Re: [Xen-devel] [PATCH v3] vpci: honor read-only devices

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Tue, Sep 03, 2019 at 02:16:52PM +0200, Jan Beulich wrote:
> On 03.09.2019 12:14, Roger Pau Monne wrote:
> > Don't allow the hardware domain write access the PCI config space of
> > devices marked as read-only.
> > 
> > Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> > ---
> > Changes since v2:
> >  - Fix test harness.
> >  - Do the RO check before the ownership one.
> > 
> > Changes since v1:
> >  - Change the approach and allow full read access, while limiting
> >    write access to devices marked RO.
> > ---
> >  tools/tests/vpci/emul.h | 3 +++
> >  xen/drivers/vpci/vpci.c | 5 +++++
> >  2 files changed, 8 insertions(+)
> > 
> > diff --git a/tools/tests/vpci/emul.h b/tools/tests/vpci/emul.h
> > index 5d47544bf7..2e1d3057c9 100644
> > --- a/tools/tests/vpci/emul.h
> > +++ b/tools/tests/vpci/emul.h
> > @@ -92,6 +92,9 @@ typedef union {
> >  #define xfree(p) free(p)
> >  
> >  #define pci_get_pdev_by_domain(...) &test_pdev
> > +#define pci_get_ro_map(...) NULL
> > +
> > +#define test_bit(...) false
> 
> The latter seems rather dangerous to me, as a further addition of
> test_bit() would silently build fine, but possibly produce a non-
> working binary. But you're the defacto maintainer of this
> harness, so if you believe it's fine so be it. (If even
> xenpaging is considered "fine" to include xc_bitops.h, I wonder
> if this harness couldn't do so too. And then there are three
> test_bit() definitions overall under tools/ - I wonder if those
> couldn't be consolidated into a single, universally usable one.)

One option would be to turn test_bit into assert(0) which should work
for the current usage, since test_bit shouldn't be called given the
current code and will trigger if it actually gets used. Would you be
fine with merging the chunk below into the current patch?

I would like to avoid including xc_bitops.h, since the xenpaging
Makefile already contains a comment regarding the wrong usage of libxc
internals.

> > --- a/xen/drivers/vpci/vpci.c
> > +++ b/xen/drivers/vpci/vpci.c
> > @@ -411,6 +411,7 @@ void vpci_write(pci_sbdf_t sbdf, unsigned int reg, 
> > unsigned int size,
> >      const struct pci_dev *pdev;
> >      const struct vpci_register *r;
> >      unsigned int data_offset = 0;
> > +    const unsigned long *ro_map = pci_get_ro_map(sbdf.seg);
> >  
> >      if ( !size )
> >      {
> > @@ -418,6 +419,10 @@ void vpci_write(pci_sbdf_t sbdf, unsigned int reg, 
> > unsigned int size,
> >          return;
> >      }
> >  
> > +    if ( ro_map && test_bit(sbdf.bdf, ro_map) )
> > +        /* Ignore writes to read-only devices. */
> > +        return;
> > +
> >      /*
> >       * Find the PCI dev matching the address.
> >       * Passthrough everything that's not trapped.
> > 
> 
> This part
> Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>

Thanks.

---8<---
diff --git a/tools/tests/vpci/emul.h b/tools/tests/vpci/emul.h
index 2e1d3057c9..796797fdc2 100644
--- a/tools/tests/vpci/emul.h
+++ b/tools/tests/vpci/emul.h
@@ -94,7 +94,7 @@ typedef union {
 #define pci_get_pdev_by_domain(...) &test_pdev
 #define pci_get_ro_map(...) NULL
 
-#define test_bit(...) false
+#define test_bit(...) ({ assert(0); false; })
 
 /* Dummy native helpers. Writes are ignored, reads return 1's. */
 #define pci_conf_read8(...)     0xff

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