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From cb72edd9757c469f3b5dc9cde374715ae8b54509 Mon Sep 17 00:00:00 2001
From: Colin Watson <cjwatson@debian.org>
Date: Sun, 9 Feb 2014 16:09:58 +0000
Subject: 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
---
auth-rhosts.c | 6 ++----
auth.c | 3 +--
misc.c | 58 ++++++++++++++++++++++++++++++++++++++++++++++-----
misc.h | 2 ++
readconf.c | 3 +--
ssh.1 | 2 ++
ssh_config.5 | 2 ++
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 687c57b42..aed3c13ac 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 554ceb0b1..75fe4dfea 100644
--- a/misc.c
+++ b/misc.c
@@ -61,8 +61,9 @@
#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
@@ -1124,6 +1125,55 @@ percent_expand(const char *string, ...)
#undef EXPAND_MAX_KEYS
}
+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)
{
@@ -1909,8 +1959,7 @@ 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;
@@ -1925,8 +1974,7 @@ 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 4a05db2da..5db594b91 100644
--- a/misc.h
+++ b/misc.h
@@ -188,6 +188,8 @@ 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 431243193..5bf0afbb4 100644
--- a/readconf.c
+++ b/readconf.c
@@ -1926,8 +1926,7 @@ 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 7a3ba31ab..a80be8efe 100644
--- a/ssh.1
+++ b/ssh.1
@@ -1509,6 +1509,8 @@ 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 85ab7447f..d814147d4 100644
--- a/ssh_config.5
+++ b/ssh_config.5
@@ -1957,6 +1957,8 @@ 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
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