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

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

[Gerd Hoffmann]

Move dst calculation into the loop, so we apply the mask on each
interation and will not overflow vga memory.

Cc: Prasad J Pandit <pjp@fedoraproject.org>
Reported-by: Niu Guoxiang <niuguoxiang@huawei.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
---
 hw/display/cirrus_vga.c | 6 ++----
 1 file changed, 2 insertions(+), 4 deletions(-)

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);
 }
-- 
2.9.3

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
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