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

[Marc-André Lureau]

Hi

On Fri, Nov 25, 2022 at 9:35 PM Philippe Mathieu-Daudé
<philmd@linaro.org> wrote:
>
> Return NULL if the requested buffer size does not fit
> within the slot memory region.
>
> 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 | 11 ++++++++++-
>  hw/display/qxl.h |  2 +-
>  2 files changed, 11 insertions(+), 2 deletions(-)
>
> diff --git a/hw/display/qxl.c b/hw/display/qxl.c
> index 231d733250..e5e162f82d 100644
> --- a/hw/display/qxl.c
> +++ b/hw/display/qxl.c
> @@ -1462,7 +1462,7 @@ static bool qxl_get_check_slot_offset(PCIQXLDevice 
> *qxl, QXLPHYSICAL pqxl,
>  void *qxl_phys2virt(PCIQXLDevice *qxl, QXLPHYSICAL pqxl, int group_id,
>                      size_t size)
>  {
> -    uint64_t offset;
> +    uint64_t offset, ptr_end_offset;
>      uint32_t slot;
>      void *ptr;
>
> @@ -1474,6 +1474,15 @@ void *qxl_phys2virt(PCIQXLDevice *qxl, QXLPHYSICAL 
> pqxl, int group_id,
>          if (!qxl_get_check_slot_offset(qxl, pqxl, &slot, &offset)) {
>              return NULL;
>          }
> +
> +        ptr_end_offset = qxl->guest_slots[slot].offset + offset + size;

This is unlikely subject to int overflow, but perhaps it's worth
considering using some int128 instead?

> +        if (ptr_end_offset > memory_region_size(qxl->guest_slots[slot].mr)) {

> +            qxl_set_guest_bug(qxl,
> +                              "slot %d offset %"PRIu64" size %zu: "
> +                              "overrun by %"PRIu64" bytes\n",
> +                              slot, offset, size, ptr_end_offset - offset);
> +            return NULL;
> +        }
>          ptr = memory_region_get_ram_ptr(qxl->guest_slots[slot].mr);
>          ptr += qxl->guest_slots[slot].offset;
>          ptr += offset;
> diff --git a/hw/display/qxl.h b/hw/display/qxl.h
> index bf03138ab4..7894bd5134 100644
> --- a/hw/display/qxl.h
> +++ b/hw/display/qxl.h
> @@ -157,7 +157,7 @@ OBJECT_DECLARE_SIMPLE_TYPE(PCIQXLDevice, PCI_QXL)
>   *
>   * Returns a host pointer to a buffer placed at offset @phys within the
>   * active slot @group_id of the PCI VGA RAM memory region associated with
> - * the @qxl device. If the slot is inactive, or the offset is out
> + * the @qxl device. If the slot is inactive, or the offset + size are out
>   * of the memory region, returns NULL.
>   *
>   * Use with care; by the time this function returns, the returned pointer is
> --
> 2.38.1
>
>


-- 
Marc-André Lureau