[PATCH 1/3] hw/display/artist: Check offset in draw_line 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.

We avoid:

  Program terminated with signal SIGSEGV, Segmentation fault.
  284             *dst &= ~plane_mask;
  (gdb) bt
  #0  0x000055d5dccdc5c0 in artist_rop8 (s=0x55d5defee510, dst=0x7f8e84ed8216 <error: Cannot access memory at address 0x7f8e84ed8216>, val=0 '\000') at hw/display/artist.c:284
  #1  0x000055d5dccdcf83 in fill_window (s=0x55d5defee510, startx=22, starty=5674, width=65, height=5697) at hw/display/artist.c:551
  #2  0x000055d5dccddfb9 in artist_reg_write (opaque=0x55d5defee510, addr=1051140, val=4265537, size=4) at hw/display/artist.c:902
  #3  0x000055d5dcb42a7c in memory_region_write_accessor (mr=0x55d5defeea10, addr=1051140, value=0x7ffe57db08c8, size=4, shift=0, mask=4294967295, attrs=...) at memory.c:483

Reported-by: LLVM libFuzzer
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
---
 hw/display/artist.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/hw/display/artist.c b/hw/display/artist.c
index 6261bfe65b..3c2550f6db 100644
--- a/hw/display/artist.c
+++ b/hw/display/artist.c
@@ -557,7 +557,7 @@ static void fill_window(ARTISTState *s, int startx, int starty,
 static void draw_line(ARTISTState *s, int x1, int y1, int x2, int y2,
                       bool update_start, int skip_pix, int max_pix)
 {
-    struct vram_buffer *buf;
+    struct vram_buffer *buf = &s->vram_buffer[ARTIST_BUFFER_AP];
     uint8_t color;
     int dx, dy, t, e, x, y, incy, diago, horiz;
     bool c1;
@@ -565,6 +565,12 @@ static void draw_line(ARTISTState *s, int x1, int y1, int x2, int y2,
 
     trace_artist_draw_line(x1, y1, x2, y2);
 
+    if (x1 * y1 >= buf->size || x2 * y2 >= buf->size) {
+        qemu_log_mask(LOG_GUEST_ERROR,
+                      "draw_line (%d,%d) (%d,%d)\n", x1, y1, x2, y2);
+        return;
+    }
+
     if (update_start) {
         s->vram_start = (x2 << 16) | y2;
     }
@@ -622,7 +628,6 @@ static void draw_line(ARTISTState *s, int x1, int y1, int x2, int y2,
     x = x1;
     y = y1;
     color = artist_get_color(s);
-    buf = &s->vram_buffer[ARTIST_BUFFER_AP];
 
     do {
         if (c1) {
-- 
2.21.3