1 /* $NetBSD: uidswap.c,v 1.10 2019/10/12 18:32:22 christos Exp $ */
2 /* $OpenBSD: uidswap.c,v 1.42 2019/06/28 13:35:04 deraadt Exp $ */
3 /*
4 * Author: Tatu Ylonen <ylo@cs.hut.fi>
5 * Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
6 * All rights reserved
7 * Code for uid-swapping.
8 *
9 * As far as I am concerned, the code I have written for this software
10 * can be used freely for any purpose. Any derived versions of this
11 * software must be clearly marked as such, and if the derived work is
12 * incompatible with the protocol description in the RFC file, it must be
13 * called by a name other than "ssh" or "Secure Shell".
14 */
15
16 #include "includes.h"
17 __RCSID("$NetBSD: uidswap.c,v 1.10 2019/10/12 18:32:22 christos Exp $");
18 #include <sys/param.h>
19 #include <errno.h>
20 #include <pwd.h>
21 #include <string.h>
22 #include <unistd.h>
23 #include <limits.h>
24 #include <stdarg.h>
25 #include <stdlib.h>
26
27 #include "log.h"
28 #include "uidswap.h"
29
30 /*
31 * Note: all these functions must work in all of the following cases:
32 * 1. euid=0, ruid=0
33 * 2. euid=0, ruid!=0
34 * 3. euid!=0, ruid!=0
35 * Additionally, they must work regardless of whether the system has
36 * POSIX saved uids or not.
37 */
38
39 /* Lets assume that posix saved ids also work with seteuid, even though that
40 is not part of the posix specification. */
41
42 /* Saved effective uid. */
43 static int privileged = 0;
44 static int temporarily_use_uid_effective = 0;
45 static uid_t saved_euid, user_groups_uid;
46 static gid_t saved_egid;
47 static gid_t saved_egroups[NGROUPS_MAX], user_groups[NGROUPS_MAX];
48 static int saved_egroupslen = -1, user_groupslen = -1;
49
50 /*
51 * Temporarily changes to the given uid. If the effective user
52 * id is not root, this does nothing. This call cannot be nested.
53 */
54 void
temporarily_use_uid(struct passwd * pw)55 temporarily_use_uid(struct passwd *pw)
56 {
57 /* Save the current euid, and egroups. */
58 saved_euid = geteuid();
59 saved_egid = getegid();
60 debug("temporarily_use_uid: %u/%u (e=%u/%u)",
61 (u_int)pw->pw_uid, (u_int)pw->pw_gid,
62 (u_int)saved_euid, (u_int)saved_egid);
63 if (saved_euid != 0) {
64 privileged = 0;
65 return;
66 }
67 privileged = 1;
68 temporarily_use_uid_effective = 1;
69 saved_egroupslen = getgroups(NGROUPS_MAX, saved_egroups);
70 if (saved_egroupslen == -1)
71 fatal("getgroups: %.100s", strerror(errno));
72
73 /* set and save the user's groups */
74 if (user_groupslen == -1 || user_groups_uid != pw->pw_uid) {
75 if (initgroups(pw->pw_name, pw->pw_gid) == -1)
76 fatal("initgroups: %s: %.100s", pw->pw_name,
77 strerror(errno));
78 user_groupslen = getgroups(NGROUPS_MAX, user_groups);
79 if (user_groupslen == -1)
80 fatal("getgroups: %.100s", strerror(errno));
81 user_groups_uid = pw->pw_uid;
82 }
83 /* Set the effective uid to the given (unprivileged) uid. */
84 if (setgroups(user_groupslen, user_groups) == -1)
85 fatal("setgroups: %.100s", strerror(errno));
86 if (setegid(pw->pw_gid) == -1)
87 fatal("setegid %u: %.100s", (u_int)pw->pw_gid,
88 strerror(errno));
89 if (seteuid(pw->pw_uid) == -1)
90 fatal("seteuid %u: %.100s", (u_int)pw->pw_uid,
91 strerror(errno));
92 }
93
94 /*
95 * Restores to the original (privileged) uid.
96 */
97 void
restore_uid(void)98 restore_uid(void)
99 {
100 /* it's a no-op unless privileged */
101 if (!privileged) {
102 debug("restore_uid: (unprivileged)");
103 return;
104 }
105 if (!temporarily_use_uid_effective)
106 fatal("restore_uid: temporarily_use_uid not effective");
107 debug("restore_uid: %u/%u", (u_int)saved_euid, (u_int)saved_egid);
108 /* Set the effective uid back to the saved privileged uid. */
109 if (seteuid(saved_euid) == -1)
110 fatal("seteuid %u: %.100s", (u_int)saved_euid, strerror(errno));
111 if (setgroups(saved_egroupslen, saved_egroups) == -1)
112 fatal("setgroups: %.100s", strerror(errno));
113 if (setegid(saved_egid) == -1)
114 fatal("setegid %u: %.100s", (u_int)saved_egid, strerror(errno));
115 temporarily_use_uid_effective = 0;
116 }
117
118 /*
119 * Permanently sets all uids to the given uid. This cannot be
120 * called while temporarily_use_uid is effective.
121 */
122 void
permanently_set_uid(struct passwd * pw)123 permanently_set_uid(struct passwd *pw)
124 {
125 if (pw == NULL)
126 fatal("permanently_set_uid: no user given");
127 if (temporarily_use_uid_effective)
128 fatal("permanently_set_uid: temporarily_use_uid effective");
129 debug("permanently_set_uid: %u/%u", (u_int)pw->pw_uid,
130 (u_int)pw->pw_gid);
131
132 if (setgid(pw->pw_gid) < 0)
133 fatal("setgid %u: %.100s", (u_int)pw->pw_gid, strerror(errno));
134 if (setegid(pw->pw_gid) < 0)
135 fatal("setegid %u: %.100s", (u_int)pw->pw_gid, strerror(errno));
136
137 if (setuid(pw->pw_uid) < 0)
138 fatal("setuid %u: %.100s", (u_int)pw->pw_uid, strerror(errno));
139 if (seteuid(pw->pw_uid) < 0)
140 fatal("seteuid %u: %.100s", (u_int)pw->pw_uid, strerror(errno));
141
142 /* Verify GID drop was successful */
143 if (getgid() != pw->pw_gid || getegid() != pw->pw_gid) {
144 fatal("%s: egid incorrect gid:%u egid:%u (should be %u)",
145 __func__, (u_int)getgid(), (u_int)getegid(),
146 (u_int)pw->pw_gid);
147 }
148
149 /* Verify UID drop was successful */
150 if (getuid() != pw->pw_uid || geteuid() != pw->pw_uid) {
151 fatal("%s: euid incorrect uid:%u euid:%u (should be %u)",
152 __func__, (u_int)getuid(), (u_int)geteuid(),
153 (u_int)pw->pw_uid);
154 }
155 }
156