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