commit c8a917711097eff2775415d1ef68058696194928
author Gilles Peskine <Gilles.Peskine@arm.com> Wed Jan 27 22:30:43 2021 +0100
committer Gilles Peskine <Gilles.Peskine@arm.com> Mon Feb 01 17:28:03 2021 +0100
tree 9818999855284282e71f94258577873801c63fd1
parent 43e89e1b1583a5847520573ddb761bd5a842e70f

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>

library/bignum.c

diff --git a/library/bignum.c b/library/bignum.c
index 9cc5d66..e74a1ad 100644
--- a/library/bignum.c
+++ b/library/bignum.c
@@ -1401,6 +1401,12 @@
     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 )