From d08cd2b0cfbedf3ccd2ec3adaef850b8d9a87e85 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: 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 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 #ifdef HAVE_PATHS_H # include -#include #endif +#include +#include #ifdef SSH_TUN_OPENBSD #include #endif @@ -1271,6 +1272,55 @@ percent_dollar_expand(const char *string, ...) return ret; } +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) { @@ -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