Re: [PATCH v3 7/8] hw/display/artist: Refactor artist_rop8() to avoid buffer over-run

[Helge Deller]

Hello Alexander,

* Alexander Bulekov <alxndr@bu.edu>:
> On 200804 2320, Helge Deller wrote:
> > * Alexander Bulekov <alxndr@bu.edu>:
> > > I applied this series and it fixes most of the problems I saw before.
> > > I still see a few crashes - I made issues for them on launchpad:
> > > https://bugs.launchpad.net/qemu/+bug/1890310
> > > https://bugs.launchpad.net/qemu/+bug/1890311
> > > https://bugs.launchpad.net/qemu/+bug/1890312
> Hi Helge, I can still reproduce this one  ^^^
> I'll fuzz it some more, but so far I am not finding anything else.

I've now updated the patch which does address all issues you found
so far. It's attached below.

If you like you can pull the full series from
https://github.com/hdeller/qemu-hppa/commits/target-hppa

I'd be happy if you could recheck if you find anything else.

In the next step I need to check if everything still works on HP-UX. If
that's Ok, I'd like to send out a new pull request.  If you could give
your Acked-by or Reviewed-by it would be great.

Thanks!
Helge

---
From 1657a7a95adc15552138c2b4d310a06128093892 Mon Sep 17 00:00:00 2001
From: Helge Deller <deller@gmx.de>
Date: Tue, 4 Aug 2020 15:35:38 +0200
Subject: [PATCH] hw/display/artist: Prevent out of VRAM buffer accesses

Simplify various bounds checks by changing parameters like row and column
numbers to become unsigned instead of signed.
With that we can check if the calculated offset is bigger than the size of the
VRAM region and bail out if not.

Reported-by: LLVM libFuzzer
Reported-by: Alexander Bulekov <alxndr@bu.edu>
Buglink: https://bugs.launchpad.net/qemu/+bug/1880326
Buglink: https://bugs.launchpad.net/qemu/+bug/1890310
Buglink: https://bugs.launchpad.net/qemu/+bug/1890311
Buglink: https://bugs.launchpad.net/qemu/+bug/1890312
Buglink: https://bugs.launchpad.net/qemu/+bug/1890370
Signed-off-by: Helge Deller <deller@gmx.de>

diff --git a/hw/display/artist.c b/hw/display/artist.c
index 47de17b9e9..570811030f 100644
--- a/hw/display/artist.c
+++ b/hw/display/artist.c
@@ -35,9 +35,9 @@
 struct vram_buffer {
     MemoryRegion mr;
     uint8_t *data;
-    int size;
-    int width;
-    int height;
+    unsigned int size;
+    unsigned int width;
+    unsigned int height;
 };

 typedef struct ARTISTState {
@@ -203,14 +203,18 @@ static int16_t artist_get_y(uint32_t reg)
 }

 static void artist_invalidate_lines(struct vram_buffer *buf,
-                                    int starty, int height)
+                                    unsigned int starty, unsigned int height)
 {
-    int start = starty * buf->width;
-    int size = height * buf->width;
+    unsigned int start, size;

-    if (start + size <= buf->size) {
-        memory_region_set_dirty(&buf->mr, start, size);
+    if (starty >= buf->height) {
+        return;
     }
+
+    start = starty * buf->width;
+    size = MIN(height * buf->width, buf->size - start);
+
+    memory_region_set_dirty(&buf->mr, start, size);
 }

 static int vram_write_pix_per_transfer(ARTISTState *s)
@@ -274,15 +278,15 @@ static artist_rop_t artist_get_op(ARTISTState *s)
 }

 static void artist_rop8(ARTISTState *s, struct vram_buffer *buf,
-                        int offset, uint8_t val)
+                        unsigned int offset, uint8_t val)
 {
     const artist_rop_t op = artist_get_op(s);
     uint8_t plane_mask;
     uint8_t *dst;

-    if (offset < 0 || offset >= buf->size) {
+    if (offset >= buf->size) {
         qemu_log_mask(LOG_GUEST_ERROR,
-                      "rop8 offset:%d bufsize:%u\n", offset, buf->size);
+                      "rop8 offset:%u bufsize:%u\n", offset, buf->size);
         return;
     }
     dst = buf->data + offset;
@@ -294,8 +298,7 @@ static void artist_rop8(ARTISTState *s, struct vram_buffer 
*buf,
         break;

     case ARTIST_ROP_COPY:
-        *dst &= ~plane_mask;
-        *dst |= val & plane_mask;
+        *dst = (*dst & ~plane_mask) | (val & plane_mask);
         break;

     case ARTIST_ROP_XOR:
@@ -349,7 +352,8 @@ static void vram_bit_write(ARTISTState *s, int posx, int 
posy, bool incr_x,
 {
     struct vram_buffer *buf;
     uint32_t vram_bitmask = s->vram_bitmask;
-    int mask, i, pix_count, pix_length, offset, width;
+    int mask, i, pix_count, pix_length;
+    unsigned int offset, width;
     uint8_t *data8, *p;

     pix_count = vram_write_pix_per_transfer(s);
@@ -364,8 +368,7 @@ static void vram_bit_write(ARTISTState *s, int posx, int 
posy, bool incr_x,
         offset = posy * width + posx;
     }

-    if (!buf->size) {
-        qemu_log("write to non-existent buffer\n");
+    if (!buf->size || offset >= buf->size) {
         return;
     }

@@ -394,7 +397,9 @@ static void vram_bit_write(ARTISTState *s, int posx, int 
posy, bool incr_x,

     case 3:
         if (s->cmap_bm_access) {
-            *(uint32_t *)(p + offset) = data;
+            if (offset + 3 < buf->size) {
+                *(uint32_t *)(p + offset) = data;
+            }
             break;
         }
         data8 = (uint8_t *)&data;
@@ -464,12 +469,14 @@ static void vram_bit_write(ARTISTState *s, int posx, int 
posy, bool incr_x,
     }
 }

-static void block_move(ARTISTState *s, int source_x, int source_y, int dest_x,
-                       int dest_y, int width, int height)
+static void block_move(ARTISTState *s,
+                       unsigned int source_x, unsigned int source_y,
+                       unsigned int dest_x,   unsigned int dest_y,
+                       unsigned int width,    unsigned int height)
 {
     struct vram_buffer *buf;
     int line, endline, lineincr, startcolumn, endcolumn, columnincr, column;
-    uint32_t dst, src;
+    unsigned int dst, src;

     trace_artist_block_move(source_x, source_y, dest_x, dest_y, width, height);

@@ -481,6 +488,12 @@ static void block_move(ARTISTState *s, int source_x, int 
source_y, int dest_x,
     }

     buf = &s->vram_buffer[ARTIST_BUFFER_AP];
+    if (height > buf->height) {
+        height = buf->height;
+    }
+    if (width > buf->width) {
+        width = buf->width;
+    }

     if (dest_y > source_y) {
         /* move down */
@@ -507,24 +520,27 @@ static void block_move(ARTISTState *s, int source_x, int 
source_y, int dest_x,
     }

     for ( ; line != endline; line += lineincr) {
-        src = source_x + ((line + source_y) * buf->width);
-        dst = dest_x + ((line + dest_y) * buf->width);
+        src = source_x + ((line + source_y) * buf->width) + startcolumn;
+        dst = dest_x + ((line + dest_y) * buf->width) + startcolumn;

         for (column = startcolumn; column != endcolumn; column += columnincr) {
-            if (dst + column > buf->size || src + column > buf->size) {
+            if (dst >= buf->size || src >= buf->size) {
                 continue;
             }
-            artist_rop8(s, buf, dst + column, buf->data[src + column]);
+            artist_rop8(s, buf, dst, buf->data[src]);
+            src += columnincr;
+            dst += columnincr;
         }
     }

     artist_invalidate_lines(buf, dest_y, height);
 }

-static void fill_window(ARTISTState *s, int startx, int starty,
-                        int width, int height)
+static void fill_window(ARTISTState *s,
+                        unsigned int startx, unsigned int starty,
+                        unsigned int width,  unsigned int height)
 {
-    uint32_t offset;
+    unsigned int offset;
     uint8_t color = artist_get_color(s);
     struct vram_buffer *buf;
     int x, y;
@@ -561,7 +577,9 @@ static void fill_window(ARTISTState *s, int startx, int 
starty,
     artist_invalidate_lines(buf, starty, height);
 }

-static void draw_line(ARTISTState *s, int x1, int y1, int x2, int y2,
+static void draw_line(ARTISTState *s,
+                      unsigned int x1, unsigned int y1,
+                      unsigned int x2, unsigned int y2,
                       bool update_start, int skip_pix, int max_pix)
 {
     struct vram_buffer *buf = &s->vram_buffer[ARTIST_BUFFER_AP];
@@ -636,7 +654,7 @@ static void draw_line(ARTISTState *s, int x1, int y1, int 
x2, int y2,
     color = artist_get_color(s);

     do {
-        int ofs;
+        unsigned int ofs;

         if (c1) {
             ofs = x * s->width + y;
@@ -768,13 +786,14 @@ static void font_write16(ARTISTState *s, uint16_t val)
     uint16_t mask;
     int i;

-    int startx = artist_get_x(s->vram_start);
-    int starty = artist_get_y(s->vram_start) + s->font_write_pos_y;
-    int offset = starty * s->width + startx;
+    unsigned int startx = artist_get_x(s->vram_start);
+    unsigned int starty = artist_get_y(s->vram_start) + s->font_write_pos_y;
+    unsigned int offset = starty * s->width + startx;

     buf = &s->vram_buffer[ARTIST_BUFFER_AP];

-    if (offset + 16 > buf->size) {
+    if (startx >= buf->width || starty >= buf->height ||
+        offset + 16 >= buf->size) {
         return;
     }

@@ -1138,7 +1157,7 @@ static void artist_vram_write(void *opaque, hwaddr addr, 
uint64_t val,
     struct vram_buffer *buf;
     int posy = (addr >> 11) & 0x3ff;
     int posx = addr & 0x7ff;
-    uint32_t offset;
+    unsigned int offset;
     trace_artist_vram_write(size, addr, val);

     if (s->cmap_bm_access) {
@@ -1159,18 +1178,28 @@ static void artist_vram_write(void *opaque, hwaddr 
addr, uint64_t val,
     }

     offset = posy * buf->width + posx;
+    if (offset >= buf->size) {
+        return;
+    }
+
     switch (size) {
     case 4:
-        *(uint32_t *)(buf->data + offset) = be32_to_cpu(val);
-        memory_region_set_dirty(&buf->mr, offset, 4);
+        if (offset + 3 < buf->size) {
+            *(uint32_t *)(buf->data + offset) = be32_to_cpu(val);
+            memory_region_set_dirty(&buf->mr, offset, 4);
+        }
         break;
     case 2:
-        *(uint16_t *)(buf->data + offset) = be16_to_cpu(val);
-        memory_region_set_dirty(&buf->mr, offset, 2);
+        if (offset + 1 < buf->size) {
+            *(uint16_t *)(buf->data + offset) = be16_to_cpu(val);
+            memory_region_set_dirty(&buf->mr, offset, 2);
+        }
         break;
     case 1:
-        *(uint8_t *)(buf->data + offset) = val;
-        memory_region_set_dirty(&buf->mr, offset, 1);
+        if (offset < buf->size) {
+            *(uint8_t *)(buf->data + offset) = val;
+            memory_region_set_dirty(&buf->mr, offset, 1);
+        }
         break;
     default:
         break;