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

[Markus Armbruster]

Alex Williamson <address@hidden> writes:

> On Wed, 11 Mar 2020 08:04:28 +0100
> Markus Armbruster <address@hidden> wrote:
>
>> Alex Williamson <address@hidden> writes:
>> 
>> > On Mon, 24 Feb 2020 14:42:17 +0800
>> > "Longpeng(Mike)" <address@hidden> wrote:
>> >  
>> >> From: Longpeng <address@hidden>
>> >> 
>> >> 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 <address@hidden>
>> >> ---
>> >>  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?

>> 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.