Re: [Qemu-devel] [PATCH v2] cirrus: fix oob access in mode4and5 write functions

[P J P]

+-- On Wed, 11 Oct 2017, Gerd Hoffmann wrote --+
| Move dst calculation into the loop, so we apply the mask on each

s/into the loop/inside for loop

| interation and will not overflow vga memory.

s/interation/iteration


| diff --git a/hw/display/cirrus_vga.c b/hw/display/cirrus_vga.c
| index afc290ab91..077a8cb74f 100644
| --- a/hw/display/cirrus_vga.c
| +++ b/hw/display/cirrus_vga.c
| @@ -2038,15 +2038,14 @@ static void 
cirrus_mem_writeb_mode4and5_8bpp(CirrusVGAState * s,
|      unsigned val = mem_value;
|      uint8_t *dst;
|  
| -    dst = s->vga.vram_ptr + (offset &= s->cirrus_addr_mask);
|      for (x = 0; x < 8; x++) {
| +        dst = s->vga.vram_ptr + ((offset + x) & s->cirrus_addr_mask);
|       if (val & 0x80) {
|           *dst = s->cirrus_shadow_gr1;
|       } else if (mode == 5) {
|           *dst = s->cirrus_shadow_gr0;
|       }
|       val <<= 1;
| -     dst++;
|      }
|      memory_region_set_dirty(&s->vga.vram, offset, 8);
|  }
| @@ -2060,8 +2059,8 @@ static void 
cirrus_mem_writeb_mode4and5_16bpp(CirrusVGAState * s,
|      unsigned val = mem_value;
|      uint8_t *dst;
|  
| -    dst = s->vga.vram_ptr + (offset &= s->cirrus_addr_mask);
|      for (x = 0; x < 8; x++) {
| +        dst = s->vga.vram_ptr + ((offset + 2 * x) & s->cirrus_addr_mask & 
~1);
|       if (val & 0x80) {
|           *dst = s->cirrus_shadow_gr1;
|           *(dst + 1) = s->vga.gr[0x11];
| @@ -2070,7 +2069,6 @@ static void 
cirrus_mem_writeb_mode4and5_16bpp(CirrusVGAState * s,
|           *(dst + 1) = s->vga.gr[0x10];
|       }
|       val <<= 1;
| -     dst += 2;
|      }
|      memory_region_set_dirty(&s->vga.vram, offset, 16);
|  }

Works to fix the issue.

Thank you.
--
Prasad J Pandit / Red Hat Product Security Team
47AF CE69 3A90 54AA 9045 1053 DD13 3D32 FE5B 041F