commit de1629aff99f2701cdec0b26170b0c6e803479f6
author Gilles Peskine <Gilles.Peskine@arm.com> Tue Nov 15 23:02:14 2022 +0100
committer Gilles Peskine <Gilles.Peskine@arm.com> Thu Nov 17 11:03:46 2022 +0100
tree 892937a19d5b951d15b073d5d35535f1b7519e92
parent 103cf59e461d03a331f27aad3cc7015f7906354d

Fix undefined behavior with the most negative mbedtls_mpi_sint

When x is the most negative value of a two's complement type,
`(unsigned_type)(-x)` has undefined behavior, whereas `-(unsigned_type)x`
has well-defined behavior and does what was intended.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>

library/bignum.c

diff --git a/library/bignum.c b/library/bignum.c
index e52afba..8ed9804 100644
--- a/library/bignum.c
+++ b/library/bignum.c
@@ -273,7 +273,7 @@
     MBEDTLS_MPI_CHK( mbedtls_mpi_grow( X, 1 ) );
     memset( X->p, 0, X->n * ciL );
 
-    X->p[0] = ( z < 0 ) ? -z : z;
+    X->p[0] = ( z < 0 ) ? -(mbedtls_mpi_uint)z : z;
     X->s    = ( z < 0 ) ? -1 : 1;
 
 cleanup:
@@ -1093,7 +1093,7 @@
     mbedtls_mpi_uint p[1];
     MPI_VALIDATE_RET( X != NULL );
 
-    *p  = ( z < 0 ) ? -z : z;
+    *p  = ( z < 0 ) ? -(mbedtls_mpi_uint)z : z;
     Y.s = ( z < 0 ) ? -1 : 1;
     Y.n = 1;
     Y.p = p;
@@ -1334,7 +1334,7 @@
     MPI_VALIDATE_RET( X != NULL );
     MPI_VALIDATE_RET( A != NULL );
 
-    p[0] = ( b < 0 ) ? -b : b;
+    p[0] = ( b < 0 ) ? -(mbedtls_mpi_uint)b : b;
     B.s = ( b < 0 ) ? -1 : 1;
     B.n = 1;
     B.p = p;
@@ -1352,7 +1352,7 @@
     MPI_VALIDATE_RET( X != NULL );
     MPI_VALIDATE_RET( A != NULL );
 
-    p[0] = ( b < 0 ) ? -b : b;
+    p[0] = ( b < 0 ) ? -(mbedtls_mpi_uint)b : b;
     B.s = ( b < 0 ) ? -1 : 1;
     B.n = 1;
     B.p = p;
@@ -1773,7 +1773,7 @@
     mbedtls_mpi_uint p[1];
     MPI_VALIDATE_RET( A != NULL );
 
-    p[0] = ( b < 0 ) ? -b : b;
+    p[0] = ( b < 0 ) ? -(mbedtls_mpi_uint)b : b;
     B.s = ( b < 0 ) ? -1 : 1;
     B.n = 1;
     B.p = p;