ECP: Prevent freeing a buffer on stack
The function ecp_mod_koblitz computed the space for the result of a
multiplication optimally for that specific case, but unfortunately
the function mbedtls_mpi_mul_mpi performs a generic, suboptimal
calculation and needs one more limb for the result. Since the result's
buffer is on the stack, the best case scenario is that the program
stops.
This only happened on 64 bit platforms.
Fixes #569
diff --git a/ChangeLog b/ChangeLog
index e26caed..e657ebe 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -13,6 +13,12 @@
characters after the footer could result in the execution of an infinite
loop. The issue can be triggered remotely. Found by Greg Zaverucha,
Microsoft.
+ * Fixed a bug that caused freeing a buffer that was allocated on the stack,
+ when verifying the validity of a key on secp224k1. This could be
+ triggered remotely for example with a maliciously constructed certificate
+ and might have led to remote code execution on some exotic embedded
+ platforms. Reported independently by rongsaws and Regina Wilson.
+ CVE-2017-2784
Bugfix
* Fix output certificate verification flags set by x509_crt_verify_top() when
diff --git a/library/ecp_curves.c b/library/ecp_curves.c
index f5afe44..db6ad3c 100644
--- a/library/ecp_curves.c
+++ b/library/ecp_curves.c
@@ -1264,7 +1264,7 @@
int ret;
size_t i;
mpi M, R;
- t_uint Mp[P_KOBLITZ_MAX + P_KOBLITZ_R];
+ t_uint Mp[P_KOBLITZ_MAX + P_KOBLITZ_R + 1];
if( N->n < p_limbs )
return( 0 );
@@ -1286,7 +1286,7 @@
memcpy( Mp, N->p + p_limbs - adjust, M.n * sizeof( t_uint ) );
if( shift != 0 )
MPI_CHK( mpi_shift_r( &M, shift ) );
- M.n += R.n - adjust; /* Make room for multiplication by R */
+ M.n += R.n; /* Make room for multiplication by R */
/* N = A0 */
if( mask != 0 )
@@ -1308,7 +1308,7 @@
memcpy( Mp, N->p + p_limbs - adjust, M.n * sizeof( t_uint ) );
if( shift != 0 )
MPI_CHK( mpi_shift_r( &M, shift ) );
- M.n += R.n - adjust; /* Make room for multiplication by R */
+ M.n += R.n; /* Make room for multiplication by R */
/* N = A0 */
if( mask != 0 )