Prevent an overflow in inet_net_pton(3) when the passed in buffer is

too small in the AF_INET6 case.
Spotted by Brad House (brad AT brad-house.com) with the c-ares
regression test.

The man page says
     Caution: The dst field should be zeroed before calling inet_net_pton() as
     the function will only fill the number of bytes necessary to encode the
     network number in network byte order.

Which seems to suggest that the function should work if the passed in
storage is big enough to hold the prefix, which might be smaller than
sizeof(in6_addr).

Input & OK tb

1 files changed, 18 insertions, 11 deletions

diff --git a/src/lib/libc/net/inet_net_pton.c b/src/lib/libc/net/inet_net_pton.c
index 2aaeac4048..aaffc1802a 100644
--- a/src/lib/libc/net/inet_net_pton.c
+++ b/src/lib/libc/net/inet_net_pton.c
@@ -1,4 +1,4 @@
-/*      $OpenBSD: inet_net_pton.c,v 1.10 2017/03/06 18:16:27 millert Exp $      */
+/*      $OpenBSD: inet_net_pton.c,v 1.11 2021/01/19 16:43:44 florian Exp $      */
 
 /*
  * Copyright (c) 2012 by Gilles Chehade <gilles@openbsd.org>
@@ -205,9 +205,10 @@ inet_net_pton_ipv4(const char *src, u_char *dst, size_t size)
 static int
 inet_net_pton_ipv6(const char *src, u_char *dst, size_t size)
 {
-        int     ret;
-        int     bits;
-        char    buf[sizeof("xxxx:xxxx:xxxx:xxxx:xxxx:xxxx:255:255:255:255/128")];
+        struct in6_addr  in6;
+        int              ret;
+        int              bits;
+        char             buf[INET6_ADDRSTRLEN + sizeof("/128")];
         char            *sep;
         const char      *errstr;
 
@@ -220,18 +221,24 @@ inet_net_pton_ipv6(const char *src, u_char *dst, size_t size)
         if (sep != NULL)
                 *sep++ = '\0';
 
-        ret = inet_pton(AF_INET6, buf, dst);
+        ret = inet_pton(AF_INET6, buf, &in6);
         if (ret != 1)
                 return (-1);
 
         if (sep == NULL)
-                return 128;
+                bits = 128;
+        else {
+                bits = strtonum(sep, 0, 128, &errstr);
+                if (errstr) {
+                        errno = EINVAL;
+                        return (-1);
+                }
+        }
 
-        bits = strtonum(sep, 0, 128, &errstr);
-        if (errstr) {
-                errno = EINVAL;
+        if ((bits + 7) / 8 > size) {
+                errno = EMSGSIZE;
                 return (-1);
         }
-
-        return bits;
+        memcpy(dst, &in6.s6_addr, size);
+        return (bits);
 }