Description: Allow harmless group-writability
 Allow ~/.ssh/config to be group-writable, provided that the group in
 question contains only the file's owner.  Rejected upstream for IMO
 incorrect reasons (e.g. a misunderstanding about the contents of
 gr->gr_mem).  Given that per-user groups and umask 002 are the default
 setup in Debian (for good reasons - this makes operating in setgid
 directories with other groups much easier), we need to permit this.
Author: Colin Watson <cjwatson@debian.org>
Bug: https://bugzilla.mindrot.org/show_bug.cgi?id=1060
Bug-Debian: http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=314347
Last-Update: 2010-02-27

Index: b/readconf.c
===================================================================
--- a/readconf.c
+++ b/readconf.c
@@ -28,6 +28,8 @@
 #include <stdio.h>
 #include <string.h>
 #include <unistd.h>
+#include <pwd.h>
+#include <grp.h>
 
 #include "xmalloc.h"
 #include "ssh.h"
@@ -1000,11 +1002,30 @@
 
 	if (checkperm) {
 		struct stat sb;
+		int bad_modes = 0;
 
 		if (fstat(fileno(f), &sb) == -1)
 			fatal("fstat %s: %s", filename, strerror(errno));
-		if (((sb.st_uid != 0 && sb.st_uid != getuid()) ||
-		    (sb.st_mode & 022) != 0))
+		if (sb.st_uid != 0 && sb.st_uid != getuid())
+			bad_modes = 1;
+		if ((sb.st_mode & 020) != 0) {
+			/* If the file is group-writable, the group in
+			 * question must have at most one member, namely the
+			 * file's owner.
+			 */
+			struct passwd *pw = getpwuid(sb.st_uid);
+			struct group *gr = getgrgid(sb.st_gid);
+			if (!pw || !gr)
+				bad_modes = 1;
+			else if (gr->gr_mem[0]) {
+				if (strcmp(pw->pw_name, gr->gr_mem[0]) ||
+				    gr->gr_mem[1])
+					bad_modes = 1;
+			}
+		}
+		if ((sb.st_mode & 002) != 0)
+			bad_modes = 1;
+		if (bad_modes)
 			fatal("Bad owner or permissions on %s", filename);
 	}
 
Index: b/ssh.1
===================================================================
--- a/ssh.1
+++ b/ssh.1
@@ -1321,6 +1321,8 @@
 .Xr ssh_config 5 .
 Because of the potential for abuse, this file must have strict permissions:
 read/write for the user, and not accessible by others.
+It may be group-writable provided that the group in question contains only
+the user.
 .Pp
 .It ~/.ssh/environment
 Contains additional definitions for environment variables; see
Index: b/ssh_config.5
===================================================================
--- a/ssh_config.5
+++ b/ssh_config.5
@@ -1208,6 +1208,8 @@
 This file is used by the SSH client.
 Because of the potential for abuse, this file must have strict permissions:
 read/write for the user, and not accessible by others.
+It may be group-writable provided that the group in question contains only
+the user.
 .It Pa /etc/ssh/ssh_config
 Systemwide configuration file.
 This file provides defaults for those