nss_dns: Check for proper A/AAAA address alignment

Reviewed-by: Carlos O'Donell <carlos@redhat.com>

2 files changed, 18 insertions, 2 deletions

diff --git a/ChangeLog b/ChangeLog
index 5f21fe463a..b46b581707 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+2019-05-24  Florian Weimer  <fweimer@redhat.com>
+
+	* resolv/nss_dns/dns-host.c (getanswer_r): Be more explicit about
+	struct in_addr/struct in6_addr alignment.
+
 2019-05-23  Joseph Myers  <joseph@codesourcery.com>
 
 	* sysdeps/unix/sysv/linux/bits/fcntl-linux.h [__USE_GNU]
diff --git a/resolv/nss_dns/dns-host.c b/resolv/nss_dns/dns-host.c
index 9c15f25f28..5af47fd10d 100644
--- a/resolv/nss_dns/dns-host.c
+++ b/resolv/nss_dns/dns-host.c
@@ -78,6 +78,7 @@
 #include <stdlib.h>
 #include <stddef.h>
 #include <string.h>
+#include <libc-pointer-arith.h>
 
 #include "nsswitch.h"
 #include <arpa/nameser.h>
@@ -947,8 +948,18 @@ getanswer_r (struct resolv_context *ctx,
 	      linebuflen -= nn;
 	    }
 
-	  linebuflen -= sizeof (align) - ((u_long) bp % sizeof (align));
-	  bp += sizeof (align) - ((u_long) bp % sizeof (align));
+	  /* Provide sufficient alignment for both address
+	     families.  */
+	  enum { align = 4 };
+	  _Static_assert ((align % __alignof__ (struct in_addr)) == 0,
+			  "struct in_addr alignment");
+	  _Static_assert ((align % __alignof__ (struct in6_addr)) == 0,
+			  "struct in6_addr alignment");
+	  {
+	    char *new_bp = PTR_ALIGN_UP (bp, align);
+	    linebuflen -= new_bp - bp;
+	    bp = new_bp;
+	  }
 
 	  if (__glibc_unlikely (n > linebuflen))
 	    goto too_small;