hash-common: fix heap overflow when writing more data after final

* tests/basic.c (check_one_md): Test writing to digest after read.
* cipher/hash-common.c (_gcry_md_block_write): Reset 'hd->count' if
greater than blocksize.
--

'_gcry_md_block_write' did not expect 'hd->count' being greater than
digest blocksize. However digest final function may set 'hd->count'
to larger value. Now, if write is called after final function and
'hd->count' gets too large value, 'copylen' parameter to buf_cpy
may have value larger than size of 'hd->buf' and cause heap overflow.

Reported-by: Tavis Ormandy <taviso@gmail.com>
Signed-off-by: Jussi Kivilinna <jussi.kivilinna@iki.fi>

2 files changed, 14 insertions, 0 deletions

diff --git a/cipher/hash-common.c b/cipher/hash-common.c
index ab486f06..ed2d7cac 100644
--- a/cipher/hash-common.c
+++ b/cipher/hash-common.c
@@ -134,6 +134,13 @@ _gcry_md_block_write (void *context, const void *inbuf_arg, size_t inlen)
   if (!hd->bwrite)
     return;
 
+  if (hd->count > blocksize)
+    {
+      /* This happens only when gcry_md_write is called after final.
+       * Writing after final is used for mitigating timing attacks. */
+      hd->count = 0;
+    }
+
   while (hd->count)
     {
       if (hd->count == blocksize)  /* Flush the buffer. */
diff --git a/tests/basic.c b/tests/basic.c
index 8b333bae..c54de78b 100644
--- a/tests/basic.c
+++ b/tests/basic.c
@@ -10223,6 +10223,8 @@ check_one_md (int algo, const char *data, int len, const char *expect, int elen,
 
   if (!xof)
     {
+      static const char buf[128];
+
       clutter_vector_registers();
       p = gcry_md_read (hd2, algo);
 
@@ -10238,6 +10240,11 @@ check_one_md (int algo, const char *data, int len, const char *expect, int elen,
 
           fail ("algo %d, digest mismatch\n", algo);
         }
+
+      /* Write after final/read is allowed for timing attack mitigation
+       * purposes. Try writing and see if we catch fire. */
+      clutter_vector_registers();
+      gcry_md_write (hd2, buf, sizeof(buf));
     }
   else
     {