Re: [PATCH] ati: mask x y display parameter values

[P J P <ppandit@redhat.com>]

  Hi,

+-- On Mon, 19 Oct 2020, BALATON Zoltan wrote --+
| On Mon, 19 Oct 2020, P J P wrote:
| >    dst_x = ... (s->regs.dst_x(=0) + 1 - s->regs.dst_width(=16383))
| >    dst_y = ... (s->regs.dst_y(=0) + 1 - s->regs.dst_height(=16383))
| >
| >  ati_2d_blt
| >    pixman_blt(0x7f03cbe00000, 0x7f03cbe00000, 131064, 131064, 32, 32,
| >       src_x=0, src_y=-16382, dst_x=0, dst_y=-16382, 16383, 16383)
| >
| > Shown as negative values due to signed '%d' conversion.
| 
| Checking the docs again I see that the allowed range of at least
| s->regs.[src|dst]_[xy] can actually be negative (-8192:8191)

* But 'struct ATIVGARegs' declares these fields as 'uint32_t' type. Ie. no 
  negativeve values.

* I guess that range applies to src->regs.dst_[width|height] too? Considering 
  it being subtracted from s->regs.dst_[xy] values above.


| >    uint8_t *end = s->vga.vram_ptr + s->vga.vram_size;
| >    if (dst_bits >= end || dst_bits + dst_x + (dst_y + s->regs.dst_height) *
| >    dst_stride >= end) {
| >        qemu_log_mask(LOG_UNIMP, "blt outside vram not implemented\n");
| >        return;
| 
| ... Could it be it overflows?

* Yes, following expression

  dst_y(=4294950914(=-16382)) + s->regs.dst_height(=16383)) overflows to => 1

Ie. (dst_bits + dst_x(=0) + (1) * dst_stride >= end) returns false.


| maybe rather add additional term for src|dst_x|y to the already existing 
| checks if their condition cannot be fixed to detect it properly.
===
diff --git a/hw/display/ati_2d.c b/hw/display/ati_2d.c
index 524bc03a83..5fa7362305 100644
--- a/hw/display/ati_2d.c
+++ b/hw/display/ati_2d.c
@@ -54,9 +54,9 @@ void ati_2d_blt(ATIVGAState *s)
...
-    if (dst_bits >= end || dst_bits + dst_x + (dst_y + s->regs.dst_height) *
-        dst_stride >= end) {
+    if (dst_x > 0x3fff || dst_y > 0x3fff || dst_bits >= end
+        || dst_bits + dst_x + (dst_y + s->regs.dst_height)
+         * dst_stride >= end) {
         qemu_log_mask(LOG_UNIMP, "blt outside vram not implemented\n");
         return;
     }
...
-        if (src_bits >= end || src_bits + src_x +
-            (src_y + s->regs.dst_height) * src_stride >= end) {
+        if (src_x > 0x3fff || src_y > 0x3ff || src_bits >= end
+            || src_bits + src_x + (src_y + s->regs.dst_height)
+             * src_stride >= end) {
             qemu_log_mask(LOG_UNIMP, "blt outside vram not implemented\n");
             return;
         }
===

* Does it look okay?


Thank you.
--
Prasad J Pandit / Red Hat Product Security Team
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