From 51c2e06eb85bf18de30d327b25c0d58e74f5de3d Mon Sep 17 00:00:00 2001
From: Gilles Peskine <Gilles.Peskine@arm.com>
Date: Mon, 20 Jan 2020 21:12:50 +0100
Subject: [PATCH] mpi_copy: make the 0 case slightly more robust

If Y was constructed through functions in this module, then Y->n == 0
iff Y->p == NULL. However we do not prevent filling mpi structures
manually, and zero may be represented with n=0 and p a valid pointer.
Most of the code can cope with such a representation, but for the
source of mbedtls_mpi_copy, this would cause an integer underflow.
Changing the test for zero from Y->p==NULL to Y->n==0 causes this case
to work at no extra cost.
---
 library/bignum.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/library/bignum.c b/library/bignum.c
index a0964e348..0aee14924 100644
--- a/library/bignum.c
+++ b/library/bignum.c
@@ -198,7 +198,7 @@ int mbedtls_mpi_copy( mbedtls_mpi *X, const mbedtls_mpi *Y )
     if( X == Y )
         return( 0 );
 
-    if( Y->p == NULL )
+    if( Y->n == 0 )
     {
         mbedtls_mpi_free( X );
         return( 0 );