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video/readers/jpeg: Block int underflow -> wild pointer write

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Certain 1 px wide images caused a wild pointer write in
grub_jpeg_ycrcb_to_rgb(). This was caused because in grub_jpeg_decode_data(),
we have the following loop:

for (; data->r1 < nr1 && (!data->dri || rst);
     data->r1++, data->bitmap_ptr += (vb * data->image_width - hb * nc1) * 3)

We did not check if vb * width >= hb * nc1.

On a 64-bit platform, if that turns out to be negative, it will underflow,
be interpreted as unsigned 64-bit, then be added to the 64-bit pointer, so
we see data->bitmap_ptr jump, e.g.:

0x6180_0000_0480 to
0x6181_0000_0498
     ^
     ~--- carry has occurred and this pointer is now far away from
          any object.

On a 32-bit platform, it will decrement the pointer, creating a pointer
that won't crash but will overwrite random data.

Catch the underflow and error out.

Fixes: CVE-2021-3697

Signed-off-by: Daniel Axtens <dja@axtens.net>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>
This commit is contained in:
Daniel Axtens 2021-07-07 15:38:19 +10:00 committed by Daniel Kiper
parent 166a4d6144
commit 22a3f97d39
1 changed files with 9 additions and 1 deletions
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10
grub-core/video/readers/jpeg.c
View File

@ -23,6 +23,7 @@
#include <grub/mm.h>
#include <grub/misc.h>
#include <grub/bufio.h>
#include <grub/safemath.h>
GRUB_MOD_LICENSE ("GPLv3+");
@ -699,6 +700,7 @@ static grub_err_t
grub_jpeg_decode_data (struct grub_jpeg_data *data)
{
unsigned c1, vb, hb, nr1, nc1;
unsigned stride_a, stride_b, stride;
int rst = data->dri;
grub_err_t err = GRUB_ERR_NONE;
@ -711,8 +713,14 @@ grub_jpeg_decode_data (struct grub_jpeg_data *data)
return grub_error (GRUB_ERR_BAD_FILE_TYPE,
"jpeg: attempted to decode data before start of stream");
if (grub_mul(vb, data->image_width, &stride_a) ||
grub_mul(hb, nc1, &stride_b) ||
grub_sub(stride_a, stride_b, &stride))
return grub_error (GRUB_ERR_BAD_FILE_TYPE,
"jpeg: cannot decode image with these dimensions");
for (; data->r1 < nr1 && (!data->dri || rst);
data->r1++, data->bitmap_ptr += (vb * data->image_width - hb * nc1) * 3)
data->r1++, data->bitmap_ptr += stride * 3)
for (c1 = 0; c1 < nc1 && (!data->dri || rst);
c1++, rst--, data->bitmap_ptr += hb * 3)
{