linux-kernel - Re: [PATCH v2] virtio: pci: sanity check bar indexes

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Message-ID: <20220323075030-mutt-send-email-mst@kernel.org>
Date:   Wed, 23 Mar 2022 08:01:42 -0400
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Jason Wang <jasowang@...hat.com>
Cc:     Keir Fraser <keirf@...gle.com>, kernel-team@...roid.com,
        virtualization <virtualization@...ts.linux-foundation.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] virtio: pci: sanity check bar indexes

On Wed, Mar 23, 2022 at 03:57:59PM +0800, Jason Wang wrote:
> On Tue, Mar 22, 2022 at 11:20 PM Keir Fraser <keirf@...gle.com> wrote:
> >
> > The bar index is used as an index into the device's resource list
> > and should be checked as within range for a standard bar.
> >
> > Also clean up an existing check to consistently use PCI_STD_NUM_BARS.
> >
> > Signed-off-by: Keir Fraser <keirf@...gle.com>
> > ---
> >  drivers/virtio/virtio_pci_modern.c     | 10 ++++++++--
> >  drivers/virtio/virtio_pci_modern_dev.c |  8 +++++++-
> >  2 files changed, 15 insertions(+), 3 deletions(-)
> >
> > diff --git a/drivers/virtio/virtio_pci_modern.c b/drivers/virtio/virtio_pci_modern.c
> > index 5455bc041fb6..84bace98dff5 100644
> > --- a/drivers/virtio/virtio_pci_modern.c
> > +++ b/drivers/virtio/virtio_pci_modern.c
> > @@ -293,7 +293,7 @@ static int virtio_pci_find_shm_cap(struct pci_dev *dev, u8 required_id,
> >
> >         for (pos = pci_find_capability(dev, PCI_CAP_ID_VNDR); pos > 0;
> >              pos = pci_find_next_capability(dev, pos, PCI_CAP_ID_VNDR)) {
> > -               u8 type, cap_len, id;
> > +               u8 type, cap_len, id, res_bar;
> >                 u32 tmp32;
> >                 u64 res_offset, res_length;
> >
> > @@ -317,7 +317,12 @@ static int virtio_pci_find_shm_cap(struct pci_dev *dev, u8 required_id,
> >
> >                 /* Type, and ID match, looks good */
> >                 pci_read_config_byte(dev, pos + offsetof(struct virtio_pci_cap,
> > -                                                        bar), bar);
> > +                                                        bar), &res_bar);
> > +               if (res_bar >= PCI_STD_NUM_BARS) {
> > +                       dev_err(&dev->dev, "%s: shm cap with bad bar: %d\n",
> > +                               __func__, res_bar);
> > +                       continue;
> > +               }
> >
> >                 /* Read the lower 32bit of length and offset */
> >                 pci_read_config_dword(dev, pos + offsetof(struct virtio_pci_cap,

In fact, the spec says such BAR values are reserved, not bad, so
the capabiluty should be ignored, they should not cause the driver to error out
or print errors.

> > @@ -337,6 +342,7 @@ static int virtio_pci_find_shm_cap(struct pci_dev *dev, u8 required_id,
> >                                                      length_hi), &tmp32);
> >                 res_length |= ((u64)tmp32) << 32;
> >
> > +               *bar = res_bar;
> >                 *offset = res_offset;
> >                 *len = res_length;
> >
> > diff --git a/drivers/virtio/virtio_pci_modern_dev.c b/drivers/virtio/virtio_pci_modern_dev.c
> > index e8b3ff2b9fbc..a6911d1e212a 100644
> > --- a/drivers/virtio/virtio_pci_modern_dev.c
> > +++ b/drivers/virtio/virtio_pci_modern_dev.c
> > @@ -35,6 +35,12 @@ vp_modern_map_capability(struct virtio_pci_modern_device *mdev, int off,
> >         pci_read_config_dword(dev, off + offsetof(struct virtio_pci_cap, length),
> >                               &length);
> >
> > +       if (bar >= PCI_STD_NUM_BARS) {
> > +               dev_err(&dev->dev,
> > +                       "virtio_pci: bad capability bar %u\n", bar);

In fact, I would say the issue is less that bar is reserved.
The real issue is that the value apparently changed since
we read it the first time. I think it's a good idea to
reflect that in the message. Maybe find_capability should return
the capability structure so we don't need to re-read it from
the device?

> > +               return NULL;
> > +       }
> > +
> >         if (length <= start) {
> >                 dev_err(&dev->dev,
> >                         "virtio_pci: bad capability len %u (>%u expected)\n",
> > @@ -120,7 +126,7 @@ static inline int virtio_pci_find_capability(struct pci_dev *dev, u8 cfg_type,
> >                                      &bar);
> >
> >                 /* Ignore structures with reserved BAR values */
> > -               if (bar > 0x5)
> > +               if (bar >= PCI_STD_NUM_BARS)
> >                         continue;
> 
> Just notice that the spec said:
> 
> "
> values 0x0 to 0x5 specify a Base Address register (BAR) belonging to
> the function located beginning at 10h in PCI Configuration Space and
> used to map the structure into Memory or I/O Space. The BAR is
> permitted to be either 32-bit or 64-bit, it can map Memory Space or
> I/O Space.
> 
> Any other value is reserved for future use.
> "
> So we probably need to stick 0x5 instead of 0x6 (PCI_STD_NUM_BARS) for
> this and other places.
> 
> Thanks

It does not matter much IMHO, the reason spec uses 0 to 0x5 is precisely
because that's the standard number of BARs. Both ways work as long as we
are consistent, and I guess PCI_STD_NUM_BARS might be preferable since
people tend to copy paste values.


> >
> >                 if (type == cfg_type) {
> > --
> > 2.35.1.894.gb6a874cedc-goog
> >