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SubjectRe: timerfd: use-after-free in timerfd_remove_cancel
On Mon, 30 Jan 2017, Dmitry Vyukov wrote:
>
> Seems that ctx->might_cancel is racy.

Yes, it is. Fix below.

8<-------------------

--- a/fs/timerfd.c
+++ b/fs/timerfd.c
@@ -40,9 +40,12 @@ struct timerfd_ctx {
short unsigned settime_flags; /* to show in fdinfo */
struct rcu_head rcu;
struct list_head clist;
- bool might_cancel;
+ unsigned long flags;
};

+/* Bit positions for ctx->flags */
+#define MIGHT_CANCEL 0
+
static LIST_HEAD(cancel_list);
static DEFINE_SPINLOCK(cancel_lock);

@@ -99,7 +102,7 @@ void timerfd_clock_was_set(void)

rcu_read_lock();
list_for_each_entry_rcu(ctx, &cancel_list, clist) {
- if (!ctx->might_cancel)
+ if (!test_bit(MIGHT_CANCEL, &ctx->flags))
continue;
spin_lock_irqsave(&ctx->wqh.lock, flags);
if (ctx->moffs != moffs) {
@@ -114,8 +117,7 @@ void timerfd_clock_was_set(void)

static void timerfd_remove_cancel(struct timerfd_ctx *ctx)
{
- if (ctx->might_cancel) {
- ctx->might_cancel = false;
+ if (test_and_clear_bit(MIGHT_CANCEL, &ctx->flags)) {
spin_lock(&cancel_lock);
list_del_rcu(&ctx->clist);
spin_unlock(&cancel_lock);
@@ -124,7 +126,7 @@ static void timerfd_remove_cancel(struct

static bool timerfd_canceled(struct timerfd_ctx *ctx)
{
- if (!ctx->might_cancel || ctx->moffs != KTIME_MAX)
+ if (!test_bit(MIGHT_CANCEL, &ctx->flags) || ctx->moffs != KTIME_MAX)
return false;
ctx->moffs = ktime_mono_to_real(0);
return true;
@@ -135,13 +137,12 @@ static void timerfd_setup_cancel(struct
if ((ctx->clockid == CLOCK_REALTIME ||
ctx->clockid == CLOCK_REALTIME_ALARM) &&
(flags & TFD_TIMER_ABSTIME) && (flags & TFD_TIMER_CANCEL_ON_SET)) {
- if (!ctx->might_cancel) {
- ctx->might_cancel = true;
+ if (test_and_set_bit(MIGHT_CANCEL, &ctx->flags)) {
spin_lock(&cancel_lock);
list_add_rcu(&ctx->clist, &cancel_list);
spin_unlock(&cancel_lock);
}
- } else if (ctx->might_cancel) {
+ } else {
timerfd_remove_cancel(ctx);
}
}
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