[SECURITY PATCH 13/13] normal/charset: Fix an integer overflow in grub_unicode_aglomerate_comb()

[Daniel Kiper]

From: Zhang Boyang <zhangboyang.id@gmail.com>

The out->ncomb is a bit-field of 8 bits. So, the max possible value is 255.
However, code in grub_unicode_aglomerate_comb() doesn't check for an
overflow when incrementing out->ncomb. If out->ncomb is already 255,
after incrementing it will get 0 instead of 256, and cause illegal
memory access in subsequent processing.

This patch introduces GRUB_UNICODE_NCOMB_MAX to represent the max
acceptable value of ncomb. The code now checks for this limit and
ignores additional combining characters when limit is reached.

Reported-by: Daniel Axtens <dja@axtens.net>
Signed-off-by: Zhang Boyang <zhangboyang.id@gmail.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>
---
 grub-core/normal/charset.c | 3 +++
 include/grub/unicode.h     | 2 ++
 2 files changed, 5 insertions(+)

diff --git a/grub-core/normal/charset.c b/grub-core/normal/charset.c
index 000e687bd..4f6647116 100644
--- a/grub-core/normal/charset.c
+++ b/grub-core/normal/charset.c
@@ -472,6 +472,9 @@ grub_unicode_aglomerate_comb (const grub_uint32_t *in, 
grub_size_t inlen,
          if (!haveout)
            continue;
 
+         if (out->ncomb == GRUB_UNICODE_NCOMB_MAX)
+           continue;
+
          if (comb_type == GRUB_UNICODE_COMB_MC
              || comb_type == GRUB_UNICODE_COMB_ME
              || comb_type == GRUB_UNICODE_COMB_MN)
diff --git a/include/grub/unicode.h b/include/grub/unicode.h
index 71a4d1a54..9360b0b97 100644
--- a/include/grub/unicode.h
+++ b/include/grub/unicode.h
@@ -147,7 +147,9 @@ struct grub_unicode_glyph
   grub_uint8_t bidi_level:6; /* minimum: 6 */
   enum grub_bidi_type bidi_type:5; /* minimum: :5 */
 
+#define GRUB_UNICODE_NCOMB_MAX ((1 << 8) - 1)
   unsigned ncomb:8;
+
   /* Hint by unicode subsystem how wide this character usually is.
      Real width is determined by font. Set only in UTF-8 stream.  */
   int estimated_width:8;
-- 
2.11.0