Merge pull request #97 from cyphar/newgidmap-secure-setgroups
newgidmap: enforce setgroups=deny if self-mapping a group
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commit
c53e4c1d77
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README
1
README
@ -44,6 +44,7 @@ a lot of mail...
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Adam Rudnicki <adam@v-lo.krakow.pl>
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Alan Curry <pacman@tardis.mars.net>
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Aleksa Sarai <cyphar@cyphar.com>
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Alexander O. Yuriev <alex@bach.cis.temple.edu>
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Algis Rudys <arudys@rice.edu>
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Andreas Jaeger <aj@arthur.rhein-neckar.de>
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@ -46,32 +46,37 @@
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*/
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const char *Prog;
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static bool verify_range(struct passwd *pw, struct map_range *range)
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static bool verify_range(struct passwd *pw, struct map_range *range, bool *allow_setgroups)
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{
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/* An empty range is invalid */
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if (range->count == 0)
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return false;
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/* Test /etc/subgid */
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if (have_sub_gids(pw->pw_name, range->lower, range->count))
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/* Test /etc/subgid. If the mapping is valid then we allow setgroups. */
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if (have_sub_gids(pw->pw_name, range->lower, range->count)) {
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*allow_setgroups = true;
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return true;
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}
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/* Allow a process to map its own gid */
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if ((range->count == 1) && (pw->pw_gid == range->lower))
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/* Allow a process to map its own gid. */
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if ((range->count == 1) && (pw->pw_gid == range->lower)) {
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/* noop -- if setgroups is enabled already we won't disable it. */
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return true;
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}
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return false;
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}
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static void verify_ranges(struct passwd *pw, int ranges,
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struct map_range *mappings)
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struct map_range *mappings, bool *allow_setgroups)
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{
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struct map_range *mapping;
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int idx;
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mapping = mappings;
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for (idx = 0; idx < ranges; idx++, mapping++) {
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if (!verify_range(pw, mapping)) {
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if (!verify_range(pw, mapping, allow_setgroups)) {
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fprintf(stderr, _( "%s: gid range [%lu-%lu) -> [%lu-%lu) not allowed\n"),
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Prog,
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mapping->upper,
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@ -89,6 +94,70 @@ static void usage(void)
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exit(EXIT_FAILURE);
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}
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void write_setgroups(int proc_dir_fd, bool allow_setgroups)
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{
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int setgroups_fd;
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char *policy, policy_buffer[4096];
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/*
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* Default is "deny", and any "allow" will out-rank a "deny". We don't
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* forcefully write an "allow" here because the process we are writing
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* mappings for may have already set themselves to "deny" (and "allow"
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* is the default anyway). So allow_setgroups == true is a noop.
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*/
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policy = "deny\n";
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if (allow_setgroups)
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return;
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setgroups_fd = openat(proc_dir_fd, "setgroups", O_RDWR|O_CLOEXEC);
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if (setgroups_fd < 0) {
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/*
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* If it's an ENOENT then we are on too old a kernel for the setgroups
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* code to exist. Emit a warning and bail on this.
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*/
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if (ENOENT == errno) {
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fprintf(stderr, _("%s: kernel doesn't support setgroups restrictions\n"), Prog);
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goto out;
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}
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fprintf(stderr, _("%s: couldn't open process setgroups: %s\n"),
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Prog,
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strerror(errno));
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exit(EXIT_FAILURE);
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}
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/*
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* Check whether the policy is already what we want. /proc/self/setgroups
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* is write-once, so attempting to write after it's already written to will
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* fail.
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*/
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if (read(setgroups_fd, policy_buffer, sizeof(policy_buffer)) < 0) {
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fprintf(stderr, _("%s: failed to read setgroups: %s\n"),
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Prog,
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strerror(errno));
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exit(EXIT_FAILURE);
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}
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if (!strncmp(policy_buffer, policy, strlen(policy)))
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goto out;
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/* Write the policy. */
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if (lseek(setgroups_fd, 0, SEEK_SET) < 0) {
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fprintf(stderr, _("%s: failed to seek setgroups: %s\n"),
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Prog,
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strerror(errno));
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exit(EXIT_FAILURE);
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}
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if (dprintf(setgroups_fd, "%s", policy) < 0) {
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fprintf(stderr, _("%s: failed to setgroups %s policy: %s\n"),
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Prog,
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policy,
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strerror(errno));
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exit(EXIT_FAILURE);
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}
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out:
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close(setgroups_fd);
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}
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/*
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* newgidmap - Set the gid_map for the specified process
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*/
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@ -103,6 +172,7 @@ int main(int argc, char **argv)
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struct stat st;
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struct passwd *pw;
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int written;
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bool allow_setgroups = false;
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Prog = Basename (argv[0]);
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@ -145,7 +215,7 @@ int main(int argc, char **argv)
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(unsigned long) getuid ()));
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return EXIT_FAILURE;
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}
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/* Get the effective uid and effective gid of the target process */
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if (fstat(proc_dir_fd, &st) < 0) {
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fprintf(stderr, _("%s: Could not stat directory for target %u\n"),
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@ -177,8 +247,9 @@ int main(int argc, char **argv)
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if (!mappings)
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usage();
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verify_ranges(pw, ranges, mappings);
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verify_ranges(pw, ranges, mappings, &allow_setgroups);
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write_setgroups(proc_dir_fd, allow_setgroups);
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write_mapping(proc_dir_fd, ranges, mappings, "gid_map");
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sub_gid_close();
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