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redo the environment inheritance to not inherit. it was intended to make
life easier, but it can be surprising or even unsafe. instead, reset just
about everything to the target user's values.
ok deraadt martijn
Thanks to Sander Bos in particular for pointing out some nasty edge cases.

/* $OpenBSD: env.c,v 1.7 2019/06/16 18:16:34 tedu Exp $ */
/*
 * Copyright (c) 2016 Ted Unangst <tedu@openbsd.org>
 *
 * Permission to use, copy, modify, and distribute this software for any
 * purpose with or without fee is hereby granted, provided that the above
 * copyright notice and this permission notice appear in all copies.
 *
 * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
 * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
 * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
 * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
 * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
 * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
 * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
 */

#include <sys/types.h>
#include <sys/tree.h>

#include <string.h>
#include <stdio.h>
#include <stdlib.h>
#include <err.h>
#include <unistd.h>
#include <errno.h>
#include <pwd.h>

#include "doas.h"

struct envnode {
	RB_ENTRY(envnode) node;
	const char *key;
	const char *value;
};

struct env {
	RB_HEAD(envtree, envnode) root;
	u_int count;
};

static void fillenv(struct env *env, const char **envlist);

static int
envcmp(struct envnode *a, struct envnode *b)
{
	return strcmp(a->key, b->key);
}
RB_GENERATE_STATIC(envtree, envnode, node, envcmp)

static struct envnode *
createnode(const char *key, const char *value)
{
	struct envnode *node;

	node = malloc(sizeof(*node));
	if (!node)
		err(1, NULL);
	node->key = strdup(key);
	node->value = strdup(value);
	if (!node->key || !node->value)
		err(1, NULL);
	return node;
}

static void
freenode(struct envnode *node)
{
	free((char *)node->key);
	free((char *)node->value);
	free(node);
}

static void
addnode(struct env *env, const char *key, const char *value)
{
	struct envnode *node;

	node = createnode(key, value);
	RB_INSERT(envtree, &env->root, node);
	env->count++;
}

static struct env *
createenv(const struct rule *rule, const struct passwd *mypw,
    const struct passwd *targpw)
{
	struct env *env;
	u_int i;

	env = malloc(sizeof(*env));
	if (!env)
		err(1, NULL);
	RB_INIT(&env->root);
	env->count = 0;

	addnode(env, "DOAS_USER", mypw->pw_name);

	if (rule->options & KEEPENV) {
		extern const char **environ;

		for (i = 0; environ[i] != NULL; i++) {
			struct envnode *node;
			const char *e, *eq;
			size_t len;
			char name[1024];

			e = environ[i];

			/* ignore invalid or overlong names */
			if ((eq = strchr(e, '=')) == NULL || eq == e)
				continue;
			len = eq - e;
			if (len > sizeof(name) - 1)
				continue;
			memcpy(name, e, len);
			name[len] = '\0';

			node = createnode(name, eq + 1);
			if (RB_INSERT(envtree, &env->root, node)) {
				/* ignore any later duplicates */
				freenode(node);
			} else {
				env->count++;
			}
		}
	} else {
		static const char *copyset[] = {
			"DISPLAY", "TERM",
			NULL
		};

		addnode(env, "HOME", targpw->pw_dir);
		addnode(env, "LOGNAME", targpw->pw_name);
		addnode(env, "PATH", getenv("PATH"));
		addnode(env, "SHELL", targpw->pw_shell);
		addnode(env, "USER", targpw->pw_name);

		fillenv(env, copyset);
	}

	return env;
}

static char **
flattenenv(struct env *env)
{
	char **envp;
	struct envnode *node;
	u_int i;

	envp = reallocarray(NULL, env->count + 1, sizeof(char *));
	if (!envp)
		err(1, NULL);
	i = 0;
	RB_FOREACH(node, envtree, &env->root) {
		if (asprintf(&envp[i], "%s=%s", node->key, node->value) == -1)
			err(1, NULL);
		i++;
	}
	envp[i] = NULL;
	return envp;
}

static void
fillenv(struct env *env, const char **envlist)
{
	struct envnode *node, key;
	const char *e, *eq;
	const char *val;
	char name[1024];
	u_int i;
	size_t len;

	for (i = 0; envlist[i]; i++) {
		e = envlist[i];

		/* parse out env name */
		if ((eq = strchr(e, '=')) == NULL)
			len = strlen(e);
		else
			len = eq - e;
		if (len > sizeof(name) - 1)
			continue;
		memcpy(name, e, len);
		name[len] = '\0';

		/* delete previous copies */
		key.key = name;
		if (*name == '-')
			key.key = name + 1;
		if ((node = RB_FIND(envtree, &env->root, &key))) {
			RB_REMOVE(envtree, &env->root, node);
			freenode(node);
			env->count--;
		}
		if (*name == '-')
			continue;

		/* assign value or inherit from environ */
		if (eq) {
			val = eq + 1;
			if (*val == '$')
				val = getenv(val + 1);
		} else {
			val = getenv(name);
		}
		/* at last, we have something to insert */
		if (val) {
			node = createnode(name, val);
			RB_INSERT(envtree, &env->root, node);
			env->count++;
		}
	}
}

char **
prepenv(const struct rule *rule, const struct passwd *mypw,
    const struct passwd *targpw)
{
	struct env *env;

	env = createenv(rule, mypw, targpw);
	if (rule->envlist)
		fillenv(env, rule->envlist);

	return flattenenv(env);
}