mbedtls_mpi_sub_abs: fix buffer overflow in error case
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>
diff --git a/ChangeLog.d/mpi_sub_abs.txt b/ChangeLog.d/mpi_sub_abs.txt
new file mode 100644
index 0000000..9f34ee7
--- /dev/null
+++ b/ChangeLog.d/mpi_sub_abs.txt
@@ -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.
diff --git a/library/bignum.c b/library/bignum.c
index 2feb727..f133f6c 100644
--- a/library/bignum.c
+++ b/library/bignum.c
@@ -1354,6 +1354,12 @@
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 )