+From fb28c99b8a66ff2605c5cb96abc0a4d975f92de0 Mon Sep 17 00:00:00 2001
+From: Aleksa Sarai <asarai@suse.de>
+Date: Thu, 15 Feb 2018 23:49:40 +1100
+Subject: [PATCH] newgidmap: enforce setgroups=deny if self-mapping a group
+
+This is necessary to match the kernel-side policy of "self-mapping in a
+user namespace is fine, but you cannot drop groups" -- a policy that was
+created in order to stop user namespaces from allowing trivial privilege
+escalation by dropping supplementary groups that were "blacklisted" from
+certain paths.
+
+This is the simplest fix for the underlying issue, and effectively makes
+it so that unless a user has a valid mapping set in /etc/subgid (which
+only administrators can modify) -- and they are currently trying to use
+that mapping -- then /proc/$pid/setgroups will be set to deny. This
+workaround is only partial, because ideally it should be possible to set
+an "allow_setgroups" or "deny_setgroups" flag in /etc/subgid to allow
+administrators to further restrict newgidmap(1).
+
+We also don't write anything in the "allow" case because "allow" is the
+default, and users may have already written "deny" even if they
+technically are allowed to use setgroups. And we don't write anything if
+the setgroups policy is already "deny".
+
+Ref: https://bugs.launchpad.net/ubuntu/+source/shadow/+bug/1729357
+Fixes: CVE-2018-7169
+Reported-by: Craig Furman <craig.furman89@gmail.com>
+Signed-off-by: Aleksa Sarai <asarai@suse.de>
+---
+ src/newgidmap.c | 89 +++++++++++++++++++++++++++++++++++++++++++++++++++------
+ 1 file changed, 80 insertions(+), 9 deletions(-)
+
+diff --git a/src/newgidmap.c b/src/newgidmap.c
+index b1e33513..59a2e75c 100644
+--- a/src/newgidmap.c
++++ b/src/newgidmap.c
+@@ -46,32 +46,37 @@
+ */
+ const char *Prog;
+
+-static bool verify_range(struct passwd *pw, struct map_range *range)
++
++static bool verify_range(struct passwd *pw, struct map_range *range, bool *allow_setgroups)
+ {
+ /* An empty range is invalid */
+ if (range->count == 0)
+ return false;
+
+- /* Test /etc/subgid */
+- if (have_sub_gids(pw->pw_name, range->lower, range->count))
++ /* Test /etc/subgid. If the mapping is valid then we allow setgroups. */
++ if (have_sub_gids(pw->pw_name, range->lower, range->count)) {
++ *allow_setgroups = true;
+ return true;
++ }
+
+- /* Allow a process to map its own gid */
+- if ((range->count == 1) && (pw->pw_gid == range->lower))
++ /* Allow a process to map its own gid. */
++ if ((range->count == 1) && (pw->pw_gid == range->lower)) {
++ /* noop -- if setgroups is enabled already we won't disable it. */
+ return true;
++ }
+
+ return false;
+ }
+
+ static void verify_ranges(struct passwd *pw, int ranges,
+- struct map_range *mappings)
++ struct map_range *mappings, bool *allow_setgroups)
+ {
+ struct map_range *mapping;
+ int idx;
+
+ mapping = mappings;
+ for (idx = 0; idx < ranges; idx++, mapping++) {
+- if (!verify_range(pw, mapping)) {
++ if (!verify_range(pw, mapping, allow_setgroups)) {
+ fprintf(stderr, _( "%s: gid range [%lu-%lu) -> [%lu-%lu) not allowed\n"),
+ Prog,
+ mapping->upper,
+@@ -89,6 +94,70 @@ static void usage(void)
+ exit(EXIT_FAILURE);
+ }
+
++void write_setgroups(int proc_dir_fd, bool allow_setgroups)
++{
++ int setgroups_fd;
++ char *policy, policy_buffer[4096];
++
++ /*
++ * Default is "deny", and any "allow" will out-rank a "deny". We don't
++ * forcefully write an "allow" here because the process we are writing
++ * mappings for may have already set themselves to "deny" (and "allow"
++ * is the default anyway). So allow_setgroups == true is a noop.
++ */
++ policy = "deny\n";
++ if (allow_setgroups)
++ return;
++
++ setgroups_fd = openat(proc_dir_fd, "setgroups", O_RDWR|O_CLOEXEC);
++ if (setgroups_fd < 0) {
++ /*
++ * If it's an ENOENT then we are on too old a kernel for the setgroups
++ * code to exist. Emit a warning and bail on this.
++ */
++ if (ENOENT == errno) {
++ fprintf(stderr, _("%s: kernel doesn't support setgroups restrictions\n"), Prog);
++ goto out;
++ }
++ fprintf(stderr, _("%s: couldn't open process setgroups: %s\n"),
++ Prog,
++ strerror(errno));
++ exit(EXIT_FAILURE);
++ }
++
++ /*
++ * Check whether the policy is already what we want. /proc/self/setgroups
++ * is write-once, so attempting to write after it's already written to will
++ * fail.
++ */
++ if (read(setgroups_fd, policy_buffer, sizeof(policy_buffer)) < 0) {
++ fprintf(stderr, _("%s: failed to read setgroups: %s\n"),
++ Prog,
++ strerror(errno));
++ exit(EXIT_FAILURE);
++ }
++ if (!strncmp(policy_buffer, policy, strlen(policy)))
++ goto out;
++
++ /* Write the policy. */
++ if (lseek(setgroups_fd, 0, SEEK_SET) < 0) {
++ fprintf(stderr, _("%s: failed to seek setgroups: %s\n"),
++ Prog,
++ strerror(errno));
++ exit(EXIT_FAILURE);
++ }
++ if (dprintf(setgroups_fd, "%s", policy) < 0) {
++ fprintf(stderr, _("%s: failed to setgroups %s policy: %s\n"),
++ Prog,
++ policy,
++ strerror(errno));
++ exit(EXIT_FAILURE);
++ }
++
++out:
++ close(setgroups_fd);
++}
++
+ /*
+ * newgidmap - Set the gid_map for the specified process
+ */
+@@ -103,6 +172,7 @@ int main(int argc, char **argv)
+ struct stat st;
+ struct passwd *pw;
+ int written;
++ bool allow_setgroups = false;
+
+ Prog = Basename (argv[0]);
+
+@@ -145,7 +215,7 @@ int main(int argc, char **argv)
+ (unsigned long) getuid ()));
+ return EXIT_FAILURE;
+ }
+-
++
+ /* Get the effective uid and effective gid of the target process */
+ if (fstat(proc_dir_fd, &st) < 0) {
+ fprintf(stderr, _("%s: Could not stat directory for target %u\n"),
+@@ -177,8 +247,9 @@ int main(int argc, char **argv)
+ if (!mappings)
+ usage();
+
+- verify_ranges(pw, ranges, mappings);
++ verify_ranges(pw, ranges, mappings, &allow_setgroups);
+
++ write_setgroups(proc_dir_fd, allow_setgroups);
+ write_mapping(proc_dir_fd, ranges, mappings, "gid_map");
+ sub_gid_close();
+
development / crossrootfs.git / commitdiff