[PATCH v10 9/9] ucounts: Set ucount_max to the largest positive value the type can hold

Alexey Gladkov
2021-04-07T17:09:14Z

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 <gladkov.alexey@gmail.com>
---
include/linux/user_namespace.h | 6 ++++++
kernel/fork.c | 8 ++++----
kernel/user_namespace.c | 8 ++++----
3 files changed, 14 insertions(+), 8 deletions(-)

diff --git a/include/linux/user_namespace.h b/include/linux/user_namespace.h
index 82851fba7278..1c778182f5d5 100644
--- a/include/linux/user_namespace.h
+++ b/include/linux/user_namespace.h
@@ -123,6 +123,12 @@ bool inc_rlimit_ucounts_and_test(struct ucounts *ucounts, enum ucount_type type,
void 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)
diff --git a/kernel/fork.c b/kernel/fork.c
index a3a5e317c3c0..2cd01c443196 100644
--- a/kernel/fork.c
+++ b/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",
diff --git a/kernel/user_namespace.c b/kernel/user_namespace.c
index 5ef0d4b182ba..df7651935fd5 100644
--- a/kernel/user_namespace.c
+++ b/kernel/user_namespace.c
@@ -121,10 +121,10 @@ 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 */
--
2.29.3