[SECURITY PATCH 14/30] video/readers/jpeg: Block int underflow -> wild pointer write

[Daniel Kiper]

From: Daniel Axtens <dja@axtens.net>

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>
---
 grub-core/video/readers/jpeg.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/grub-core/video/readers/jpeg.c b/grub-core/video/readers/jpeg.c
index 579bbe8a4..09596fbf5 100644
--- a/grub-core/video/readers/jpeg.c
+++ b/grub-core/video/readers/jpeg.c
@@ -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)
       {
-- 
2.11.0