From 984fd07c53ec2b753cfe68fa317ca3f8240e2536 Mon Sep 17 00:00:00 2001
From: Gilles Peskine <Gilles.Peskine@arm.com>
Date: Sat, 3 Apr 2021 18:26:13 +0200
Subject: [PATCH] Fix and simplify sign handling in mbedtls_mpi_read_string

Move the handling of the sign out of the base-specific loops. This
both simplifies the code, and corrects an edge case: the code in the
non-hexadecimal case depended on mbedtls_mpi_mul_int() preserving the
sign bit when multiplying a "negative zero" MPI by an integer, which
used to be the case but stopped with PR #2512.

Fix #4295. Thanks to Guido Vranken for analyzing the cause of the bug.
Credit to OSS-Fuzz.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
---
 library/bignum.c | 32 +++++++++++---------------------
 1 file changed, 11 insertions(+), 21 deletions(-)

diff --git a/library/bignum.c b/library/bignum.c
index f133f6c13..bd352e1bb 100644
--- a/library/bignum.c
+++ b/library/bignum.c
@@ -500,6 +500,7 @@ int mbedtls_mpi_read_string( mbedtls_mpi *X, int radix, const char *s )
 {
     int ret;
     size_t i, j, slen, n;
+    int sign = 1;
     mbedtls_mpi_uint d;
     mbedtls_mpi T;
     MPI_VALIDATE_RET( X != NULL );
@@ -510,6 +511,12 @@ int mbedtls_mpi_read_string( mbedtls_mpi *X, int radix, const char *s )
 
     mbedtls_mpi_init( &T );
 
+    if( s[0] == '-' )
+    {
+        ++s;
+        sign = -1;
+    }
+
     slen = strlen( s );
 
     if( radix == 16 )
@@ -524,12 +531,6 @@ int mbedtls_mpi_read_string( mbedtls_mpi *X, int radix, const char *s )
 
         for( i = slen, j = 0; i > 0; i--, j++ )
         {
-            if( i == 1 && s[i - 1] == '-' )
-            {
-                X->s = -1;
-                break;
-            }
-
             MBEDTLS_MPI_CHK( mpi_get_digit( &d, radix, s[i - 1] ) );
             X->p[j / ( 2 * ciL )] |= d << ( ( j % ( 2 * ciL ) ) << 2 );
         }
@@ -540,26 +541,15 @@ int mbedtls_mpi_read_string( mbedtls_mpi *X, int radix, const char *s )
 
         for( i = 0; i < slen; i++ )
         {
-            if( i == 0 && s[i] == '-' )
-            {
-                X->s = -1;
-                continue;
-            }
-
             MBEDTLS_MPI_CHK( mpi_get_digit( &d, radix, s[i] ) );
             MBEDTLS_MPI_CHK( mbedtls_mpi_mul_int( &T, X, radix ) );
-
-            if( X->s == 1 )
-            {
-                MBEDTLS_MPI_CHK( mbedtls_mpi_add_int( X, &T, d ) );
-            }
-            else
-            {
-                MBEDTLS_MPI_CHK( mbedtls_mpi_sub_int( X, &T, d ) );
-            }
+            MBEDTLS_MPI_CHK( mbedtls_mpi_add_int( X, &T, d ) );
         }
     }
 
+    if( sign < 0 && mbedtls_mpi_bitlen( X ) != 0 )
+        X->s = -1;
+
 cleanup:
 
     mbedtls_mpi_free( &T );