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 10:46, Philippe Mathieu-Daudé <philmd@linaro.org> wrote:
>
> On 28/11/22 16:32, Stefan Hajnoczi wrote:
> > 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.
>
> Or I could use Int128 to do arithmetic, but various other places do it
> this way without checking overflow with memory_region_size(). Such API
> change should be global and is out of the scope of this CVE fix IMO.
>
> > Aside from concerns about -DNDEBUG, which builds without assertions,
>
> This isn't an issue anymore since 262a69f428 ("osdep.h: Prohibit
> disabling assert() in supported builds").

I didn't know about that. Thanks!

Stefan