep 17 00:00:00 2001
> From: Halil Pasic <pasic at linux.vnet.ibm.com>
> Date: Thu, 26 Jan 2017 00:06:15 +0100
> Subject: [PATCH] vhost: remove useless/dangerous reset of is_le
> 
> The reset of is_le does no good, but it contributes its fair share to a
> bug in vhost_net, which occurs if we have some oldubufs when stopping and
> setting a fd = -1 as a backend. Instead of doing something convoluted in
> vhost_net, let's just get rid of the reset.
> 
> Signed-off-by: Halil Pasic <pasic at linux.vnet.ibm.com>
> Fixes: commit 2751c9882b94 
> ---
>  drivers/vhost/vhost.c | 4 +---
>  1 file changed, 1 insertion(+), 3 deletions(-)
> 
> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> index d643260..08072a2 100644
> --- a/drivers/vhost/vhost.c
> +++ b/drivers/vhost/vhost.c
> @@ -1714,10 +1714,8 @@ int vhost_vq_init_access(struct vhost_virtqueue *vq)
>         int r;
>         bool is_le = vq->is_le;
> 
> -       if (!vq->private_data) {
> -               vhost_reset_is_le(vq);
> +       if (!vq->private_data)
>                 return 0;
> -       }
> 
>         vhost_init_is_le(vq);

I think you do need to reset it, just maybe within vhost_init_is_le.

        if (vhost_has_feature(vq, VIRTIO_F_VERSION_1))
                vq->is_le = true;
	else
		vhost_reset_is_le(vq);


> -- 
> 2.8.4

On 01/26/2017 08:20 PM, Michael S. Tsirkin wrote:
> On Thu, Jan 26, 2017 at 06:39:14PM +0100, Halil Pasic wrote:
>>
>> Hi!
>>
>> Recently I have been investigating some strange migration problems on
>> s390x.
>>
>> It turned out under certain circumstances vhost_net corrupts avail.idx
by
>> using wrong endianness.
[..]
>> -------------------------8<--------------
>> >From b26e2bbdc03832a0204ee2b42967a1b49a277dc8 Mon Sep 17 00:00:00
2001
>> From: Halil Pasic <pasic at linux.vnet.ibm.com>
>> Date: Thu, 26 Jan 2017 00:06:15 +0100
>> Subject: [PATCH] vhost: remove useless/dangerous reset of is_le
>>
>> The reset of is_le does no good, but it contributes its fair share to a
>> bug in vhost_net, which occurs if we have some oldubufs when stopping
and
>> setting a fd = -1 as a backend. Instead of doing something convoluted
in
>> vhost_net, let's just get rid of the reset.
>>
>> Signed-off-by: Halil Pasic <pasic at linux.vnet.ibm.com>
>> Fixes: commit 2751c9882b94 
>> ---
>>  drivers/vhost/vhost.c | 4 +---
>>  1 file changed, 1 insertion(+), 3 deletions(-)
>>
>> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
>> index d643260..08072a2 100644
>> --- a/drivers/vhost/vhost.c
>> +++ b/drivers/vhost/vhost.c
>> @@ -1714,10 +1714,8 @@ int vhost_vq_init_access(struct vhost_virtqueue
*vq)
>>         int r;
>>         bool is_le = vq->is_le;
>>
>> -       if (!vq->private_data) {
>> -               vhost_reset_is_le(vq);
>> +       if (!vq->private_data)
>>                 return 0;
>> -       }
>>
>>         vhost_init_is_le(vq);
> 
> 
> I think you do need to reset it, just maybe within vhost_init_is_le.
> 
>         if (vhost_has_feature(vq, VIRTIO_F_VERSION_1))
>                 vq->is_le = true;
> 	else
> 		vhost_reset_is_le(vq);
> 
> 
That is a very good point! I have overlooked that while the 
CONFIG_VHOST_CROSS_ENDIAN_LEGACY variant

static void vhost_init_is_le(struct vhost_virtqueue *vq)
{
        /* Note for legacy virtio: user_be is initialized at reset time
         * according to the host endianness. If userspace does not set an
         * explicit endianness, the default behavior is native endian, as
         * expected by legacy virtio.
         */
        vq->is_le = vhost_has_feature(vq, VIRTIO_F_VERSION_1) ||
!vq->user_be;
}

is fine the other variant 

static void vhost_init_is_le(struct vhost_virtqueue *vq)
{
        if (vhost_has_feature(vq, VIRTIO_F_VERSION_1))
                vq->is_le = true;
}
is a very strange initializer (makes assumptions about the state
to be initialized).

I agree, setting native endianness there sounds very reasonable.

I have a question regarding readability. IMHO the relationship
of reset_is_le and int_is_le is a bit confusing, and I'm afraid
it could become even more confusing with using reset in one of
the init_is_le's.

How about we do the following?

static void vhost_init_is_le(struct vhost_virtqueue *vq)
{
        if (vhost_has_feature(vq, VIRTIO_F_VERSION_1))
                vq->is_le = true;
+       else
+               vq->is_le = virtio_legacy_is_little_endian();

}

static void vhost_reset_is_le(struct vhost_virtqueue *vq)
{
-       vq->is_le = virtio_legacy_is_little_endian();
+       vhost_init_is_le(vq);
}

That way we would have correct endianness both after reset
and after init, I think :).

Thank you very much!

Halil