Commit 040757f7 authored by Eric W. Biederman's avatar Eric W. Biederman

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.

Fixes: f6b2db1a ("userns: Make the count of user namespaces per user")
Reported-by: default avatarJongHwan Kim <>
Reported-by: default avatarDmitry Vyukov <>
Reviewed-by: default avatarAndrei Vagin <>
Signed-off-by: default avatar"Eric W. Biederman" <>
parent c1ae3cfa
......@@ -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];
......@@ -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;
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;
ucounts->count += 1;
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)
spin_unlock_irqrestore(&ucounts_lock, flags);
ucounts = NULL;
spin_unlock_irqrestore(&ucounts_lock, flags);
static inline bool atomic_inc_below(atomic_t *v, int u)
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