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Description: 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.
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
@@ -30,6 +30,8 @@
#include <stdio.h>
#include <string.h>
#include <unistd.h>
+#include <pwd.h>
+#include <grp.h>
#include "xmalloc.h"
#include "ssh.h"
@@ -1085,8 +1087,7 @@
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);
}
Index: b/ssh.1
===================================================================
--- a/ssh.1
+++ b/ssh.1
@@ -1293,6 +1293,8 @@
.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 Pa ~/.ssh/environment
Contains additional definitions for environment variables; see
Index: b/ssh_config.5
===================================================================
--- a/ssh_config.5
+++ b/ssh_config.5
@@ -1299,6 +1299,8 @@
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
Index: b/auth.c
===================================================================
--- a/auth.c
+++ b/auth.c
@@ -392,8 +392,7 @@
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);
@@ -454,8 +453,7 @@
/* check the open file to avoid races */
if (fstat(fileno(f), &st) < 0 ||
- (st.st_uid != 0 && st.st_uid != uid) ||
- (st.st_mode & 022) != 0) {
+ !secure_permissions(&st, uid)) {
snprintf(err, errlen, "bad ownership or modes for file %s",
buf);
return -1;
@@ -471,8 +469,7 @@
debug3("secure_filename: checking '%s'", buf);
if (stat(buf, &st) < 0 ||
- (st.st_uid != 0 && 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;
Index: b/misc.c
===================================================================
--- a/misc.c
+++ b/misc.c
@@ -48,8 +48,9 @@
#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
@@ -642,6 +643,55 @@
}
int
+secure_permissions(struct stat *st, uid_t uid)
+{
+ if (st->st_uid != 0 && 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)
{
#if defined(CUSTOM_SYS_TUN_OPEN)
Index: b/misc.h
===================================================================
--- a/misc.h
+++ b/misc.h
@@ -102,4 +102,6 @@
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 */
Index: b/auth-rhosts.c
===================================================================
--- a/auth-rhosts.c
+++ b/auth-rhosts.c
@@ -256,8 +256,7 @@
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: "
@@ -283,8 +282,7 @@
* 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);
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