Re: [RFC PATCH-for-7.2 4/5] hw/display/qxl: Avoid buffer overrun in qxl_phys2virt (CVE-2022-4144)

[Stefan Hajnoczi]

On Mon, 28 Nov 2022 at 10:25, Philippe Mathieu-Daudé <philmd@linaro.org> wrote:
>
> On 28/11/22 16:16, Stefan Hajnoczi wrote:
> > On Mon, 28 Nov 2022 at 08:53, Philippe Mathieu-Daudé <philmd@linaro.org> 
> > wrote:
> >>
> >> Have qxl_get_check_slot_offset() return false if the requested
> >> buffer size does not fit within the slot memory region.
> >>
> >> Similarly qxl_phys2virt() now returns NULL in such case, and
> >> qxl_dirty_one_surface() aborts.
> >>
> >> This avoids buffer overrun in the host pointer returned by
> >> memory_region_get_ram_ptr().
> >>
> >> Fixes: CVE-2022-4144 (out-of-bounds read)
> >> Reported-by: Wenxu Yin (@awxylitol)
> >> Resolves: https://gitlab.com/qemu-project/qemu/-/issues/1336
> >> Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
> >> ---
> >>   hw/display/qxl.c | 22 ++++++++++++++++++----
> >>   hw/display/qxl.h |  2 +-
> >>   2 files changed, 19 insertions(+), 5 deletions(-)
> >>
> >> diff --git a/hw/display/qxl.c b/hw/display/qxl.c
> >> index 231d733250..afa157d327 100644
> >> --- a/hw/display/qxl.c
> >> +++ b/hw/display/qxl.c
> >> @@ -1424,11 +1424,13 @@ static void qxl_reset_surfaces(PCIQXLDevice *d)
> >>
> >>   /* can be also called from spice server thread context */
> >>   static bool qxl_get_check_slot_offset(PCIQXLDevice *qxl, QXLPHYSICAL 
> >> pqxl,
> >> -                                      uint32_t *s, uint64_t *o)
> >> +                                      uint32_t *s, uint64_t *o,
> >> +                                      size_t size_requested)
> >>   {
> >>       uint64_t phys   = le64_to_cpu(pqxl);
> >>       uint32_t slot   = (phys >> (64 -  8)) & 0xff;
> >>       uint64_t offset = phys & 0xffffffffffff;
> >> +    uint64_t size_available;
> >>
> >>       if (slot >= NUM_MEMSLOTS) {
> >>           qxl_set_guest_bug(qxl, "slot too large %d >= %d", slot,
> >> @@ -1453,6 +1455,18 @@ static bool qxl_get_check_slot_offset(PCIQXLDevice 
> >> *qxl, QXLPHYSICAL pqxl,
> >>           return false;
> >>       }
> >>
> >> +    size_available = memory_region_size(qxl->guest_slots[slot].mr);
> >> +    assert(qxl->guest_slots[slot].offset + offset < size_available);
> >
> > Can this assertion be triggered by the guest (via an invalid pqxl
> > value)? I think the answer is no, but I don't know the the qxl code
> > well enough to be sure.
>
> 'qxl->guest_slots[slot].offset' is initialized in qxl_add_memslot()
> (host); 'size_available' also comes from the host, but 'offset'
> comes from the guest via 'QXLPHYSICAL pqxl' IIUC.
>
> I added this check to avoid overflow, but it can be changed to return
> an error.

Yes, please.

Aside from concerns about -DNDEBUG, which builds without assertions,
there is also a DoS issue with nested virt where an L2 guest shouldn't
be able to abort the L1 guest's QEMU by triggering an assertion in a
pass through device.

Guest input validation should use explicit error checking code instead
of assert(3).

Thanks,
Stefan