Add some sanity checks around usage of t_fromb64()

The internal SRP function t_fromb64() converts from base64 to binary. It
does not validate that the size of the destination is sufficiently large -
that is up to the callers. In some places there was such a check, but not
in others.

Add an argument to t_fromb64() to provide the size of the destination
buffer and validate that we don't write too much data. Also add some sanity
checks to the callers where appropriate.

With thanks to Shi Lei for reporting this issue.
Reviewed-by: NRichard Levitte <levitte@openssl.org>

crypto/srp/srp_vfy.c

@@ -30,7 +30,7 @@ static char b64table[] =
 /*
  * Convert a base64 string into raw byte array representation.
  */
-static int t_fromb64(unsigned char *a, const char *src)
+static int t_fromb64(unsigned char *a, size_t alen, const char *src)
 {
     char *loc;
     int i, j;
@@ -39,6 +39,9 @@ static int t_fromb64(unsigned char *a, const char *src)
     while (*src && (*src == ' ' || *src == '\t' || *src == '\n'))
         ++src;
     size = strlen(src);
+    if (alen > INT_MAX || size > (int)alen)
+        return -1;
+
     i = 0;
     while (i < size) {
         loc = strchr(b64table, src[i]);
@@ -181,13 +184,25 @@ static int SRP_user_pwd_set_sv(SRP_user_pwd *vinfo, const char *s,
     unsigned char tmp[MAX_LEN];
     int len;
 
-    if (strlen(s) > MAX_LEN || strlen(v) > MAX_LEN)
+    vinfo->v = NULL;
+    vinfo->s = NULL;
+
+    len = t_fromb64(tmp, sizeof(tmp), v);
+    if (len < 0)
         return 0;
-    len = t_fromb64(tmp, v);
     if (NULL == (vinfo->v = BN_bin2bn(tmp, len, NULL)))
         return 0;
-    len = t_fromb64(tmp, s);
-    return ((vinfo->s = BN_bin2bn(tmp, len, NULL)) != NULL);
+    len = t_fromb64(tmp, sizeof(tmp), s);
+    if (len < 0)
+        goto err;
+    vinfo->s = BN_bin2bn(tmp, len, NULL);
+    if (vinfo->s == NULL)
+        goto err;
+    return 1;
+ err:
+    BN_free(vinfo->v);
+    vinfo->v = NULL;
+    return 0;
 }
 
 static int SRP_user_pwd_set_sv_BN(SRP_user_pwd *vinfo, BIGNUM *s, BIGNUM *v)
@@ -257,10 +272,13 @@ static SRP_gN_cache *SRP_gN_new_init(const char *ch)
     if (newgN == NULL)
         return NULL;
 
+    len = t_fromb64(tmp, sizeof(tmp), ch);
+    if (len < 0)
+        goto err;
+
     if ((newgN->b64_bn = OPENSSL_strdup(ch)) == NULL)
         goto err;
 
-    len = t_fromb64(tmp, ch);
     if ((newgN->bn = BN_bin2bn(tmp, len, NULL)))
         return newgN;
 
@@ -539,11 +557,11 @@ char *SRP_create_verifier(const char *user, const char *pass, char **salt,
         goto err;
 
     if (N) {
-        if ((len = t_fromb64(tmp, N)) == 0)
+        if ((len = t_fromb64(tmp, sizeof(tmp), N)) <= 0)
             goto err;
         N_bn_alloc = BN_bin2bn(tmp, len, NULL);
         N_bn = N_bn_alloc;
-        if ((len = t_fromb64(tmp, g)) == 0)
+        if ((len = t_fromb64(tmp, sizeof(tmp) ,g)) <= 0)
             goto err;
         g_bn_alloc = BN_bin2bn(tmp, len, NULL);
         g_bn = g_bn_alloc;
@@ -563,7 +581,7 @@ char *SRP_create_verifier(const char *user, const char *pass, char **salt,
 
         s = BN_bin2bn(tmp2, SRP_RANDOM_SALT_LEN, NULL);
     } else {
-        if ((len = t_fromb64(tmp2, *salt)) == 0)
+        if ((len = t_fromb64(tmp2, sizeof(tmp2), *salt)) <= 0)
             goto err;
         s = BN_bin2bn(tmp2, len, NULL);
     }