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Fix null pointer dereference in mbedtls_mpi_exp_mod

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Fix a null pointer dereference in mbedtls_mpi_exp_mod(X, A, N, E, _RR) when
A is the value 0 represented with 0 limbs.

Make the code a little more robust against similar bugs.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
This commit is contained in:
Gilles Peskine 2021-06-08 23:17:42 +02:00
parent bcfc83f7c8
commit f643e8e8a9
2 changed files with 25 additions and 6 deletions
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27
library/bignum.c
View file

@ -203,7 +203,13 @@ static int mbedtls_mpi_resize_clear( mbedtls_mpi *X, size_t limbs )
}
/*
* Copy the contents of Y into X
* Copy the contents of Y into X.
*
* This function is not constant-time. Leading zeros in Y may be removed.
*
* Ensure that X does not shrink. This is not guaranteed by the public API,
* but some code in the bignum module relies on this property, for example
* in mbedtls_mpi_exp_mod().
*/
int mbedtls_mpi_copy( mbedtls_mpi *X, const mbedtls_mpi *Y )
{
@ -217,7 +223,11 @@ int mbedtls_mpi_copy( mbedtls_mpi *X, const mbedtls_mpi *Y )
if( Y->n == 0 )
{
mbedtls_mpi_free( X );
if( X->n != 0 )
{
X->s = 1;
memset( X->p, 0, X->n * ciL );
}
return( 0 );
}
@ -2175,6 +2185,11 @@ int mbedtls_mpi_exp_mod( mbedtls_mpi *X, const mbedtls_mpi *A,
#endif
j = N->n + 1;
/* All W[i] and X must have at least N->n limbs for the mpi_montmul()
* and mpi_montred() calls later. Here we ensure that W[1] and X are
* large enough, and later we'll grow other W[i] to the same length.
* They must not be shrunk midway through this function!
*/
MBEDTLS_MPI_CHK( mbedtls_mpi_grow( X, j ) );
MBEDTLS_MPI_CHK( mbedtls_mpi_grow( &W[1], j ) );
MBEDTLS_MPI_CHK( mbedtls_mpi_grow( &T, j * 2 ) );
@ -2209,10 +2224,18 @@ int mbedtls_mpi_exp_mod( mbedtls_mpi *X, const mbedtls_mpi *A,
* W[1] = A * R^2 * R^-1 mod N = A * R mod N
*/
if( mbedtls_mpi_cmp_mpi( A, N ) >= 0 )
{
MBEDTLS_MPI_CHK( mbedtls_mpi_mod_mpi( &W[1], A, N ) );
/* This should be a no-op because W[1] is already that large before
* mbedtls_mpi_mod_mpi(), but it's necessary to avoid an overflow
* in mpi_montmul() below, so let's make sure. */
MBEDTLS_MPI_CHK( mbedtls_mpi_grow( &W[1], N->n +1 ) );
}
else
MBEDTLS_MPI_CHK( mbedtls_mpi_copy( &W[1], A ) );
/* Note that this is safe because W[1] always has at least N->n limbs
* (it grew above and was preserved by mbedtls_mpi_copy()). */
mpi_montmul( &W[1], &RR, N, mm, &T );
/*

4
tests/suites/test_suite_mpi.data
View file

@ -1085,19 +1085,15 @@ Base test mbedtls_mpi_exp_mod #6 (Negative base + exponent)
mbedtls_mpi_exp_mod:10:"-23":10:"-13":10:"29":10:"0":MBEDTLS_ERR_MPI_BAD_INPUT_DATA
Test mbedtls_mpi_exp_mod: 0 (null) ^ 0 (null) mod 9
depends_on:TEST_TEMPORARILY_DISABLED_BECAUSE_IT_CAUSES_A_CRASH
mbedtls_mpi_exp_mod:16:"":16:"":16:"09":16:"1":0
Test mbedtls_mpi_exp_mod: 0 (null) ^ 0 (1 limb) mod 9
depends_on:TEST_TEMPORARILY_DISABLED_BECAUSE_IT_CAUSES_A_CRASH
mbedtls_mpi_exp_mod:16:"":16:"00":16:"09":16:"1":0
Test mbedtls_mpi_exp_mod: 0 (null) ^ 1 mod 9
depends_on:TEST_TEMPORARILY_DISABLED_BECAUSE_IT_CAUSES_A_CRASH
mbedtls_mpi_exp_mod:16:"":16:"01":16:"09":16:"":0
Test mbedtls_mpi_exp_mod: 0 (null) ^ 2 mod 9
depends_on:TEST_TEMPORARILY_DISABLED_BECAUSE_IT_CAUSES_A_CRASH
mbedtls_mpi_exp_mod:16:"":16:"02":16:"09":16:"":0
Test mbedtls_mpi_exp_mod: 0 (1 limb) ^ 0 (null) mod 9