Re: [PATCH RESEND 1/3] vfio/pci: fix a null pointer reference in vfio_rom_read

[Alex Williamson <alex.williamson@redhat.com>]

On Thu, 12 Mar 2020 06:50:30 +0100
Markus Armbruster <armbru@redhat.com> wrote:

> Alex Williamson <alex.williamson@redhat.com> writes:
> 
> > On Wed, 11 Mar 2020 08:04:28 +0100
> > Markus Armbruster <armbru@redhat.com> wrote:
> >  
> >> Alex Williamson <alex.williamson@redhat.com> writes:
> >>   
> >> > On Mon, 24 Feb 2020 14:42:17 +0800
> >> > "Longpeng(Mike)" <longpeng2@huawei.com> wrote:
> >> >    
> >> >> From: Longpeng <longpeng2@huawei.com>
> >> >> 
> >> >> vfio_pci_load_rom() maybe failed and then the vdev->rom is NULL in
> >> >> some situation (though I've not encountered yet), maybe we should
> >> >> avoid the VM abort.    
> >> 
> >> What "VM abort" exactly?  
> >
> > There is none because memcpy() does something sane when size is zero,
> > but to be ISO whatever spec compliant we shouldn't rely on that.
> >  
> >> >> 
> >> >> Signed-off-by: Longpeng <longpeng2@huawei.com>
> >> >> ---
> >> >>  hw/vfio/pci.c | 13 ++++++++-----
> >> >>  1 file changed, 8 insertions(+), 5 deletions(-)
> >> >> 
> >> >> diff --git a/hw/vfio/pci.c b/hw/vfio/pci.c
> >> >> index 5e75a95..ed798ae 100644
> >> >> --- a/hw/vfio/pci.c
> >> >> +++ b/hw/vfio/pci.c
> >> >> @@ -768,7 +768,7 @@ static void vfio_update_msi(VFIOPCIDevice *vdev)
> >> >>      }
> >> >>  }
> >> >>  
> >> >> -static void vfio_pci_load_rom(VFIOPCIDevice *vdev)
> >> >> +static bool vfio_pci_load_rom(VFIOPCIDevice *vdev)
> >> >>  {
> >> >>      struct vfio_region_info *reg_info;
> >> >>      uint64_t size;
> >> >> @@ -778,7 +778,7 @@ static void vfio_pci_load_rom(VFIOPCIDevice *vdev)
> >> >>      if (vfio_get_region_info(&vdev->vbasedev,
> >> >>                               VFIO_PCI_ROM_REGION_INDEX, &reg_info)) {
> >> >>          error_report("vfio: Error getting ROM info: %m");
> >> >> -        return;
> >> >> +        return false;
> >> >>      }
> >> >>  
> >> >>      trace_vfio_pci_load_rom(vdev->vbasedev.name, (unsigned long)reg_info->size,
> >> >> @@ -797,7 +797,7 @@ static void vfio_pci_load_rom(VFIOPCIDevice *vdev)
> >> >>          error_printf("Device option ROM contents are probably invalid "
> >> >>                      "(check dmesg).\nSkip option ROM probe with rombar=0, "
> >> >>                      "or load from file with romfile=\n");
> >> >> -        return;
> >> >> +        return false;
> >> >>      }
> >> >>  
> >> >>      vdev->rom = g_malloc(size);
> >> >> @@ -849,6 +849,8 @@ static void vfio_pci_load_rom(VFIOPCIDevice *vdev)
> >> >>              data[6] = -csum;
> >> >>          }
> >> >>      }
> >> >> +
> >> >> +    return true;
> >> >>  }
> >> >>  
> >> >>  static uint64_t vfio_rom_read(void *opaque, hwaddr addr, unsigned size)
> >> >> @@ -863,8 +865,9 @@ static uint64_t vfio_rom_read(void *opaque, hwaddr addr, unsigned size)    
> >>     {
> >>         VFIOPCIDevice *vdev = opaque;
> >>         union {
> >>             uint8_t byte;
> >>             uint16_t word;
> >>             uint32_t dword;
> >>             uint64_t qword;
> >>         } val;  
> >> >>      uint64_t data = 0;
> >> >>  
> >> >>      /* Load the ROM lazily when the guest tries to read it */
> >> >> -    if (unlikely(!vdev->rom && !vdev->rom_read_failed)) {
> >> >> -        vfio_pci_load_rom(vdev);
> >> >> +    if (unlikely(!vdev->rom && !vdev->rom_read_failed) &&
> >> >> +        !vfio_pci_load_rom(vdev)) {
> >> >> +        return 0;
> >> >>      }
> >> >>  
> >> >>      memcpy(&val, vdev->rom + addr,    
> >> >
> >> > Looks like an obvious bug, until you look at the rest of this memcpy():
> >> >
> >> > memcpy(&val, vdev->rom + addr,
> >> >            (addr < vdev->rom_size) ? MIN(size, vdev->rom_size - addr) : 0);
> >> >
> >> > IOW, we'll do a zero sized memcpy() if rom_size is zero, so there's no
> >> > risk of the concern identified in the commit log.  This patch is
> >> > unnecessary.  Thanks,    
> >> 
> >> I'm blind: why does !vdev->rom imply !vdev->rom_size?  
> >
> > See vfio_pci_load_rom(), rom_size and rom are set and allocated
> > together.  
> 
> What if vfio_pci_load_rom() isn't called, or returns before it sets
> these guys?

vfio_pci_load_rom() is called from this read function if they're not
set and we haven't triggered a read failure.  They're zero allocated.
The intention is that any vfio_pci_load_rom() failure will leave these
unset and trigger a zero sized memcpy, or now skip the mempcy as in the
patch below.  Thanks,

Alex

> >> Moreover, when MIN(size, vdev->rom_size - addr) < size, we seem to read
> >> uninitialized data from @val:  
> >
> > This is fixed in my patch
> > https://lists.gnu.org/archive/html/qemu-devel/2020-03/msg02778.html  
> 
> Yes.
> 
> >> 
> >>         switch (size) {
> >>         case 1:
> >>             data = val.byte;
> >>             break;
> >>         case 2:
> >>             data = le16_to_cpu(val.word);
> >>             break;
> >>         case 4:
> >>             data = le32_to_cpu(val.dword);
> >>             break;
> >>         default:
> >>             hw_error("vfio: unsupported read size, %d bytes\n", size);
> >>             break;
> >>         }
> >> 
> >>         trace_vfio_rom_read(vdev->vbasedev.name, addr, size, data);
> >> 
> >>         return data;
> >>     }
> >> 
> >> Why is that okay?
> >> 
> >> Why do we initialize @data?  
> >
> > Bug.  The switch was only added later (75bd0c7253f3) and we failed to
> > catch it.  Prior to that we were initializing val and the memcpy() only
> > overwrote it as necessary.  In any case, getting back garbage for the
> > rom when there isn't one generally works ok since the chances of
> > generating a proper rom signature are infinitesimal.  Clearly not what
> > was intended though.
> >  
> >> How can we get to the default case?  If we can get there, is hw_error()
> >> really the right thing to do?  It almost never is...  If getting there
> >> is the guest's fault, we need to tell it off the same way physical
> >> hardware does.  If we should not ever get there (i.e. it's a QEMU bug),
> >> then a plain abort() would be clearer.  
> >
> > AFAIK this is relatively standard, if not somewhat paranoid, handling
> > for a MemoryRegion ops callback.  The MemoryRegionOps code only allows
> > certain size accesses, so it would effectively be an internal error to
> > hit the default case, which seems to be not an uncommon use case of
> > hw_error.  Thanks,  
> 
> Using hw_error() for such programming errors is not helpful.  Everything
> it adds to abort() is useless or misleading.
> 
> In fact, most uses of hw_error() are not helpful.
> 
> But you're going with the flow here.  I accept that.