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From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
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From: Daniel Axtens <dja@axtens.net>
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Date: Wed, 7 Jul 2021 15:38:19 +1000
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Subject: [PATCH] video/readers/jpeg: Block int underflow -> wild pointer write
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Certain 1 px wide images caused a wild pointer write in
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grub_jpeg_ycrcb_to_rgb(). This was caused because in grub_jpeg_decode_data(),
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we have the following loop:
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for (; data->r1 < nr1 && (!data->dri || rst);
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data->r1++, data->bitmap_ptr += (vb * data->image_width - hb * nc1) * 3)
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We did not check if vb * width >= hb * nc1.
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On a 64-bit platform, if that turns out to be negative, it will underflow,
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be interpreted as unsigned 64-bit, then be added to the 64-bit pointer, so
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we see data->bitmap_ptr jump, e.g.:
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0x6180_0000_0480 to
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0x6181_0000_0498
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^
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~--- carry has occurred and this pointer is now far away from
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any object.
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On a 32-bit platform, it will decrement the pointer, creating a pointer
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that won't crash but will overwrite random data.
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Catch the underflow and error out.
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Fixes: CVE-2021-3697
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Signed-off-by: Daniel Axtens <dja@axtens.net>
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Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>
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(cherry picked from commit 41aeb2004db9924fecd9f2dd64bc2a5a5594a4b5)
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(cherry picked from commit 5f9582490792108306d047379fed2371bee286f8)
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---
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grub-core/video/readers/jpeg.c | 4 ++++
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1 file changed, 4 insertions(+)
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diff --git a/grub-core/video/readers/jpeg.c b/grub-core/video/readers/jpeg.c
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index 1df1171d78..2da04094b3 100644
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--- a/grub-core/video/readers/jpeg.c
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+++ b/grub-core/video/readers/jpeg.c
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@@ -705,6 +705,10 @@ grub_jpeg_decode_data (struct grub_jpeg_data *data)
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return grub_error (GRUB_ERR_BAD_FILE_TYPE,
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"jpeg: attempted to decode data before start of stream");
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+ if (vb * data->image_width <= hb * nc1)
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+ return grub_error (GRUB_ERR_BAD_FILE_TYPE,
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+ "jpeg: cannot decode image with these dimensions");
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+
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for (; data->r1 < nr1 && (!data->dri || rst);
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data->r1++, data->bitmap_ptr += (vb * data->image_width - hb * nc1) * 3)
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for (c1 = 0; c1 < nc1 && (!data->dri || rst);
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