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

["Philippe Mathieu-Daudé" <philmd@linaro.org>]

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.

Thanks,

Phil.