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mpi_exp_mod: remove memory ownership confusion

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Elements of W didn't all have the same owner: all were owned by this
function, except W[x_index]. It is more robust if we make a proper copy
of X.

Signed-off-by: Janos Follath <janos.follath@arm.com>
This commit is contained in:
Janos Follath 2022-11-21 14:31:22 +00:00
parent f08b40eaab
commit 844614814e
1 changed files with 19 additions and 17 deletions
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36
library/bignum.c
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@ -2012,16 +2012,6 @@ int mbedtls_mpi_exp_mod( mbedtls_mpi *X, const mbedtls_mpi *A,
wsize = MBEDTLS_MPI_WINDOW_SIZE;
#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 ) );
/*
* If we call mpi_montmul() without doing a table lookup first, we leak
* through timing side channels the fact that a squaring is happening. In
@ -2030,14 +2020,23 @@ int mbedtls_mpi_exp_mod( mbedtls_mpi *X, const mbedtls_mpi *A,
* To prevent this leak, we append the output variable to the end of the
* table. This allows as to always do a constant time lookup whenever we
* call mpi_montmul().
*
* To achieve this, we make a copy of X and we use the table entry in each
* calculation from this point on.
*/
const size_t x_index = w_count - 1;
/*
* To prevent the leak, we need to use the table entry in each calculation
* from this point on. This makes it safe to load X into the table by a
* simple assignment.
mbedtls_mpi_init( &W[x_index] );
mbedtls_mpi_copy( &W[x_index], X );
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!
*/
W[x_index] = *X;
MBEDTLS_MPI_CHK( mbedtls_mpi_grow( &W[x_index], j ) );
MBEDTLS_MPI_CHK( mbedtls_mpi_grow( &W[1], j ) );
MBEDTLS_MPI_CHK( mbedtls_mpi_grow( &T, j * 2 ) );
/*
* Compensate for negative A (and correct at the end)
@ -2214,14 +2213,17 @@ int mbedtls_mpi_exp_mod( mbedtls_mpi *X, const mbedtls_mpi *A,
/*
* Load the result in the output variable.
*/
*X = W[x_index];
mbedtls_mpi_copy( X, &W[x_index] );
cleanup:
for( i = ( one << ( wsize - 1 ) ); i < ( one << wsize ); i++ )
mbedtls_mpi_free( &W[i] );
mbedtls_mpi_free( &W[1] ); mbedtls_mpi_free( &T ); mbedtls_mpi_free( &Apos );
mbedtls_mpi_free( &W[1] );
mbedtls_mpi_free( &W[x_index] );
mbedtls_mpi_free( &T );
mbedtls_mpi_free( &Apos );
mbedtls_mpi_free( &WW );
if( prec_RR == NULL || prec_RR->p == NULL )