Re: [PATCH] hw/qxl: move check of slot_id before accessing guest_slots

[Philippe Mathieu-Daudé]

Hi Anastasia,

On 14/9/23 11:27, Anastasia Belova wrote:
> If slot_id >= NUM_MEMSLOTS, buffer overflow is possible.

overflow: unlikely. Do you mean over-read?

Did you found that by code audit? I can't see where this
function get slot_id >= NUM_MEMSLOTS.

This isn't guest triggerable and seems an API invalid use,
so developer error, which we can catch with assertions:

-- >8 --
diff --git a/hw/display/qxl.c b/hw/display/qxl.c
index 7bb00d68f5..443f034168 100644
--- a/hw/display/qxl.c
+++ b/hw/display/qxl.c
@@ -1309,16 +1309,13 @@ static int qxl_add_memslot(PCIQXLDevice *d, 
uint32_t slot_id, uint64_t delta,
     QXLDevMemSlot memslot;
     int i;

+    assert(slot_id < NUM_MEMSLOTS);
+
     guest_start = le64_to_cpu(d->guest_slots[slot_id].slot.mem_start);
     guest_end   = le64_to_cpu(d->guest_slots[slot_id].slot.mem_end);

     trace_qxl_memslot_add_guest(d->id, slot_id, guest_start, guest_end);

-    if (slot_id >= NUM_MEMSLOTS) {
-        qxl_set_guest_bug(d, "%s: slot_id >= NUM_MEMSLOTS %d >= %d", 
__func__,
-                      slot_id, NUM_MEMSLOTS);
-        return 1;
-    }
     if (guest_start > guest_end) {
         qxl_set_guest_bug(d, "%s: guest_start > guest_end 0x%" PRIx64
                          " > 0x%" PRIx64, __func__, guest_start, 
guest_end);
---

Note, qxl_add_memslot() return value is ignored...

> So the check should be upper than d->guest_slots[slot_id]
> where size of d->guest_slots is NUM_MEMSLOTS.
>
> Fixes: e954ea2873 ("qxl: qxl_add_memslot: remove guest trigerrable panics")
> Signed-off-by: Anastasia Belova <abelova@astralinux.ru>
> ---
>   hw/display/qxl.c | 11 ++++++-----
>   1 file changed, 6 insertions(+), 5 deletions(-)
>
> diff --git a/hw/display/qxl.c b/hw/display/qxl.c
> index 7bb00d68f5..dc618727c0 100644
> --- a/hw/display/qxl.c
> +++ b/hw/display/qxl.c
> @@ -1309,16 +1309,17 @@ static int qxl_add_memslot(PCIQXLDevice *d, uint32_t 
> slot_id, uint64_t delta,
>       QXLDevMemSlot memslot;
>       int i;
>   
> -    guest_start = le64_to_cpu(d->guest_slots[slot_id].slot.mem_start);
> -    guest_end   = le64_to_cpu(d->guest_slots[slot_id].slot.mem_end);
> -
> -    trace_qxl_memslot_add_guest(d->id, slot_id, guest_start, guest_end);
> -
>       if (slot_id >= NUM_MEMSLOTS) {
>           qxl_set_guest_bug(d, "%s: slot_id >= NUM_MEMSLOTS %d >= %d", __func__,
>                         slot_id, NUM_MEMSLOTS);
>           return 1;
>       }
> +
> +    guest_start = le64_to_cpu(d->guest_slots[slot_id].slot.mem_start);
> +    guest_end   = le64_to_cpu(d->guest_slots[slot_id].slot.mem_end);
> +
> +    trace_qxl_memslot_add_guest(d->id, slot_id, guest_start, guest_end);
> +
>       if (guest_start > guest_end) {
>           qxl_set_guest_bug(d, "%s: guest_start > guest_end 0x%" PRIx64
>                            " > 0x%" PRIx64, __func__, guest_start, guest_end);