Reimplement RLIMIT_NPROC on top of ucounts

The rlimit counter is tied to uid in the user_namespace. This allows
rlimit values to be specified in userns even if they are already
globally exceeded by the user. However, the value of the previous
user_namespaces cannot be exceeded.

To illustrate the impact of rlimits, let's say there is a program that
does not fork. Some service-A wants to run this program as user X in
multiple containers. Since the program never fork the service wants to
set RLIMIT_NPROC=1.

service-A
 \- program (uid=1000, container1, rlimit_nproc=1)
 \- program (uid=1000, container2, rlimit_nproc=1)

The service-A sets RLIMIT_NPROC=1 and runs the program in container1.
When the service-A tries to run a program with RLIMIT_NPROC=1 in
container2 it fails since user X already has one running process.

We cannot use existing inc_ucounts / dec_ucounts because they do not
allow us to exceed the maximum for the counter. Some rlimits can be
overlimited by root or if the user has the appropriate capability.

Changelog

v11:
* Change inc_rlimit_ucounts() which now returns top value of ucounts.
* Drop inc_rlimit_ucounts_and_test() because the return code of
  inc_rlimit_ucounts() can be checked.

Signed-off-by: Alexey Gladkov <legion@kernel.org>
Link: https://lkml.kernel.org/r/c5286a8aa16d2d698c222f7532f3d735c82bc6bc.1619094428.git.legion@kernel.org
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>

1 files changed, 44 insertions, 0 deletions

diff --git a/kernel/ucount.c b/kernel/ucount.c
index 365865f368ec..6caa56f7dec8 100644
--- a/kernel/ucount.c
+++ b/kernel/ucount.c
@@ -80,6 +80,7 @@ static struct ctl_table user_table[] = {
 	UCOUNT_ENTRY("max_inotify_instances"),
 	UCOUNT_ENTRY("max_inotify_watches"),
 #endif
+	{ },
 	{ }
 };
 #endif /* CONFIG_SYSCTL */
@@ -240,6 +241,48 @@ void dec_ucount(struct ucounts *ucounts, enum ucount_type type)
 	put_ucounts(ucounts);
 }
 
+long inc_rlimit_ucounts(struct ucounts *ucounts, enum ucount_type type, long v)
+{
+	struct ucounts *iter;
+	long ret = 0;
+
+	for (iter = ucounts; iter; iter = iter->ns->ucounts) {
+		long max = READ_ONCE(iter->ns->ucount_max[type]);
+		long new = atomic_long_add_return(v, &iter->ucount[type]);
+		if (new < 0 || new > max)
+			ret = LONG_MAX;
+		else if (iter == ucounts)
+			ret = new;
+	}
+	return ret;
+}
+
+bool dec_rlimit_ucounts(struct ucounts *ucounts, enum ucount_type type, long v)
+{
+	struct ucounts *iter;
+	long new;
+	for (iter = ucounts; iter; iter = iter->ns->ucounts) {
+		long dec = atomic_long_add_return(-v, &iter->ucount[type]);
+		WARN_ON_ONCE(dec < 0);
+		if (iter == ucounts)
+			new = dec;
+	}
+	return (new == 0);
+}
+
+bool is_ucounts_overlimit(struct ucounts *ucounts, enum ucount_type type, unsigned long max)
+{
+	struct ucounts *iter;
+	if (get_ucounts_value(ucounts, type) > max)
+		return true;
+	for (iter = ucounts; iter; iter = iter->ns->ucounts) {
+		max = READ_ONCE(iter->ns->ucount_max[type]);
+		if (get_ucounts_value(iter, type) > max)
+			return true;
+	}
+	return false;
+}
+
 static __init int user_namespace_sysctl_init(void)
 {
 #ifdef CONFIG_SYSCTL
@@ -256,6 +299,7 @@ static __init int user_namespace_sysctl_init(void)
 	BUG_ON(!setup_userns_sysctls(&init_user_ns));
 #endif
 	hlist_add_ucounts(&init_ucounts);
+	inc_rlimit_ucounts(&init_ucounts, UCOUNT_RLIMIT_NPROC, 1);
 	return 0;
 }
 subsys_initcall(user_namespace_sysctl_init);