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Merge pull request #97 from cyphar/newgidmap-secure-setgroups

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newgidmap: enforce setgroups=deny if self-mapping a group
This commit is contained in:
Serge Hallyn 2018-02-16 08:40:39 -06:00 committed by GitHub
commit c53e4c1d77
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GPG Key ID: 4AEE18F83AFDEB23
2 changed files with 81 additions and 9 deletions
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1
README
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@ -44,6 +44,7 @@ a lot of mail...
Adam Rudnicki <adam@v-lo.krakow.pl>
Alan Curry <pacman@tardis.mars.net>
Aleksa Sarai <cyphar@cyphar.com>
Alexander O. Yuriev <alex@bach.cis.temple.edu>
Algis Rudys <arudys@rice.edu>
Andreas Jaeger <aj@arthur.rhein-neckar.de>

89
src/newgidmap.c
View File

@ -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();