Re: [PATCH] hw/display/ati_2d: Fix buffer overflow in ati_2d_blt (CVE-2021-3638)

["Philippe Mathieu-Daudé" <philmd@redhat.com>]

(Forgot to Cc Alex for eventual reproducer)

On 9/6/21 6:44 PM, Mauro Matteo Cascella wrote:
> On Mon, Sep 6, 2021 at 5:31 PM Philippe Mathieu-Daudé <philmd@redhat.com> wrote:
>>
>> When building QEMU with DEBUG_ATI defined then running with
>> '-device ati-vga,romfile="" -d unimp,guest_errors -trace ati\*'
>> we get:
>>
>>   ati_mm_write 4 0x16c0 DP_CNTL <- 0x1
>>   ati_mm_write 4 0x146c DP_GUI_MASTER_CNTL <- 0x2
>>   ati_mm_write 4 0x16c8 DP_MIX <- 0xff0000
>>   ati_mm_write 4 0x16c4 DP_DATATYPE <- 0x2
>>   ati_mm_write 4 0x224 CRTC_OFFSET <- 0x0
>>   ati_mm_write 4 0x142c DST_PITCH_OFFSET <- 0xfe00000
>>   ati_mm_write 4 0x1420 DST_Y <- 0x3fff
>>   ati_mm_write 4 0x1410 DST_HEIGHT <- 0x3fff
>>   ati_mm_write 4 0x1588 DST_WIDTH_X <- 0x3fff3fff
>>   ati_2d_blt: vram:0x7fff5fa00000 addr:0 ds:0x7fff61273800 stride:2560 bpp:32 rop:0xff
>>   ati_2d_blt: 0 0 0, 0 127 0, (0,0) -> (16383,16383) 16383x16383 > ^
>>   ati_2d_blt: pixman_fill(dst:0x7fff5fa00000, stride:254, bpp:8, x:16383, y:16383, w:16383, h:16383, xor:0xff000000)
>>   Thread 3 "qemu-system-i38" received signal SIGSEGV, Segmentation fault.
>>   (gdb) bt
>>   #0  0x00007ffff7f62ce0 in sse2_fill.lto_priv () at /lib64/libpixman-1.so.0
>>   #1  0x00007ffff7f09278 in pixman_fill () at /lib64/libpixman-1.so.0
>>   #2  0x0000555557b5a9af in ati_2d_blt (s=0x631000028800) at hw/display/ati_2d.c:196
>>   #3  0x0000555557b4b5a2 in ati_mm_write (opaque=0x631000028800, addr=5512, data=1073692671, size=4) at hw/display/ati.c:843
>>   #4  0x0000555558b90ec4 in memory_region_write_accessor (mr=0x631000039cc0, addr=5512, ..., size=4, ...) at softmmu/memory.c:492
>>
>> Commit 584acf34cb0 ("ati-vga: Fix reverse bit blts") introduced
>> the local dst_x and dst_y which adjust the (x, y) coordinates
>> depending on the direction in the SRCCOPY ROP3 operation, but
>> forgot to address the same issue for the PATCOPY, BLACKNESS and
>> WHITENESS operations, which also call pixman_fill().
>>
>> Fix that now by using the adjusted coordinates in the pixman_fill
>> call, and update the related debug printf().
>>
>> Reported-by: Qiang Liu <qiangliu@zju.edu.cn>
>> Fixes: 584acf34cb0 ("ati-vga: Fix reverse bit blts")
>> Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
>> ---
>>  hw/display/ati_2d.c | 6 +++---
>>  1 file changed, 3 insertions(+), 3 deletions(-)
>>
>> diff --git a/hw/display/ati_2d.c b/hw/display/ati_2d.c
>> index 4dc10ea7952..692bec91de4 100644
>> --- a/hw/display/ati_2d.c
>> +++ b/hw/display/ati_2d.c
>> @@ -84,7 +84,7 @@ void ati_2d_blt(ATIVGAState *s)
>>      DPRINTF("%d %d %d, %d %d %d, (%d,%d) -> (%d,%d) %dx%d %c %c\n",
>>              s->regs.src_offset, s->regs.dst_offset, s->regs.default_offset,
>>              s->regs.src_pitch, s->regs.dst_pitch, s->regs.default_pitch,
>> -            s->regs.src_x, s->regs.src_y, s->regs.dst_x, s->regs.dst_y,
>> +            s->regs.src_x, s->regs.src_y, dst_x, dst_y,
>>              s->regs.dst_width, s->regs.dst_height,
>>              (s->regs.dp_cntl & DST_X_LEFT_TO_RIGHT ? '>' : '<'),
>>              (s->regs.dp_cntl & DST_Y_TOP_TO_BOTTOM ? 'v' : '^'));
>> @@ -180,11 +180,11 @@ void ati_2d_blt(ATIVGAState *s)
>>          dst_stride /= sizeof(uint32_t);
>>          DPRINTF("pixman_fill(%p, %d, %d, %d, %d, %d, %d, %x)\n",
>>                  dst_bits, dst_stride, bpp,
>> -                s->regs.dst_x, s->regs.dst_y,
>> +                dst_x, dst_y,
>>                  s->regs.dst_width, s->regs.dst_height,
>>                  filler);
>>          pixman_fill((uint32_t *)dst_bits, dst_stride, bpp,
>> -                    s->regs.dst_x, s->regs.dst_y,
>> +                    dst_x, dst_y,
>>                      s->regs.dst_width, s->regs.dst_height,
>>                      filler);
>>          if (dst_bits >= s->vga.vram_ptr + s->vga.vbe_start_addr &&
>> --
>> 2.31.1
>>
> 
> Tested-by: Mauro Matteo Cascella <mcascell@redhat.com>

Thanks. I wouldn't be surprise if we get another CVE in this code /
file / function ASAP this patch get merged... The code calls for a
rewrite, as per this function comment in its header:

void ati_2d_blt(ATIVGAState *s)
{
    /* FIXME it is probably more complex than this and may need to be */
    /* rewritten but for now as a start just to get some output: */

Regards,

Phil.