Date:   Thu, 14 Oct 2021 10:30:33 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     virtualization <virtualization@...ts.linux-foundation.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        "Hetzelt, Felicitas" <f.hetzelt@...berlin.de>,
        "kaplan, david" <david.kaplan@....com>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
Subject: Re: [PATCH V2 09/12] virtio_ring: validate used buffer length

On Wed, Oct 13, 2021 at 6:02 PM Michael S. Tsirkin <mst@...hat.com> wrote:
>
> On Tue, Oct 12, 2021 at 02:52:24PM +0800, Jason Wang wrote:
> > This patch validate the used buffer length provided by the device
> > before trying to use it. This is done by record the in buffer length
> > in a new field in desc_state structure during virtqueue_add(), then we
> > can fail the virtqueue_get_buf() when we find the device is trying to
> > give us a used buffer length which is greater than the in buffer
> > length.
> >
> > Since some drivers have already done the validation by itself, this
>
> by themselves
>
> > patch tries to makes the core validation optional. For the driver that
> > doesn't want the validation, it can set the validate_used to be
> > true (which could be overridden by force_used_validation). To be more
> > efficient, a dedicate array is used for storing the validate used
> > length, this helps to eliminate the cache stress if validation is done
> > by the driver.
> >
> > Signed-off-by: Jason Wang <jasowang@...hat.com>
> > ---
> >  drivers/virtio/virtio_ring.c | 56 ++++++++++++++++++++++++++++++++++++
> >  include/linux/virtio.h       |  2 ++
> >  2 files changed, 58 insertions(+)
> >
> > diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> > index d2ca0a7365f8..dee962bd8b04 100644
> > --- a/drivers/virtio/virtio_ring.c
> > +++ b/drivers/virtio/virtio_ring.c
> > @@ -14,6 +14,9 @@
> >  #include <linux/spinlock.h>
> >  #include <xen/xen.h>
> >
> > +static bool force_used_validation = false;
> > +module_param(force_used_validation, bool, 0444);
> > +
> >  #ifdef DEBUG
> >  /* For development, we want to crash whenever the ring is screwed. */
> >  #define BAD_RING(_vq, fmt, args...)                          \
> > @@ -182,6 +185,9 @@ struct vring_virtqueue {
> >               } packed;
> >       };
> >
> > +     /* Per-descriptor in buffer length */
> > +     u32 *buflen;
> > +
> >       /* How to notify other side. FIXME: commonalize hcalls! */
> >       bool (*notify)(struct virtqueue *vq);
> >
> > @@ -490,6 +496,7 @@ static inline int virtqueue_add_split(struct virtqueue *_vq,
> >       unsigned int i, n, avail, descs_used, prev, err_idx;
> >       int head;
> >       bool indirect;
> > +     u32 buflen = 0;
> >
> >       START_USE(vq);
> >
> > @@ -571,6 +578,7 @@ static inline int virtqueue_add_split(struct virtqueue *_vq,
> >                                                    VRING_DESC_F_NEXT |
> >                                                    VRING_DESC_F_WRITE,
> >                                                    indirect);
> > +                     buflen += sg->length;
> >               }
> >       }
> >       /* Last one doesn't continue. */
> > @@ -610,6 +618,10 @@ static inline int virtqueue_add_split(struct virtqueue *_vq,
> >       else
> >               vq->split.desc_state[head].indir_desc = ctx;
> >
> > +     /* Store in buffer length if necessary */
> > +     if (vq->buflen)
> > +             vq->buflen[head] = buflen;
> > +
> >       /* Put entry in available array (but don't update avail->idx until they
> >        * do sync). */
> >       avail = vq->split.avail_idx_shadow & (vq->split.vring.num - 1);
> > @@ -784,6 +796,11 @@ static void *virtqueue_get_buf_ctx_split(struct virtqueue *_vq,
> >               BAD_RING(vq, "id %u is not a head!\n", i);
> >               return NULL;
> >       }
> > +     if (vq->buflen && unlikely(*len > vq->buflen[i])) {
> > +             BAD_RING(vq, "used len %d is larger than in buflen %u\n",
> > +                     *len, vq->buflen[i]);
> > +             return NULL;
> > +     }
> >
> >       /* detach_buf_split clears data, so grab it now. */
> >       ret = vq->split.desc_state[i].data;
> > @@ -1062,6 +1079,7 @@ static int virtqueue_add_indirect_packed(struct vring_virtqueue *vq,
> >       unsigned int i, n, err_idx;
> >       u16 head, id;
> >       dma_addr_t addr;
> > +     u32 buflen = 0;
> >
> >       head = vq->packed.next_avail_idx;
> >       desc = alloc_indirect_packed(total_sg, gfp);
> > @@ -1089,6 +1107,8 @@ static int virtqueue_add_indirect_packed(struct vring_virtqueue *vq,
> >                       desc[i].addr = cpu_to_le64(addr);
> >                       desc[i].len = cpu_to_le32(sg->length);
> >                       i++;
> > +                     if (n >= out_sgs)
> > +                             buflen += sg->length;
> >               }
> >       }
> >
> > @@ -1142,6 +1162,10 @@ static int virtqueue_add_indirect_packed(struct vring_virtqueue *vq,
> >       vq->packed.desc_state[id].indir_desc = desc;
> >       vq->packed.desc_state[id].last = id;
> >
> > +     /* Store in buffer length if necessary */
> > +     if (vq->buflen)
> > +             vq->buflen[id] = buflen;
> > +
> >       vq->num_added += 1;
> >
> >       pr_debug("Added buffer head %i to %p\n", head, vq);
> > @@ -1176,6 +1200,7 @@ static inline int virtqueue_add_packed(struct virtqueue *_vq,
> >       unsigned int i, n, c, descs_used, err_idx;
> >       __le16 head_flags, flags;
> >       u16 head, id, prev, curr, avail_used_flags;
> > +     u32 buflen = 0;
> >
> >       START_USE(vq);
> >
> > @@ -1250,6 +1275,8 @@ static inline int virtqueue_add_packed(struct virtqueue *_vq,
> >                                       1 << VRING_PACKED_DESC_F_AVAIL |
> >                                       1 << VRING_PACKED_DESC_F_USED;
> >                       }
> > +                     if (n >= out_sgs)
> > +                             buflen += sg->length;
> >               }
> >       }
> >
> > @@ -1269,6 +1296,10 @@ static inline int virtqueue_add_packed(struct virtqueue *_vq,
> >       vq->packed.desc_state[id].indir_desc = ctx;
> >       vq->packed.desc_state[id].last = prev;
> >
> > +     /* Store in buffer length if necessary */
> > +     if (vq->buflen)
> > +             vq->buflen[id] = buflen;
> > +
> >       /*
> >        * A driver MUST NOT make the first descriptor in the list
> >        * available before all subsequent descriptors comprising
> > @@ -1455,6 +1486,11 @@ static void *virtqueue_get_buf_ctx_packed(struct virtqueue *_vq,
> >               BAD_RING(vq, "id %u is not a head!\n", id);
> >               return NULL;
> >       }
> > +     if (vq->buflen && unlikely(*len > vq->buflen[id])) {
> > +             BAD_RING(vq, "used len %d is larger than in buflen %u\n",
> > +                     *len, vq->buflen[id]);
> > +             return NULL;
> > +     }
> >
> >       /* detach_buf_packed clears data, so grab it now. */
> >       ret = vq->packed.desc_state[id].data;
> > @@ -1660,6 +1696,7 @@ static struct virtqueue *vring_create_virtqueue_packed(
> >       struct vring_virtqueue *vq;
> >       struct vring_packed_desc *ring;
> >       struct vring_packed_desc_event *driver, *device;
> > +     struct virtio_driver *drv = drv_to_virtio(vdev->dev.driver);
> >       dma_addr_t ring_dma_addr, driver_event_dma_addr, device_event_dma_addr;
> >       size_t ring_size_in_bytes, event_size_in_bytes;
> >
> > @@ -1749,6 +1786,13 @@ static struct virtqueue *vring_create_virtqueue_packed(
> >       if (!vq->packed.desc_extra)
> >               goto err_desc_extra;
> >
> > +     if (!drv->validate_used || force_used_validation) {
> > +             vq->buflen = kmalloc_array(num, sizeof(*vq->buflen),
> > +                                        GFP_KERNEL);
> > +             if (!vq->buflen)
> > +                     goto err_buflen;
> > +     }
> > +
> >       /* No callback?  Tell other side not to bother us. */
> >       if (!callback) {
> >               vq->packed.event_flags_shadow = VRING_PACKED_EVENT_FLAG_DISABLE;
> > @@ -1761,6 +1805,8 @@ static struct virtqueue *vring_create_virtqueue_packed(
> >       spin_unlock(&vdev->vqs_list_lock);
> >       return &vq->vq;
> >
> > +err_buflen:
> > +     kfree(vq->packed.desc_extra);
> >  err_desc_extra:
> >       kfree(vq->packed.desc_state);
> >  err_desc_state:
> > @@ -2168,6 +2214,7 @@ struct virtqueue *__vring_new_virtqueue(unsigned int index,
> >                                       void (*callback)(struct virtqueue *),
> >                                       const char *name)
> >  {
> > +     struct virtio_driver *drv = drv_to_virtio(vdev->dev.driver);
> >       struct vring_virtqueue *vq;
> >
> >       if (virtio_has_feature(vdev, VIRTIO_F_RING_PACKED))
> > @@ -2227,6 +2274,13 @@ struct virtqueue *__vring_new_virtqueue(unsigned int index,
> >       if (!vq->split.desc_extra)
> >               goto err_extra;
> >
> > +     if (!drv->validate_used || force_used_validation) {
> > +             vq->buflen = kmalloc_array(vring.num, sizeof(*vq->buflen),
> > +                                        GFP_KERNEL);
> > +             if (!vq->buflen)
> > +                     goto err_buflen;
> > +     }
> > +
> >       /* Put everything in free lists. */
> >       vq->free_head = 0;
> >       memset(vq->split.desc_state, 0, vring.num *
> > @@ -2237,6 +2291,8 @@ struct virtqueue *__vring_new_virtqueue(unsigned int index,
> >       spin_unlock(&vdev->vqs_list_lock);
> >       return &vq->vq;
> >
> > +err_buflen:
> > +     kfree(vq->split.desc_extra);
> >  err_extra:
> >       kfree(vq->split.desc_state);
> >  err_state:
> > diff --git a/include/linux/virtio.h b/include/linux/virtio.h
> > index 0cd8685aeba4..79e4f6765e3a 100644
> > --- a/include/linux/virtio.h
> > +++ b/include/linux/virtio.h
> > @@ -153,6 +153,7 @@ size_t virtio_max_dma_size(struct virtio_device *vdev);
> >   * @feature_table_legacy: same as feature_table but when working in legacy mode.
> >   * @feature_table_size_legacy: number of entries in feature table legacy array.
> >   * @validate: optional function to do early checks
> > + * @validate_used: whether the driver can validate used buffer length
>
> So you set it to true to *not* have core validate used. Ugh.
> I understand completely why it's like this: to have the default false
> be safe. But let's call this out.
>
>         @suppress_used_validation: set to not have core validate used length

Ok.

Thanks

>
>
> >   * @probe: the function to call when a device is found.  Returns 0 or -errno.
> >   * @scan: optional function to call after successful probe; intended
> >   *    for virtio-scsi to invoke a scan.
> > @@ -169,6 +170,7 @@ struct virtio_driver {
> >       unsigned int feature_table_size;
> >       const unsigned int *feature_table_legacy;
> >       unsigned int feature_table_size_legacy;
> > +     bool validate_used;
> >       int (*validate)(struct virtio_device *dev);
> >       int (*probe)(struct virtio_device *dev);
> >       void (*scan)(struct virtio_device *dev);
> > --
> > 2.25.1
>

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