kernel: make groups_sort calling a responsibility group_info allocators

In testing, we found that nfsd threads may call set_groups in parallel
for the same entry cached in auth.unix.gid, racing in the call of
groups_sort, corrupting the groups for that entry and leading to
permission denials for the client.

This patch:
- Make groups_sort globally visible.
- Move the call to groups_sort to the modifiers of group_info
- Remove the call to groups_sort from set_groups

Link: http://lkml.kernel.org/r/[e-mail]
Signed-off-by: Thiago Rafael Becker <[e-mail]>
Reviewed-by: Matthew Wilcox <[e-mail]>
Reviewed-by: NeilBrown <[e-mail]>
Acked-by: "J. Bruce Fields" <[e-mail]>
Cc: Al Viro <[e-mail]>
Cc: Martin Schwidefsky <[e-mail]>
Cc: <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

arch/s390/kernel/compat_linux.c

···
 		return retval;
 	}
 
+	groups_sort(group_info);
 	retval = set_current_groups(group_info);
 	put_group_info(group_info);
 

fs/nfsd/auth.c

···
 				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);

include/linux/cred.h

···
 extern void set_groups(struct cred *, struct group_info *);
 extern int groups_search(const struct group_info *, kgid_t);
 extern bool may_setgroups(void);
+extern void groups_sort(struct group_info *);
 
 /*
  * The security context of a task

kernel/groups.c

···
 	return gid_gt(a, b) - gid_lt(a, b);
 }
 
-static void groups_sort(struct group_info *group_info)
+void groups_sort(struct group_info *group_info)
 {
 	sort(group_info->gid, group_info->ngroups, sizeof(*group_info->gid),
 	     gid_cmp, NULL);
 }
+EXPORT_SYMBOL(groups_sort);
 
 /* a simple bsearch */
 int groups_search(const struct group_info *group_info, kgid_t grp)
···
 void set_groups(struct cred *new, struct group_info *group_info)
 {
 	put_group_info(new->group_info);
-	groups_sort(group_info);
 	get_group_info(group_info);
 	new->group_info = group_info;
 }
···
 		return retval;
 	}
 
+	groups_sort(group_info);
 	retval = set_current_groups(group_info);
 	put_group_info(group_info);
 

kernel/uid16.c

···
 		return retval;
 	}
 
+	groups_sort(group_info);
 	retval = set_current_groups(group_info);
 	put_group_info(group_info);
 

net/sunrpc/auth_gss/gss_rpc_xdr.c

···
 			goto out_free_groups;
 		creds->cr_group_info->gid[i] = kgid;
 	}
+	groups_sort(creds->cr_group_info);
 
 	return 0;
 out_free_groups:

net/sunrpc/auth_gss/svcauth_gss.c

···
 				goto out;
 			rsci.cred.cr_group_info->gid[i] = kgid;
 		}
+		groups_sort(rsci.cred.cr_group_info);
 
 		/* mech name */
 		len = qword_get(&mesg, buf, mlen);

net/sunrpc/svcauth_unix.c

···
 		ug.gi->gid[i] = kgid;
 	}
 
+	groups_sort(ug.gi);
 	ugp = unix_gid_lookup(cd, uid);
 	if (ugp) {
 		struct cache_head *ch;
···
 		kgid_t kgid = make_kgid(&init_user_ns, svc_getnl(argv));
 		cred->cr_group_info->gid[i] = kgid;
 	}
+	groups_sort(cred->cr_group_info);
 	if (svc_getu32(argv) != htonl(RPC_AUTH_NULL) || svc_getu32(argv) != 0) {
 		*authp = rpc_autherr_badverf;
 		return SVC_DENIED;