From 7b931d36ad36a93d2b1811858ca29408ec44ecae Mon Sep 17 00:00:00 2001 From: Colin Watson 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: 2017-10-04 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 57296e1f6..546aa0495 100644 --- a/auth-rhosts.c +++ b/auth-rhosts.c @@ -261,8 +261,7 @@ 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: " @@ -288,8 +287,7 @@ 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 d8e6b4a3d..9d1d453f1 100644 --- a/auth.c +++ b/auth.c @@ -473,8 +473,7 @@ 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 bdc06fdb3..5159e6692 100644 --- a/misc.c +++ b/misc.c @@ -58,8 +58,9 @@ #include #ifdef HAVE_PATHS_H # include -#include #endif +#include +#include #ifdef SSH_TUN_OPENBSD #include #endif @@ -1028,6 +1029,55 @@ 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) { @@ -1786,8 +1836,7 @@ 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; @@ -1802,8 +1851,7 @@ safe_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.h b/misc.h index 31b207a8d..aaf966e65 100644 --- a/misc.h +++ b/misc.h @@ -168,6 +168,8 @@ int safe_path_fd(int, const char *, struct passwd *, char *read_passphrase(const char *, int); int ask_permission(const char *, ...) __attribute__((format(printf, 1, 2))); +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 052d4b1ac..6b01f20d2 100644 --- a/readconf.c +++ b/readconf.c @@ -1820,8 +1820,7 @@ 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 7760c3075..81f29af43 100644 --- a/ssh.1 +++ b/ssh.1 @@ -1485,6 +1485,8 @@ 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 54e143c93..7d55fa820 100644 --- a/ssh_config.5 +++ b/ssh_config.5 @@ -1835,6 +1835,8 @@ 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