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 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 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 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