Allow harmless group-writability

Allow secure files (~/.ssh/config, ~/.ssh/authorized_keys, etc.) 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 by
default.

Bug: https://bugzilla.mindrot.org/show_bug.cgi?id=1060
Bug-Debian: http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=314347
Last-Update: 2013-09-14

Patch-Name: user-group-modes.patch

8 files changed, 80 insertions, 29 deletions

diff --git a/auth-rhosts.c b/auth-rhosts.c
index 0ef344712..c17c13cc1 100644
--- a/auth-rhosts.c
+++ b/auth-rhosts.c
@@ -273,8 +273,7 @@ auth_rhosts2_raw(struct passwd *pw, const char *client_user, const char *hostnam
                 return 0;
         }
         if (options.strict_modes &&
-            ((st.st_uid != 0 && st.st_uid != pw->pw_uid) ||
-            (st.st_mode & 022) != 0)) {
+            !secure_permissions(&st, pw->pw_uid)) {
                 logit("Rhosts authentication refused for %.100s: "
                     "bad ownership or modes for home directory.", pw->pw_name);
                 auth_debug_add("Rhosts authentication refused for %.100s: "
@@ -300,8 +299,7 @@ auth_rhosts2_raw(struct passwd *pw, const char *client_user, const char *hostnam
                  * allowing access to their account by anyone.
                  */
                 if (options.strict_modes &&
-                    ((st.st_uid != 0 && st.st_uid != pw->pw_uid) ||
-                    (st.st_mode & 022) != 0)) {
+                    !secure_permissions(&st, pw->pw_uid)) {
                         logit("Rhosts authentication refused for %.100s: bad modes for %.200s",
                             pw->pw_name, buf);
                         auth_debug_add("Bad file modes for %.200s", buf);
diff --git a/auth.c b/auth.c
index f56dcc6cf..3f8b348fc 100644
--- a/auth.c
+++ b/auth.c
@@ -435,8 +435,7 @@ check_key_in_hostfiles(struct passwd *pw, Key *key, const char *host,
                 user_hostfile = tilde_expand_filename(userfile, pw->pw_uid);
                 if (options.strict_modes &&
                     (stat(user_hostfile, &st) == 0) &&
-                    ((st.st_uid != 0 && st.st_uid != pw->pw_uid) ||
-                    (st.st_mode & 022) != 0)) {
+                    !secure_permissions(&st, pw->pw_uid)) {
                         logit("Authentication refused for %.100s: "
                             "bad owner or modes for %.200s",
                             pw->pw_name, user_hostfile);
@@ -498,8 +497,7 @@ auth_secure_path(const char *name, struct stat *stp, const char *pw_dir,
                 snprintf(err, errlen, "%s is not a regular file", buf);
                 return -1;
         }
-        if ((!platform_sys_dir_uid(stp->st_uid) && stp->st_uid != uid) ||
-            (stp->st_mode & 022) != 0) {
+        if (!secure_permissions(stp, uid)) {
                 snprintf(err, errlen, "bad ownership or modes for file %s",
                     buf);
                 return -1;
@@ -514,8 +512,7 @@ auth_secure_path(const char *name, struct stat *stp, const char *pw_dir,
                 strlcpy(buf, cp, sizeof(buf));
 
                 if (stat(buf, &st) < 0 ||
-                    (!platform_sys_dir_uid(st.st_uid) && st.st_uid != uid) ||
-                    (st.st_mode & 022) != 0) {
+                    !secure_permissions(&st, uid)) {
                         snprintf(err, errlen,
                             "bad ownership or modes for directory %s", buf);
                         return -1;
diff --git a/misc.c b/misc.c
index 9421b4d39..68efb2b38 100644
--- a/misc.c
+++ b/misc.c
@@ -51,8 +51,9 @@
 #include <netdb.h>
 #ifdef HAVE_PATHS_H
 # include <paths.h>
-#include <pwd.h>
 #endif
+#include <pwd.h>
+#include <grp.h>
 #ifdef SSH_TUN_OPENBSD
 #include <net/if.h>
 #endif
@@ -61,6 +62,7 @@
 #include "misc.h"
 #include "log.h"
 #include "ssh.h"
+#include "platform.h"
 
 /* remove newline at end of string */
 char *
@@ -708,6 +710,71 @@ read_keyfile_line(FILE *f, const char *filename, char *buf, size_t bufsz,
         return -1;
 }
 
+/*
+ * return 1 if the specified uid is a uid that may own a system directory
+ * otherwise 0.
+ */
+int
+platform_sys_dir_uid(uid_t uid)
+{
+        if (uid == 0)
+                return 1;
+#ifdef PLATFORM_SYS_DIR_UID
+        if (uid == PLATFORM_SYS_DIR_UID)
+                return 1;
+#endif
+        return 0;
+}
+
+int
+secure_permissions(struct stat *st, uid_t uid)
+{
+        if (!platform_sys_dir_uid(st->st_uid) && st->st_uid != uid)
+                return 0;
+        if ((st->st_mode & 002) != 0)
+                return 0;
+        if ((st->st_mode & 020) != 0) {
+                /* If the file is group-writable, the group in question must
+                 * have exactly one member, namely the file's owner.
+                 * (Zero-member groups are typically used by setgid
+                 * binaries, and are unlikely to be suitable.)
+                 */
+                struct passwd *pw;
+                struct group *gr;
+                int members = 0;
+
+                gr = getgrgid(st->st_gid);
+                if (!gr)
+                        return 0;
+
+                /* Check primary group memberships. */
+                while ((pw = getpwent()) != NULL) {
+                        if (pw->pw_gid == gr->gr_gid) {
+                                ++members;
+                                if (pw->pw_uid != uid)
+                                        return 0;
+                        }
+                }
+                endpwent();
+
+                pw = getpwuid(st->st_uid);
+                if (!pw)
+                        return 0;
+
+                /* Check supplementary group memberships. */
+                if (gr->gr_mem[0]) {
+                        ++members;
+                        if (strcmp(pw->pw_name, gr->gr_mem[0]) ||
+                            gr->gr_mem[1])
+                                return 0;
+                }
+
+                if (!members)
+                        return 0;
+        }
+        return 1;
+}
+
 int
 tun_open(int tun, int mode)
 {
diff --git a/misc.h b/misc.h
index 7c76a6a72..42cd95e42 100644
--- a/misc.h
+++ b/misc.h
@@ -139,4 +139,6 @@ char	*read_passphrase(const char *, int);
 int      ask_permission(const char *, ...) __attribute__((format(printf, 1, 2)));
 int      read_keyfile_line(FILE *, const char *, char *, size_t, u_long *);
 
+int      secure_permissions(struct stat *st, uid_t uid);
+
 #endif /* _MISC_H */
diff --git a/platform.c b/platform.c
index 483170680..2ce4dbf81 100644
--- a/platform.c
+++ b/platform.c
@@ -199,19 +199,3 @@ platform_krb5_get_principal_name(const char *pw_name)
         return NULL;
 #endif
 }
-
-/*
- * return 1 if the specified uid is a uid that may own a system directory
- * otherwise 0.
- */
-int
-platform_sys_dir_uid(uid_t uid)
-{
-        if (uid == 0)
-                return 1;
-#ifdef PLATFORM_SYS_DIR_UID
-        if (uid == PLATFORM_SYS_DIR_UID)
-                return 1;
-#endif
-        return 0;
-}
diff --git a/readconf.c b/readconf.c
index 3a6c67ba5..f6b4c8f1e 100644
--- a/readconf.c
+++ b/readconf.c
@@ -1753,8 +1753,7 @@ read_config_file_depth(const char *filename, struct passwd *pw,
 
                 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 (!secure_permissions(&sb, getuid()))
                         fatal("Bad owner or permissions on %s", filename);
         }
 
diff --git a/ssh.1 b/ssh.1
index 4011c65aa..feef81a54 100644
--- a/ssh.1
+++ b/ssh.1
@@ -1484,6 +1484,8 @@ The file format and configuration options are described in
 .Xr ssh_config 5 .
 Because of the potential for abuse, this file must have strict permissions:
 read/write for the user, and not writable by others.
+It may be group-writable provided that the group in question contains only
+the user.
 .Pp
 .It Pa ~/.ssh/environment
 Contains additional definitions for environment variables; see
diff --git a/ssh_config.5 b/ssh_config.5
index efc265ab7..5dd26bcaa 100644
--- a/ssh_config.5
+++ b/ssh_config.5
@@ -1903,6 +1903,8 @@ The format of this file is described above.
 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