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ECP: Prevent freeing a buffer on stack

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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
This commit is contained in:
Janos Follath
2017-01-27 16:05:20 +00:00
committed by Simon Butcher
parent 28fff14113
commit 7dadc2f259
2 changed files with 11 additions and 3 deletions
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6
library/ecp_curves.c
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@ -1213,7 +1213,7 @@ static inline int ecp_mod_koblitz( mbedtls_mpi *N, mbedtls_mpi_uint *Rp, size_t
int ret;
size_t i;
mbedtls_mpi M, R;
mbedtls_mpi_uint Mp[P_KOBLITZ_MAX + P_KOBLITZ_R];
mbedtls_mpi_uint Mp[P_KOBLITZ_MAX + P_KOBLITZ_R + 1];
if( N->n < p_limbs )
return( 0 );
@ -1235,7 +1235,7 @@ static inline int ecp_mod_koblitz( mbedtls_mpi *N, mbedtls_mpi_uint *Rp, size_t
memcpy( Mp, N->p + p_limbs - adjust, M.n * sizeof( mbedtls_mpi_uint ) );
if( shift != 0 )
MBEDTLS_MPI_CHK( mbedtls_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 )
@ -1257,7 +1257,7 @@ static inline int ecp_mod_koblitz( mbedtls_mpi *N, mbedtls_mpi_uint *Rp, size_t
memcpy( Mp, N->p + p_limbs - adjust, M.n * sizeof( mbedtls_mpi_uint ) );
if( shift != 0 )
MBEDTLS_MPI_CHK( mbedtls_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 )