ucounts: Set ucount_max to the largest positive value the type can hold

The ns->ucount_max[] is signed long which is less than the rlimit size.
We have to protect ucount_max[] from overflow and only use the largest
value that we can hold.

On 32bit using "long" instead of "unsigned long" to hold the counts has
the downside that RLIMIT_MSGQUEUE and RLIMIT_MEMLOCK are limited to 2GiB
instead of 4GiB. I don't think anyone cares but it should be mentioned
in case someone does.

The RLIMIT_NPROC and RLIMIT_SIGPENDING used atomic_t so their maximum
hasn't changed.

Signed-off-by: Alexey Gladkov <legion@kernel.org>
Link: https://lkml.kernel.org/r/1825a5dfa18bc5a570e79feb05e2bd07fd57e7e3.1619094428.git.legion@kernel.org
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>

Author: legionus

include/linux/user_namespace.h

@@ -125,6 +125,12 @@ long inc_rlimit_ucounts(struct ucounts *ucounts, enum ucount_type type, long v);
 bool dec_rlimit_ucounts(struct ucounts *ucounts, enum ucount_type type, long v);
 bool is_ucounts_overlimit(struct ucounts *ucounts, enum ucount_type type, unsigned long max);
 
+static inline void set_rlimit_ucount_max(struct user_namespace *ns,
+		enum ucount_type type, unsigned long max)
+{
+	ns->ucount_max[type] = max <= LONG_MAX ? max : LONG_MAX;
+}
+
 #ifdef CONFIG_USER_NS
 
 static inline struct user_namespace *get_user_ns(struct user_namespace *ns)

kernel/fork.c

@@ -822,10 +822,10 @@ void __init fork_init(void)
 	for (i = 0; i < MAX_PER_NAMESPACE_UCOUNTS; i++)
 		init_user_ns.ucount_max[i] = max_threads/2;
 
-	init_user_ns.ucount_max[UCOUNT_RLIMIT_NPROC] = task_rlimit(&init_task, RLIMIT_NPROC);
-	init_user_ns.ucount_max[UCOUNT_RLIMIT_MSGQUEUE] = task_rlimit(&init_task, RLIMIT_MSGQUEUE);
-	init_user_ns.ucount_max[UCOUNT_RLIMIT_SIGPENDING] = task_rlimit(&init_task, RLIMIT_SIGPENDING);
-	init_user_ns.ucount_max[UCOUNT_RLIMIT_MEMLOCK] = task_rlimit(&init_task, RLIMIT_MEMLOCK);
+	set_rlimit_ucount_max(&init_user_ns, UCOUNT_RLIMIT_NPROC, task_rlimit(&init_task, RLIMIT_NPROC));
+	set_rlimit_ucount_max(&init_user_ns, UCOUNT_RLIMIT_MSGQUEUE, task_rlimit(&init_task, RLIMIT_MSGQUEUE));
+	set_rlimit_ucount_max(&init_user_ns, UCOUNT_RLIMIT_SIGPENDING, task_rlimit(&init_task, RLIMIT_SIGPENDING));
+	set_rlimit_ucount_max(&init_user_ns, UCOUNT_RLIMIT_MEMLOCK, task_rlimit(&init_task, RLIMIT_MEMLOCK));
 
 #ifdef CONFIG_VMAP_STACK
 	cpuhp_setup_state(CPUHP_BP_PREPARE_DYN, "fork:vm_stack_cache",

kernel/user_namespace.c

@@ -122,10 +122,10 @@ int create_user_ns(struct cred *new)
 	for (i = 0; i < MAX_PER_NAMESPACE_UCOUNTS; i++) {
 		ns->ucount_max[i] = INT_MAX;
 	}
-	ns->ucount_max[UCOUNT_RLIMIT_NPROC] = rlimit(RLIMIT_NPROC);
-	ns->ucount_max[UCOUNT_RLIMIT_MSGQUEUE] = rlimit(RLIMIT_MSGQUEUE);
-	ns->ucount_max[UCOUNT_RLIMIT_SIGPENDING] = rlimit(RLIMIT_SIGPENDING);
-	ns->ucount_max[UCOUNT_RLIMIT_MEMLOCK] = rlimit(RLIMIT_MEMLOCK);
+	set_rlimit_ucount_max(ns, UCOUNT_RLIMIT_NPROC, rlimit(RLIMIT_NPROC));
+	set_rlimit_ucount_max(ns, UCOUNT_RLIMIT_MSGQUEUE, rlimit(RLIMIT_MSGQUEUE));
+	set_rlimit_ucount_max(ns, UCOUNT_RLIMIT_SIGPENDING, rlimit(RLIMIT_SIGPENDING));
+	set_rlimit_ucount_max(ns, UCOUNT_RLIMIT_MEMLOCK, rlimit(RLIMIT_MEMLOCK));
 	ns->ucounts = ucounts;
 
 	/* Inherit USERNS_SETGROUPS_ALLOWED from our parent */