From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001 From: Daniel Axtens Date: Wed, 7 Jul 2021 15:38:19 +1000 Subject: [PATCH] video/readers/jpeg: Block int underflow -> wild pointer write 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 Reviewed-by: Daniel Kiper (cherry picked from commit 41aeb2004db9924fecd9f2dd64bc2a5a5594a4b5) (cherry picked from commit 5f9582490792108306d047379fed2371bee286f8) (cherry picked from commit 7e4bf25d9bb5219fbf11c523296dc3bd78b80698) --- grub-core/video/readers/jpeg.c | 4 ++++ 1 file changed, 4 insertions(+) diff --git a/grub-core/video/readers/jpeg.c b/grub-core/video/readers/jpeg.c index 1df1171d78..2da04094b3 100644 --- a/grub-core/video/readers/jpeg.c +++ b/grub-core/video/readers/jpeg.c @@ -705,6 +705,10 @@ 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 (vb * data->image_width <= hb * nc1) + 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) for (c1 = 0; c1 < nc1 && (!data->dri || rst);