diff -up ./src/copy_file.c.symbolic-link-attack-4 ./src/copy_file.c

--- ./src/copy_file.c.symbolic-link-attack-4	2021-02-02 16:35:18.453036846 +0100

+++ ./src/copy_file.c	2021-02-02 16:38:09.430731749 +0100

@@ -23,6 +23,7 @@

 #include <config.h>

+#include <sys/stat.h>

 #include <sys/types.h>

 #include <stdio.h>

@@ -126,3 +127,35 @@ write_error:

 	debug_return_int(-1);

     }

 }

+

+#ifdef HAVE_SELINUX

+bool

+sudo_check_temp_file(int tfd, const char *tfile, uid_t uid, struct stat *sb)

+{

+    struct stat sbuf;

+    debug_decl(sudo_check_temp_file, SUDO_DEBUG_UTIL);

+

+    if (sb == NULL)

+	sb = &sbu;;

+

+    if (fstat(tfd, sb) == -1) {

+	sudo_warn(U_("unable to stat %s"), tfile);

+	debug_return_bool(false);

+    }

+    if (!S_ISREG(sb->st_mode)) {

+	sudo_warnx(U_("%s: not a regular file"), tfile);

+	debug_return_bool(false);

+    }

+    if ((sb->st_mode & ALLPERMS) != (S_IRUSR|S_IWUSR)) {

+	sudo_warnx(U_("%s: bad file mode: 0%o"), tfile,

+	    (unsigned int)(sb->st_mode & ALLPERMS));

+	debug_return_bool(false);

+    }

+    if (sb->st_uid != uid) {

+	sudo_warnx(U_("%s is owned by uid %u, should be %u"),

+	    tfile, (unsigned int)sb->st_uid, (unsigned int)uid);

+	debug_return_bool(false);

+    }

+    debug_return_bool(true);

+}

+#endif /* SELINUX */

diff -up ./src/sesh.c.symbolic-link-attack-4 ./src/sesh.c

--- ./src/sesh.c.symbolic-link-attack-4	2021-02-02 16:35:18.450036887 +0100

+++ ./src/sesh.c	2021-02-02 16:38:52.907146897 +0100

@@ -134,7 +134,7 @@ main(int argc, char *argv[], char *envp[

 static int

 sesh_sudoedit(int argc, char *argv[])

 {

-    int i, oflags_dst, post, ret = SESH_ERR_FAILURE;

+    int i, oflags_src, oflags_dst, post, ret = SESH_ERR_FAILURE;

     int fd_src = -1, fd_dst = -1, follow = 0;

     ssize_t nread, nwritten;

     struct stat sb;

@@ -178,10 +178,12 @@ sesh_sudoedit(int argc, char *argv[])

 	debug_return_int(SESH_ERR_BAD_PATHS);

     /*

-     * Use O_EXCL if we are not in the post editing stage

-     * so that it's ensured that the temporary files are

-     * created by us and that we are not opening any symlinks.

+     * In the pre-editing stage, use O_EXCL to ensure that the temporary

+     * files are created by us and that we are not opening any symlinks.

+     * In the post-editing stage, use O_NOFOLLOW so we don't follow symlinks

+     * when opening the temporary files.

      */

+    oflags_src = O_RDONLY|(post ? O_NONBLOCK|O_NOFOLLOW : follow);

     oflags_dst = O_WRONLY|O_CREAT|(post ? follow : O_EXCL);

     for (i = 0; i < argc - 1; i += 2) {

 	const char *path_src = argv[i];

@@ -191,7 +193,7 @@ sesh_sudoedit(int argc, char *argv[])

 	 * doesn't exist, that's OK, we'll create an empty

 	 * destination file.

 	 */

-	if ((fd_src = open(path_src, O_RDONLY|follow, S_IRUSR|S_IWUSR)) < 0) {

+	if ((fd_src = open(path_src, oflags_src, S_IRUSR|S_IWUSR)) < 0) {

 	    if (errno != ENOENT) {

 		sudo_warn("%s", path_src);

 		if (post) {

@@ -201,6 +203,14 @@ sesh_sudoedit(int argc, char *argv[])

 		    goto cleanup_0;

 	    }

 	}

+	if (post) {

+	    /* Make sure the temporary file is safe and has the proper owner. */

+	    if (!sudo_check_temp_file(fd_src, path_src, geteuid(), &sb)) {

+		ret = SESH_ERR_SOME_FILES;

+		goto nocleanup;

+	    }

+	    fcntl(fd_src, F_SETFL, fcntl(fd_src, F_GETFL, 0) & ~O_NONBLOCK);

+	}

 	if ((fd_dst = open(path_dst, oflags_dst, post ?

 	    (S_IRUSR|S_IWUSR|S_IRGRP|S_IROTH) : (S_IRUSR|S_IWUSR))) < 0) {

@@ -218,10 +228,7 @@ sesh_sudoedit(int argc, char *argv[])

 	    off_t len_dst = -1;

 	    if (post) {

-		if (fstat(fd_src, &sb) != 0) {

-		    ret = SESH_ERR_SOME_FILES;

-		    goto nocleanup;

-		}

+		/* sudo_check_temp_file() filled in sb for us. */

 		len_src = sb.st_size;

 		if (fstat(fd_dst, &sb) != 0) {

 		    ret = SESH_ERR_SOME_FILES;

diff -up ./src/sudo_edit.c.symbolic-link-attack-4 ./src/sudo_edit.c

--- ./src/sudo_edit.c.symbolic-link-attack-4	2021-02-02 16:35:18.452036860 +0100

+++ ./src/sudo_edit.c	2021-02-02 16:54:25.943429580 +0100

@@ -253,8 +253,10 @@ sudo_edit_mktemp(const char *ofile, char

     } else {

 	len = asprintf(tfile, "%s/%s.XXXXXXXX", edit_tmpdir, cp);

     }

-    if (len == -1)

-	sudo_fatalx(U_("%s: %s"), __func__, U_("unable to allocate memory"));

+    if (len == -1) {

+	sudo_warnx(U_("%s: %s"), __func__, U_("unable to allocate memory"));

+	debug_return_int(-1);

+    }

     tfd = mkstemps(*tfile, suff ? strlen(suff) : 0);

     sudo_debug_printf(SUDO_DEBUG_INFO|SUDO_DEBUG_LINENO,

 	"%s -> %s, fd %d", ofile, *tfile, tfd);

@@ -757,7 +759,8 @@ bad:

 #ifdef HAVE_SELINUX

 static int

-selinux_run_helper(char *argv[], char *envp[])

+selinux_run_helper(uid_t uid, gid_t gid, int ngroups, GETGROUPS_T *groups,

+    char *const argv[], char *const envp[])

 {

     int status, ret = SESH_ERR_FAILURE;

     const char *sesh;

@@ -777,8 +780,10 @@ selinux_run_helper(char *argv[], char *e

 	break;

     case 0:

 	/* child runs sesh in new context */

-	if (selinux_setcon() == 0)

+	if (selinux_setcon() == 0) {

+	    switch_user(uid, gid, ngroups, groups);

 	    execve(sesh, argv, envp);

+	}

 	_exit(SESH_ERR_FAILURE);

     default:

 	/* parent waits */

@@ -797,7 +802,7 @@ selinux_edit_create_tfiles(struct comman

     struct tempfile *tf, char *files[], int nfiles)

 {

     char **sesh_args, **sesh_ap;

-    int i, rc, sesh_nargs;

+    int i, rc, error, sesh_nargs, ret = -1;

     struct stat sb;

     debug_decl(selinux_edit_create_tfiles, SUDO_DEBUG_EDIT)

@@ -809,7 +814,7 @@ selinux_edit_create_tfiles(struct comman

     sesh_args = sesh_ap = reallocarray(NULL, sesh_nargs, sizeof(char *));

     if (sesh_args == NULL) {

 	sudo_warnx(U_("%s: %s"), __func__, U_("unable to allocate memory"));

-	debug_return_int(-1);

+	goto done;

     }

     *sesh_ap++ = "sesh";

     *sesh_ap++ = "-e";

@@ -817,7 +822,6 @@ selinux_edit_create_tfiles(struct comman

 	*sesh_ap++ = "-h";

     *sesh_ap++ = "0";

-    /* XXX - temp files should be created with user's context */

     for (i = 0; i < nfiles; i++) {

 	char *tfile, *ofile = files[i];

 	int tfd;

@@ -835,8 +839,7 @@ selinux_edit_create_tfiles(struct comman

 	if (tfd == -1) {

 	    sudo_warn("mkstemps");

 	    free(tfile);

-	    free(sesh_args);

-	    debug_return_int(-1);

+	    goto done;

 	}

 	/* Helper will re-create temp file with proper security context. */

 	close(tfd);

@@ -847,8 +850,10 @@ selinux_edit_create_tfiles(struct comman

     *sesh_ap = NULL;

     /* Run sesh -e [-h] 0 <o1> <t1> ... <on> <tn> */

-    rc = selinux_run_helper(sesh_args, command_details->envp);

-    switch (rc) {

+    error = selinux_run_helper(command_details->uid, command_details->gid,

+	command_details->ngroups, command_details->groups, sesh_args,

+	command_details->envp);

+    switch (error) {

     case SESH_SUCCESS:

 	break;

     case SESH_ERR_BAD_PATHS:

@@ -858,21 +863,34 @@ selinux_edit_create_tfiles(struct comman

     case SESH_ERR_KILLED:

 	sudo_fatalx(U_("sesh: killed by a signal"));

     default:

-	sudo_fatalx(U_("sesh: unknown error %d"), rc);

+	sudo_fatalx(U_("sesh: unknown error %d"), error);

+    goto done;

     }

-    /* Chown to user's UID so they can edit the temporary files. */

     for (i = 0; i < nfiles; i++) {

-	if (chown(tf[i].tfile, user_details.uid, user_details.gid) != 0) {

-	    sudo_warn("unable to chown(%s) to %d:%d for editing",

-		tf[i].tfile, user_details.uid, user_details.gid);

-	}

+        int tfd = open(tf[i].tfile, O_RDONLY|O_NONBLOCK|O_NOFOLLOW);

+        if (tfd == -1) {

+            sudo_warn(U_("unable to open %s"), tf[i].tfile);

+            goto done;

+        }

+        if (!sudo_check_temp_file(tfd, tf[i].tfile, command_details->uid, NULL)) {

+            close(tfd);

+            goto done;

+        }

+        if (fchown(tfd, user_details.uid, user_details.gid) != 0) {

+            sudo_warn("unable to chown(%s) to %d:%d for editing",

+                      tf[i].tfile, user_details.uid, user_details.gid);

+            close(tfd);

+            goto done;

+        }

+        close(tfd);

     }

+done:

     /* Contents of tf will be freed by caller. */

     free(sesh_args);

-    return (nfiles);

+    debug_return_int(ret);

 }

 static int

@@ -880,7 +898,8 @@ selinux_edit_copy_tfiles(struct command_

     struct tempfile *tf, int nfiles, struct timespec *times)

 {

     char **sesh_args, **sesh_ap;

-    int i, rc, sesh_nargs, ret = 1;

+    int i, rc, error, sesh_nargs, ret = 1;

+    int tfd = -1;

     struct timespec ts;

     struct stat sb;

     debug_decl(selinux_edit_copy_tfiles, SUDO_DEBUG_EDIT)

@@ -901,33 +920,43 @@ selinux_edit_copy_tfiles(struct command_

     /* Construct args for sesh -e 1 */

     for (i = 0; i < nfiles; i++) {

-	if (stat(tf[i].tfile, &sb) == 0) {

-	    mtim_get(&sb, ts);

-	    if (tf[i].osize == sb.st_size && sudo_timespeccmp(&tf[i].omtim, &ts, ==)) {

-		/*

-		 * If mtime and size match but the user spent no measurable

-		 * time in the editor we can't tell if the file was changed.

-		 */

-		if (sudo_timespeccmp(&times[0], &times[1], !=)) {

-		    sudo_warnx(U_("%s unchanged"), tf[i].ofile);

-		    unlink(tf[i].tfile);

-		    continue;

-		}

+	if (tfd != -1)

+	    close(tfd);

+	if ((tfd = open(tf[i].tfile, O_RDONLY|O_NONBLOCK|O_NOFOLLOW)) == -1) {

+	    sudo_warn(U_("unable to open %s"), tf[i].tfile);

+	    continue;

+	}

+	if (!sudo_check_temp_file(tfd, tf[i].tfile, user_details.uid, &sb))

+	    continue;

+	mtim_get(&sb, ts);

+	if (tf[i].osize == sb.st_size && sudo_timespeccmp(&tf[i].omtim, &ts, ==)) {

+	    /*

+	     * If mtime and size match but the user spent no measurable

+	     * time in the editor we can't tell if the file was changed.

+	     */

+	    if (sudo_timespeccmp(&times[0], &times[1], !=)) {

+		sudo_warnx(U_("%s unchanged"), tf[i].ofile);

+		unlink(tf[i].tfile);

+		continue;

 	    }

 	}

 	*sesh_ap++ = tf[i].tfile;

 	*sesh_ap++ = tf[i].ofile;

-	if (chown(tf[i].tfile, command_details->uid, command_details->gid) != 0) {

+	if (fchown(tfd, command_details->uid, command_details->gid) != 0) {

 	    sudo_warn("unable to chown(%s) back to %d:%d", tf[i].tfile,

 		command_details->uid, command_details->gid);

 	}

     }

     *sesh_ap = NULL;

+    if (tfd != -1)

+	close(tfd);

     if (sesh_ap - sesh_args > 3) {

 	/* Run sesh -e 1 <t1> <o1> ... <tn> <on> */

-	rc = selinux_run_helper(sesh_args, command_details->envp);

-	switch (rc) {

+	error = selinux_run_helper(command_details->uid, command_details->gid,

+	    command_details->ngroups, command_details->groups, sesh_args,

+	    command_details->envp);

+	switch (error) {

 	case SESH_SUCCESS:

 	    ret = 0;

 	    break;

@@ -941,7 +970,7 @@ selinux_edit_copy_tfiles(struct command_

 	    sudo_warnx(U_("sesh: killed by a signal"));

 	    break;

 	default:

-	    sudo_warnx(U_("sesh: unknown error %d"), rc);

+	    sudo_warnx(U_("sesh: unknown error %d"), error);

 	    break;

 	}

 	if (ret != 0)

@@ -963,7 +992,7 @@ sudo_edit(struct command_details *comman

 {

     struct command_details saved_command_details;

     char **nargv = NULL, **ap, **files = NULL;

-    int errors, i, ac, nargc, rc;

+    int errors, i, ac, nargc, ret;

     int editor_argc = 0, nfiles = 0;

     struct timespec times[2];

     struct tempfile *tf = NULL;

@@ -1058,7 +1087,7 @@ sudo_edit(struct command_details *comman

     command_details->ngroups = user_details.ngroups;

     command_details->groups = user_details.groups;

     command_details->argv = nargv;

-    rc = run_command(command_details);

+    ret = run_command(command_details);

     if (sudo_gettime_real(&times[1]) == -1) {

 	sudo_warn(U_("unable to read the clock"));

 	goto cleanup;

@@ -1080,14 +1109,16 @@ sudo_edit(struct command_details *comman

     else

 #endif

 	errors = sudo_edit_copy_tfiles(command_details, tf, nfiles, times);

-    if (errors)

-	goto cleanup;

+    if (errors) {

+        /* Preserve the edited temporary files. */

+        ret = W_EXITCODE(1, 0);

+    }

     for (i = 0; i < nfiles; i++)

 	free(tf[i].tfile);

     free(tf);

     free(nargv);

-    debug_return_int(rc);

+    debug_return_int(ret);

 cleanup:

     /* Clean up temp files and return. */

diff -up ./src/sudo_exec.h.symbolic-link-attack-4 ./src/sudo_exec.h

--- ./src/sudo_exec.h.symbolic-link-attack-4	2021-02-02 16:35:18.452036860 +0100

+++ ./src/sudo_exec.h	2021-02-02 16:35:18.454036833 +0100

@@ -1,7 +1,7 @@

 /*

  * SPDX-License-Identifier: ISC

  *

- * Copyright (c) 2010-2016 Todd C. Miller <Todd.Miller@sudo.ws>

+ * Copyright (c) 2010-2017, 2020-2021 Todd C. Miller <Todd.Miller@sudo.ws>

  *

  * Permission to use, copy, modify, and distribute this software for any

  * purpose with or without fee is hereby granted, provided that the above

@@ -84,9 +84,11 @@

  */

 struct command_details;

 struct command_status;

+struct stat;

 /* copy_file.c */

 int sudo_copy_file(const char *src, int src_fd, off_t src_len, const char *dst, int dst_fd, off_t dst_len);

+bool sudo_check_temp_file(int tfd, const char *tname, uid_t uid, struct stat *sb);

 /* exec.c */

 void exec_cmnd(struct command_details *details, int errfd);