From 22a3f97d39f6a10b08ad7fd1cc47c4dcd10413f6 Mon Sep 17 00:00:00 2001 From: Daniel Axtens Date: Wed, 7 Jul 2021 15:38:19 +1000 Subject: [PATCH 8/8] 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 --- 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 #include #include +#include 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) { -- 2.33.5