Commit message: nfsd: auth: Fix gid sorting when rootsquash enabled

Commit bdcf0a423ea1 ("kernel: make groups_sort calling a responsibility
group_info allocators") appears to break nfsd rootsquash in a pretty
major way.

It adds a call to groups_sort() inside the loop that copies/squashes
gids, which means the valid gids are sorted along with the following
garbage.  The net result is that the highest numbered valid gids are
replaced with any lower-valued garbage gids, possibly including 0.

We should sort only once, after filling in all the gids.

Fixes: bdcf0a423ea1 ("kernel: make groups_sort calling a responsibility ...")
Signed-off-by: Ben Hutchings <ben.hutchings@codethink.co.uk>
Acked-by: J. Bruce Fields <bfields@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
CVE: CVE-2018-1000028
CWE: CWE-269
Vulnerability description: Linux kernel version after commit bdcf0a423ea1 - 4.15-rc4+, 4.14.8+, 4.9.76+, 4.4.111+ contains a Incorrect Access Control vulnerability in NFS server (nfsd) that can result in remote users reading or writing files they should not be able to via NFS. This attack appear to be exploitable via NFS server must export a filesystem with the "rootsquash" options enabled. This vulnerability appears to have been fixed in after commit 1995266727fa.

Project: linux
File: auth.c

int nfsd_setuser(struct svc_rqst *rqstp, struct svc_export *exp)
{
	struct group_info *rqgi;
	struct group_info *gi;
	struct cred *new;
	int i;
	int flags = nfsexp_flags(rqstp, exp);

	validate_process_creds();

	/* discard any old override before preparing the new set */
	revert_creds(get_cred(current_real_cred()));
	new = prepare_creds();
	if (!new)
		return -ENOMEM;

	new->fsuid = rqstp->rq_cred.cr_uid;
	new->fsgid = rqstp->rq_cred.cr_gid;

	rqgi = rqstp->rq_cred.cr_group_info;

	if (flags & NFSEXP_ALLSQUASH) {
		new->fsuid = exp->ex_anon_uid;
		new->fsgid = exp->ex_anon_gid;
		gi = groups_alloc(0);
		if (!gi)
			goto oom;
	} else if (flags & NFSEXP_ROOTSQUASH) {
		if (uid_eq(new->fsuid, GLOBAL_ROOT_UID))
			new->fsuid = exp->ex_anon_uid;
		if (gid_eq(new->fsgid, GLOBAL_ROOT_GID))
			new->fsgid = exp->ex_anon_gid;

		gi = groups_alloc(rqgi->ngroups);
		if (!gi)
			goto oom;

		for (i = 0; i < rqgi->ngroups; i++) {
			if (gid_eq(GLOBAL_ROOT_GID, rqgi->gid[i]))
				gi->gid[i] = exp->ex_anon_gid;
			else
				gi->gid[i] = rqgi->gid[i];
		}

		/* Each thread allocates its own gi, no race */
		groups_sort(gi);
	} else {
		gi = get_group_info(rqgi);
	}

	if (uid_eq(new->fsuid, INVALID_UID))
		new->fsuid = exp->ex_anon_uid;
	if (gid_eq(new->fsgid, INVALID_GID))
		new->fsgid = exp->ex_anon_gid;

	set_groups(new, gi);
	put_group_info(gi);

	if (!uid_eq(new->fsuid, GLOBAL_ROOT_UID))
		new->cap_effective = cap_drop_nfsd_set(new->cap_effective);
	else
		new->cap_effective = cap_raise_nfsd_set(new->cap_effective,
							new->cap_permitted);
	validate_process_creds();
	put_cred(override_creds(new));
	put_cred(new);
	validate_process_creds();
	return 0;

oom:
	abort_creds(new);
	return -ENOMEM;
}

Project: linux
File: auth.c

int nfsexp_flags(struct svc_rqst *rqstp, struct svc_export *exp)
{
	struct exp_flavor_info *f;
	struct exp_flavor_info *end = exp->ex_flavors + exp->ex_nflavors;

	for (f = exp->ex_flavors; f < end; f++) {
		if (f->pseudoflavor == rqstp->rq_cred.cr_flavor)
			return f->flags;
	}
	return exp->ex_flags;

}