84 lines
2.6 KiB
Diff
84 lines
2.6 KiB
Diff
From: "Eric W. Biederman" <ebiederm@xmission.com>
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Date: Sun, 5 Mar 2017 15:03:22 -0600
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Subject: ucount: Remove the atomicity from ucount->count
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Origin: https://git.kernel.org/linus/040757f738e13caaa9c5078bca79aa97e11dde88
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Bug-Debian-Security: https://security-tracker.debian.org/tracker/CVE-2017-6874
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Always increment/decrement ucount->count under the ucounts_lock. The
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increments are there already and moving the decrements there means the
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locking logic of the code is simpler. This simplification in the
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locking logic fixes a race between put_ucounts and get_ucounts that
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could result in a use-after-free because the count could go zero then
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be found by get_ucounts and then be freed by put_ucounts.
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A bug presumably this one was found by a combination of syzkaller and
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KASAN. JongWhan Kim reported the syzkaller failure and Dmitry Vyukov
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spotted the race in the code.
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Cc: stable@vger.kernel.org
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Fixes: f6b2db1a3e8d ("userns: Make the count of user namespaces per user")
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Reported-by: JongHwan Kim <zzoru007@gmail.com>
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Reported-by: Dmitry Vyukov <dvyukov@google.com>
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Reviewed-by: Andrei Vagin <avagin@gmail.com>
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Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
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---
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include/linux/user_namespace.h | 2 +-
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kernel/ucount.c | 18 +++++++++++-------
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2 files changed, 12 insertions(+), 8 deletions(-)
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--- a/include/linux/user_namespace.h
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+++ b/include/linux/user_namespace.h
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@@ -65,7 +65,7 @@ struct ucounts {
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struct hlist_node node;
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struct user_namespace *ns;
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kuid_t uid;
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- atomic_t count;
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+ int count;
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atomic_t ucount[UCOUNT_COUNTS];
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};
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--- a/kernel/ucount.c
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+++ b/kernel/ucount.c
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@@ -139,7 +139,7 @@ static struct ucounts *get_ucounts(struc
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new->ns = ns;
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new->uid = uid;
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- atomic_set(&new->count, 0);
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+ new->count = 0;
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spin_lock_irq(&ucounts_lock);
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ucounts = find_ucounts(ns, uid, hashent);
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@@ -150,8 +150,10 @@ static struct ucounts *get_ucounts(struc
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ucounts = new;
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}
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}
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- if (!atomic_add_unless(&ucounts->count, 1, INT_MAX))
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+ if (ucounts->count == INT_MAX)
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ucounts = NULL;
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+ else
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+ ucounts->count += 1;
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spin_unlock_irq(&ucounts_lock);
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return ucounts;
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}
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@@ -160,13 +162,15 @@ static void put_ucounts(struct ucounts *
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{
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unsigned long flags;
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- if (atomic_dec_and_test(&ucounts->count)) {
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- spin_lock_irqsave(&ucounts_lock, flags);
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+ spin_lock_irqsave(&ucounts_lock, flags);
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+ ucounts->count -= 1;
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+ if (!ucounts->count)
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hlist_del_init(&ucounts->node);
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- spin_unlock_irqrestore(&ucounts_lock, flags);
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+ else
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+ ucounts = NULL;
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+ spin_unlock_irqrestore(&ucounts_lock, flags);
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- kfree(ucounts);
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- }
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+ kfree(ucounts);
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}
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static inline bool atomic_inc_below(atomic_t *v, int u)
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