ucount: Remove the atomicity from ucount->count

Always increment/decrement ucount->count under the ucounts_lock.  The
increments are there already and moving the decrements there means the
locking logic of the code is simpler.  This simplification in the
locking logic fixes a race between put_ucounts and get_ucounts that
could result in a use-after-free because the count could go zero then
be found by get_ucounts and then be freed by put_ucounts.

A bug presumably this one was found by a combination of syzkaller and
KASAN.  JongWhan Kim reported the syzkaller failure and Dmitry Vyukov
spotted the race in the code.

Cc: stable@vger.kernel.org
Fixes: f6b2db1a3e8d ("userns: Make the count of user namespaces per user")
Reported-by: JongHwan Kim <zzoru007@gmail.com>
Reported-by: Dmitry Vyukov <dvyukov@google.com>
Reviewed-by: Andrei Vagin <avagin@gmail.com>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>

2 files changed, 12 insertions, 8 deletions

diff --git a/include/linux/user_namespace.h b/include/linux/user_namespace.h
index be765234c0a2..32354b4b4b2b 100644
--- a/include/linux/user_namespace.h
+++ b/include/linux/user_namespace.h
@@ -72,7 +72,7 @@ struct ucounts {
         struct hlist_node node;
         struct user_namespace *ns;
         kuid_t uid;
-        atomic_t count;
+        int count;
         atomic_t ucount[UCOUNT_COUNTS];
 };
 
diff --git a/kernel/ucount.c b/kernel/ucount.c
index 62630a40ab3a..b4eeee03934f 100644
--- a/kernel/ucount.c
+++ b/kernel/ucount.c
@@ -144,7 +144,7 @@ static struct ucounts *get_ucounts(struct user_namespace *ns, kuid_t uid)
 
                 new->ns = ns;
                 new->uid = uid;
-                atomic_set(&new->count, 0);
+                new->count = 0;
 
                 spin_lock_irq(&ucounts_lock);
                 ucounts = find_ucounts(ns, uid, hashent);
@@ -155,8 +155,10 @@ static struct ucounts *get_ucounts(struct user_namespace *ns, kuid_t uid)
                         ucounts = new;
                 }
         }
-        if (!atomic_add_unless(&ucounts->count, 1, INT_MAX))
+        if (ucounts->count == INT_MAX)
                 ucounts = NULL;
+        else
+                ucounts->count += 1;
         spin_unlock_irq(&ucounts_lock);
         return ucounts;
 }
@@ -165,13 +167,15 @@ static void put_ucounts(struct ucounts *ucounts)
 {
         unsigned long flags;
 
-        if (atomic_dec_and_test(&ucounts->count)) {
-                spin_lock_irqsave(&ucounts_lock, flags);
+        spin_lock_irqsave(&ucounts_lock, flags);
+        ucounts->count -= 1;
+        if (!ucounts->count)
                 hlist_del_init(&ucounts->node);
-                spin_unlock_irqrestore(&ucounts_lock, flags);
+        else
+                ucounts = NULL;
+        spin_unlock_irqrestore(&ucounts_lock, flags);
 
-                kfree(ucounts);
-        }
+        kfree(ucounts);
 }
 
 static inline bool atomic_inc_below(atomic_t *v, int u)