From: Francis Deslauriers Date: Fri, 23 Jul 2021 20:27:00 +0000 (-0400) Subject: Fix: runas: supplementary groups are ignored on lttng save X-Git-Url: http://drtracing.org/?a=commitdiff_plain;h=55f2155d3a7ddd376cc705dc8431c95ee233b0f9;p=lttng-tools.git Fix: runas: supplementary groups are ignored on lttng save Observed issue ============== On `lttng save` the following is reported to the user: $ sudo -u my_user lttng save -o /tmp/my_dir my_session_name Error: Permission denied Note that: * the running lttng-sessiond is root, * "my_user" is part of the tracing group, * "my_user" primary group is "my_user" and is part of group "my_dummy_group" * The "/tmp/my_dir" has the following permissions: drwxrwx--- 2 root my_dummy_group 4096 Jul 26 16:39 /tmp/my_dir/ Cause ===== The supplementary groups are not initialized when the run-as process demote itself to the user "my_user" to perform the recursive mkdir required by the `lttng save` command. From the point of the view the kernel, at the moment of performing the mkdir call the permissions looks like this: euid: uid of "my_user" egid: primary gid of "my_user" supplementary group list: "root" Note that the kernel does not treat the presence of the root group in the supplementary group list in any special way. Since "root gid" != "my_dummy_group gid" the directory creation is refused. Solution ======== Use initgroups(3) to initialize the supplementary group list. Known drawbacks ========= None. Signed-off-by: Francis Deslauriers Signed-off-by: Jonathan Rajotte Signed-off-by: Jérémie Galarneau Change-Id: I58656a3107e4f7b59a2391a4759988401cad7a2b --- diff --git a/src/common/runas.c b/src/common/runas.c index 04aedc3e6..ab43423b1 100644 --- a/src/common/runas.c +++ b/src/common/runas.c @@ -8,19 +8,20 @@ */ #define _LGPL_SOURCE +#include +#include +#include #include +#include +#include +#include #include #include #include -#include -#include #include +#include +#include #include -#include -#include -#include -#include -#include #include #include @@ -41,6 +42,8 @@ #include "runas.h" +#define GETPW_BUFFER_FALLBACK_SIZE 4096 + struct run_as_data; struct run_as_ret; typedef int (*run_as_fct)(struct run_as_data *data, struct run_as_ret *ret_value); @@ -899,18 +902,207 @@ end: return ret; } +static int get_user_infos_from_uid( + uid_t uid, char **username, gid_t *primary_gid) +{ + int ret; + char *buf = NULL; + size_t buf_size; + struct passwd pwd; + struct passwd *result = NULL; + + /* Fetch the max size for the temporary buffer. */ + errno = 0; + buf_size = sysconf(_SC_GETPW_R_SIZE_MAX); + if (buf_size < 0) { + if (errno != 0) { + PERROR("Failed to query _SC_GETPW_R_SIZE_MAX"); + goto error; + } + + /* Limit is indeterminate. */ + WARN("Failed to query _SC_GETPW_R_SIZE_MAX as it is " + "indeterminate; falling back to default buffer size"); + buf_size = GETPW_BUFFER_FALLBACK_SIZE; + } + + buf = zmalloc(buf_size); + if (buf == NULL) { + PERROR("Failed to allocate buffer to get password file entries"); + goto error; + } + + ret = getpwuid_r(uid, &pwd, buf, buf_size, &result); + if (ret < 0) { + PERROR("Failed to get user information for user: uid = %d", + (int) uid); + goto error; + } + + if (result == NULL) { + ERR("Failed to find user information in password entries: uid = %d", + (int) uid); + ret = -1; + goto error; + } + + *username = strdup(result->pw_name); + if (*username == NULL) { + PERROR("Failed to copy user name"); + goto error; + } + + *primary_gid = result->pw_gid; + +end: + free(buf); + return ret; +error: + *username = NULL; + *primary_gid = -1; + ret = -1; + goto end; +} + +static int demote_creds( + uid_t prev_uid, gid_t prev_gid, uid_t new_uid, gid_t new_gid) +{ + int ret = 0; + gid_t primary_gid; + char *username = NULL; + + /* Change the group id. */ + if (prev_gid != new_gid) { + ret = setegid(new_gid); + if (ret < 0) { + PERROR("Failed to set effective group id: new_gid = %d", + (int) new_gid); + goto end; + } + } + + /* Change the user id. */ + if (prev_uid != new_uid) { + ret = get_user_infos_from_uid(new_uid, &username, &primary_gid); + if (ret < 0) { + goto end; + } + + /* + * Initialize the supplementary group access list. + * + * This is needed to handle cases where the supplementary groups + * of the user the process is demoting-to would give it access + * to a given file/folder, but not it's primary group. + * + * e.g + * username: User1 + * Primary Group: User1 + * Secondary group: Disk, Network + * + * mkdir inside the following directory must work since User1 + * is part of the Network group. + * + * drwxrwx--- 2 root Network 4096 Jul 23 17:17 /tmp/my_folder/ + * + * + * The order of the following initgroups and seteuid calls is + * important here; + * Only a root process or one with CAP_SETGID capability can + * call the the initgroups() function. We must initialize the + * supplementary groups before we change the effective + * UID to a less-privileged user. + */ + ret = initgroups(username, primary_gid); + if (ret < 0) { + PERROR("Failed to init the supplementary group access list: " + "username = `%s`, primary gid = %d", username, + (int) primary_gid); + goto end; + } + + ret = seteuid(new_uid); + if (ret < 0) { + PERROR("Failed to set effective user id: new_uid = %d", + (int) new_uid); + goto end; + } + } +end: + free(username); + return ret; +} + +static int promote_creds( + uid_t prev_uid, gid_t prev_gid, uid_t new_uid, gid_t new_gid) +{ + int ret = 0; + gid_t primary_gid; + char *username = NULL; + + /* Change the group id. */ + if (prev_gid != new_gid) { + ret = setegid(new_gid); + if (ret < 0) { + PERROR("Failed to set effective group id: new_gid = %d", + (int) new_gid); + goto end; + } + } + + /* Change the user id. */ + if (prev_uid != new_uid) { + ret = get_user_infos_from_uid(new_uid, &username, &primary_gid); + if (ret < 0) { + goto end; + } + + /* + * seteuid call must be done before the initgroups call because + * we need to be privileged (CAP_SETGID) to call initgroups(). + */ + ret = seteuid(new_uid); + if (ret < 0) { + PERROR("Failed to set effective user id: new_uid = %d", + (int) new_uid); + goto end; + } + + /* + * Initialize the supplementary group access list. + * + * There is a possibility the groups we set in the following + * initgroups() call are not exactly the same as the ones we + * had when we originally demoted. This can happen if the + * /etc/group file is modified after the runas process is + * forked. This is very unlikely. + */ + ret = initgroups(username, primary_gid); + if (ret < 0) { + PERROR("Failed to init the supplementary group access " + "list: username = `%s`, primary gid = %d", + username, (int) primary_gid) + goto end; + } + } +end: + free(username); + return ret; +} + /* * Return < 0 on error, 0 if OK, 1 on hangup. */ static int handle_one_cmd(struct run_as_worker *worker) { - int ret = 0; + int ret = 0, promote_ret; struct run_as_data data = {}; ssize_t readlen, writelen; struct run_as_ret sendret = {}; run_as_fct cmd; - uid_t prev_euid; + uid_t prev_ruid; + gid_t prev_rgid; /* * Stage 1: Receive run_as_data struct from the master. @@ -948,24 +1140,12 @@ int handle_one_cmd(struct run_as_worker *worker) goto end; } - prev_euid = getuid(); - if (data.gid != getegid()) { - ret = setegid(data.gid); - if (ret < 0) { - sendret._error = true; - sendret._errno = errno; - PERROR("setegid"); - goto write_return; - } - } - if (data.uid != prev_euid) { - ret = seteuid(data.uid); - if (ret < 0) { - sendret._error = true; - sendret._errno = errno; - PERROR("seteuid"); - goto write_return; - } + prev_ruid = getuid(); + prev_rgid = getgid(); + + ret = demote_creds(prev_ruid, prev_rgid, data.uid, data.gid); + if (ret < 0) { + goto write_return; } /* @@ -985,7 +1165,7 @@ write_return: ret = cleanup_received_fds(&data); if (ret < 0) { ERR("Error cleaning up FD"); - goto end; + goto promote_back; } /* @@ -997,7 +1177,7 @@ write_return: if (writelen < sizeof(sendret)) { PERROR("lttcomm_send_unix_sock error"); ret = -1; - goto end; + goto promote_back; } /* @@ -1006,15 +1186,17 @@ write_return: ret = send_fds_to_master(worker, data.cmd, &sendret); if (ret < 0) { DBG("Sending FD to master returned an error"); - goto end; } - if (seteuid(prev_euid) < 0) { - PERROR("seteuid"); - ret = -1; - goto end; - } ret = 0; + +promote_back: + /* Return to previous uid/gid. */ + promote_ret = promote_creds(data.uid, data.gid, prev_ruid, prev_rgid); + if (promote_ret < 0) { + ERR("Failed to promote back to the initial credentials"); + } + end: return ret; }