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: 2019-10-09

Patch-Name: user-group-modes.patch

7 files changed, 63 insertions, 13 deletions

diff --git a/auth-rhosts.c b/auth-rhosts.c
index e81321b49..3bcc73766 100644
--- a/auth-rhosts.c
+++ b/auth-rhosts.c
@@ -260,8 +260,7 @@ auth_rhosts2(struct passwd *pw, const char *client_user, const char *hostname,
                 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: "
@@ -287,8 +286,7 @@ auth_rhosts2(struct passwd *pw, const char *client_user, const char *hostname,
                  * 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 3d31ec860..4152d9c44 100644
--- a/auth.c
+++ b/auth.c
@@ -474,8 +474,7 @@ check_key_in_hostfiles(struct passwd *pw, struct sshkey *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);
diff --git a/misc.c b/misc.c
index 4623b5755..c75a795c2 100644
--- a/misc.c
+++ b/misc.c
@@ -55,8 +55,9 @@
 #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
@@ -1272,6 +1273,55 @@ percent_dollar_expand(const char *string, ...)
 }
 
 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, char **ifname)
 {
 #if defined(CUSTOM_SYS_TUN_OPEN)
@@ -2056,8 +2106,7 @@ safe_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;
@@ -2072,8 +2121,7 @@ safe_path(const char *name, struct stat *stp, const char *pw_dir,
                 strlcpy(buf, cp, sizeof(buf));
 
                 if (stat(buf, &st) == -1 ||
-                    (!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.h b/misc.h
index ab94a79c0..b34c798e7 100644
--- a/misc.h
+++ b/misc.h
@@ -192,6 +192,8 @@ struct notifier_ctx *notify_start(int, const char *, ...)
         __attribute__((format(printf, 2, 3)));
 void    notify_complete(struct notifier_ctx *);
 
+int      secure_permissions(struct stat *st, uid_t uid);
+
 #define MINIMUM(a, b)   (((a) < (b)) ? (a) : (b))
 #define MAXIMUM(a, b)   (((a) > (b)) ? (a) : (b))
 #define ROUNDUP(x, y)   ((((x)+((y)-1))/(y))*(y))
diff --git a/readconf.c b/readconf.c
index 3d0a812b3..f4f273c96 100644
--- a/readconf.c
+++ b/readconf.c
@@ -1967,8 +1967,7 @@ 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 be8e964f0..5d613076c 100644
--- a/ssh.1
+++ b/ssh.1
@@ -1528,6 +1528,8 @@ 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 3ceb800ba..190e1d927 100644
--- a/ssh_config.5
+++ b/ssh_config.5
@@ -2010,6 +2010,8 @@ 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 writable 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