[PATCH 2/3] hw/display/artist: Refactor artist_rop8() to avoid buffer over-run

["Philippe Mathieu-Daudé" <f4bug@amsat.org>]

Invalid I/O writes can craft an offset out of the vram_buffer
range.
Instead of passing an unsafe pointer to artist_rop8(), pass the
vram_buffer and the offset. We can now check if the offset is
in range before accessing it.

We avoid:

  Program terminated with signal SIGSEGV, Segmentation fault.
  284             *dst &= ~plane_mask;
  (gdb) bt
  #0  0x000056367b2085c0 in artist_rop8 (s=0x56367d38b510, dst=0x7f9f972fffff <error: Cannot access memory at address 0x7f9f972fffff>, val=0 '\000') at hw/display/artist.c:284
  #1  0x000056367b209325 in draw_line (s=0x56367d38b510, x1=-20480, y1=-1, x2=0, y2=17920, update_start=true, skip_pix=-1, max_pix=-1) at hw/display/artist.c:646

Reported-by: LLVM libFuzzer
Buglink: https://bugs.launchpad.net/qemu/+bug/1880326
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
---
 hw/display/artist.c | 40 +++++++++++++++++++++++++---------------
 1 file changed, 25 insertions(+), 15 deletions(-)

diff --git a/hw/display/artist.c b/hw/display/artist.c
index 3c2550f6db..6f61b85a24 100644
--- a/hw/display/artist.c
+++ b/hw/display/artist.c
@@ -273,11 +273,20 @@ static artist_rop_t artist_get_op(ARTISTState *s)
     return (s->image_bitmap_op >> 8) & 0xf;
 }
 
-static void artist_rop8(ARTISTState *s, uint8_t *dst, uint8_t val)
+static void artist_rop8(ARTISTState *s, struct vram_buffer *buf,
+                        int offset, uint8_t val)
 {
-
     const artist_rop_t op = artist_get_op(s);
-    uint8_t plane_mask = s->plane_mask & 0xff;
+    uint8_t plane_mask;
+    uint8_t *dst;
+
+    if (offset < 0 || offset >= buf->size) {
+        qemu_log_mask(LOG_GUEST_ERROR,
+                      "rop8 offset:%d bufsize:%u\n", offset, buf->size);
+        return;
+    }
+    dst = buf->data + offset;
+    plane_mask = s->plane_mask & 0xff;
 
     switch (op) {
     case ARTIST_ROP_CLEAR:
@@ -381,7 +390,7 @@ static void vram_bit_write(ARTISTState *s, int posx, int posy, bool incr_x,
         }
 
         for (i = 0; i < pix_count; i++) {
-            artist_rop8(s, p + offset + pix_count - 1 - i,
+            artist_rop8(s, buf, offset + pix_count - 1 - i,
                         (data & 1) ? (s->plane_mask >> 24) : 0);
             data >>= 1;
         }
@@ -398,7 +407,7 @@ static void vram_bit_write(ARTISTState *s, int posx, int posy, bool incr_x,
         for (i = 3; i >= 0; i--) {
             if (!(s->image_bitmap_op & 0x20000000) ||
                 s->vram_bitmask & (1 << (28 + i))) {
-                artist_rop8(s, p + offset + 3 - i, data8[ROP8OFF(i)]);
+                artist_rop8(s, buf, offset + 3 - i, data8[ROP8OFF(i)]);
             }
         }
         memory_region_set_dirty(&buf->mr, offset, 3);
@@ -426,10 +435,10 @@ static void vram_bit_write(ARTISTState *s, int posx, int posy, bool incr_x,
             if (!(s->image_bitmap_op & 0x20000000) ||
                 (vram_bitmask & mask)) {
                 if (data & mask) {
-                    artist_rop8(s, p + offset + i, s->fg_color);
+                    artist_rop8(s, buf, offset + i, s->fg_color);
                 } else {
                     if (!(s->image_bitmap_op & 0x10000002)) {
-                        artist_rop8(s, p + offset + i, s->bg_color);
+                        artist_rop8(s, buf, offset + i, s->bg_color);
                     }
                 }
             }
@@ -507,7 +516,7 @@ static void block_move(ARTISTState *s, int source_x, int source_y, int dest_x,
             if (dst + column > buf->size || src + column > buf->size) {
                 continue;
             }
-            artist_rop8(s, buf->data + dst + column, buf->data[src + column]);
+            artist_rop8(s, buf, dst + column, buf->data[src + column]);
         }
     }
 
@@ -548,7 +557,7 @@ static void fill_window(ARTISTState *s, int startx, int starty,
         offset = y * s->width;
 
         for (x = startx; x < startx + width; x++) {
-            artist_rop8(s, buf->data + offset + x, color);
+            artist_rop8(s, buf, offset + x, color);
         }
     }
     artist_invalidate_lines(buf, starty, height);
@@ -561,7 +570,6 @@ static void draw_line(ARTISTState *s, int x1, int y1, int x2, int y2,
     uint8_t color;
     int dx, dy, t, e, x, y, incy, diago, horiz;
     bool c1;
-    uint8_t *p;
 
     trace_artist_draw_line(x1, y1, x2, y2);
 
@@ -630,16 +638,18 @@ static void draw_line(ARTISTState *s, int x1, int y1, int x2, int y2,
     color = artist_get_color(s);
 
     do {
+        int ofs;
+
         if (c1) {
-            p = buf->data + x * s->width + y;
+            ofs = x * s->width + y;
         } else {
-            p = buf->data + y * s->width + x;
+            ofs = y * s->width + x;
         }
 
         if (skip_pix > 0) {
             skip_pix--;
         } else {
-            artist_rop8(s, p, color);
+            artist_rop8(s, buf, ofs, color);
         }
 
         if (e > 0) {
@@ -773,10 +783,10 @@ static void font_write16(ARTISTState *s, uint16_t val)
     for (i = 0; i < 16; i++) {
         mask = 1 << (15 - i);
         if (val & mask) {
-            artist_rop8(s, buf->data + offset + i, color);
+            artist_rop8(s, buf, offset + i, color);
         } else {
             if (!(s->image_bitmap_op & 0x20000000)) {
-                artist_rop8(s, buf->data + offset + i, s->bg_color);
+                artist_rop8(s, buf, offset + i, s->bg_color);
             }
         }
     }
-- 
2.21.3