Add a reference to ucounts for each cred

For RLIMIT_NPROC and some other rlimits the user_struct that holds the
global limit is kept alive for the lifetime of a process by keeping it
in struct cred. Adding a pointer to ucounts in the struct cred will
allow to track RLIMIT_NPROC not only for user in the system, but for
user in the user_namespace.

Updating ucounts may require memory allocation which may fail. So, we
cannot change cred.ucounts in the commit_creds() because this function
cannot fail and it should always return 0. For this reason, we modify
cred.ucounts before calling the commit_creds().

Changelog

v6:
* Fix null-ptr-deref in is_ucounts_overlimit() detected by trinity. This
  error was caused by the fact that cred_alloc_blank() left the ucounts
  pointer empty.

Reported-by: kernel test robot <oliver.sang@intel.com>
Signed-off-by: Alexey Gladkov <legion@kernel.org>
Link: https://lkml.kernel.org/r/b37aaef28d8b9b0d757e07ba6dd27281bbe39259.1619094428.git.legion@kernel.org
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>

1 files changed, 37 insertions, 3 deletions

diff --git a/kernel/ucount.c b/kernel/ucount.c
index 04c561751af1..50cc1dfb7d28 100644
--- a/kernel/ucount.c
+++ b/kernel/ucount.c
@@ -8,6 +8,12 @@
 #include <linux/kmemleak.h>
 #include <linux/user_namespace.h>
 
+struct ucounts init_ucounts = {
+	.ns    = &init_user_ns,
+	.uid   = GLOBAL_ROOT_UID,
+	.count = 1,
+};
+
 #define UCOUNTS_HASHTABLE_BITS 10
 static struct hlist_head ucounts_hashtable[(1 << UCOUNTS_HASHTABLE_BITS)];
 static DEFINE_SPINLOCK(ucounts_lock);
@@ -125,7 +131,15 @@ static struct ucounts *find_ucounts(struct user_namespace *ns, kuid_t uid, struc
 	return NULL;
 }
 
-static struct ucounts *get_ucounts(struct user_namespace *ns, kuid_t uid)
+static void hlist_add_ucounts(struct ucounts *ucounts)
+{
+	struct hlist_head *hashent = ucounts_hashentry(ucounts->ns, ucounts->uid);
+	spin_lock_irq(&ucounts_lock);
+	hlist_add_head(&ucounts->node, hashent);
+	spin_unlock_irq(&ucounts_lock);
+}
+
+struct ucounts *alloc_ucounts(struct user_namespace *ns, kuid_t uid)
 {
 	struct hlist_head *hashent = ucounts_hashentry(ns, uid);
 	struct ucounts *ucounts, *new;
@@ -160,7 +174,26 @@ static struct ucounts *get_ucounts(struct user_namespace *ns, kuid_t uid)
 	return ucounts;
 }
 
-static void put_ucounts(struct ucounts *ucounts)
+struct ucounts *get_ucounts(struct ucounts *ucounts)
+{
+	unsigned long flags;
+
+	if (!ucounts)
+		return NULL;
+
+	spin_lock_irqsave(&ucounts_lock, flags);
+	if (ucounts->count == INT_MAX) {
+		WARN_ONCE(1, "ucounts: counter has reached its maximum value");
+		ucounts = NULL;
+	} else {
+		ucounts->count += 1;
+	}
+	spin_unlock_irqrestore(&ucounts_lock, flags);
+
+	return ucounts;
+}
+
+void put_ucounts(struct ucounts *ucounts)
 {
 	unsigned long flags;
 
@@ -194,7 +227,7 @@ struct ucounts *inc_ucount(struct user_namespace *ns, kuid_t uid,
 {
 	struct ucounts *ucounts, *iter, *bad;
 	struct user_namespace *tns;
-	ucounts = get_ucounts(ns, uid);
+	ucounts = alloc_ucounts(ns, uid);
 	for (iter = ucounts; iter; iter = tns->ucounts) {
 		long max;
 		tns = iter->ns;
@@ -237,6 +270,7 @@ static __init int user_namespace_sysctl_init(void)
 	BUG_ON(!user_header);
 	BUG_ON(!setup_userns_sysctls(&init_user_ns));
 #endif
+	hlist_add_ucounts(&init_ucounts);
 	return 0;
 }
 subsys_initcall(user_namespace_sysctl_init);