diff options
Diffstat (limited to 'ucount-Remove-the-atomicity-from-ucount-count.patch')
| -rw-r--r-- | ucount-Remove-the-atomicity-from-ucount-count.patch | 89 |
1 files changed, 89 insertions, 0 deletions
diff --git a/ucount-Remove-the-atomicity-from-ucount-count.patch b/ucount-Remove-the-atomicity-from-ucount-count.patch new file mode 100644 index 000000000..1b4e3c03f --- /dev/null +++ b/ucount-Remove-the-atomicity-from-ucount-count.patch @@ -0,0 +1,89 @@ +From 040757f738e13caaa9c5078bca79aa97e11dde88 Mon Sep 17 00:00:00 2001 +From: "Eric W. Biederman" <ebiederm@xmission.com> +Date: Sun, 5 Mar 2017 15:03:22 -0600 +Subject: [PATCH] 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> +--- + include/linux/user_namespace.h | 2 +- + kernel/ucount.c | 18 +++++++++++------- + 2 files changed, 12 insertions(+), 8 deletions(-) + +diff --git a/include/linux/user_namespace.h b/include/linux/user_namespace.h +index be76523..32354b4 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 62630a4..b4eeee0 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) +-- +2.9.3 + |