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mbedtls_mpi_sub_abs: fix buffer overflow in error case

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Fix a buffer overflow in mbedtls_mpi_sub_abs() when calculating
|A| - |B| where |B| is larger than |A| and has more limbs (so the
function should return MBEDTLS_ERR_MPI_NEGATIVE_VALUE).

Fix #4042

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
This commit is contained in:
Gilles Peskine 2021-01-27 22:30:43 +01:00
parent 9a3cf3174d
commit 6260b70717
2 changed files with 13 additions and 0 deletions
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7
ChangeLog.d/mpi_sub_abs.txt Normal file
View file

@ -0,0 +1,7 @@
Security
* Fix a buffer overflow in mbedtls_mpi_sub_abs() when calculating
|A| - |B| where |B| is larger than |A| and has more limbs (so the
function should return MBEDTLS_ERR_MPI_NEGATIVE_VALUE). Only
applications calling mbedtls_mpi_sub_abs() directly are affected:
all calls inside the library were safe since this function is
only called with |A| >= |B|. Reported by Guido Vranken in #4042.

6
library/bignum.c
View file

@ -1354,6 +1354,12 @@ int mbedtls_mpi_sub_abs( mbedtls_mpi *X, const mbedtls_mpi *A, const mbedtls_mpi
for( n = B->n; n > 0; n-- )
if( B->p[n - 1] != 0 )
break;
if( n > A->n )
{
/* B >= (2^ciL)^n > A */
ret = MBEDTLS_ERR_MPI_NEGATIVE_VALUE;
goto cleanup;
}
carry = mpi_sub_hlp( n, X->p, B->p );
if( carry != 0 )