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SubjectRe: timerfd: use-after-free in timerfd_remove_cancel
On Mon, Jan 30, 2017 at 07:41:59PM +0100, Dmitry Vyukov wrote:
> Hello,
>
> The following program triggers use-after-free in timerfd_remove_cancel:
> https://gist.githubusercontent.com/dvyukov/202576d437c84ffbbe52e9ccd77e1b44/raw/5562bff8626a73627157331ea2b837f59080ac84/gistfile1.txt
>
> BUG: KASAN: use-after-free in __list_del include/linux/list.h:104
> [inline] at addr ffff88006bab1410
> BUG: KASAN: use-after-free in __list_del_entry
> include/linux/list.h:119 [inline] at addr ffff88006bab1410
> BUG: KASAN: use-after-free in list_del_rcu include/linux/rculist.h:129
> [inline] at addr ffff88006bab1410
> BUG: KASAN: use-after-free in timerfd_remove_cancel fs/timerfd.c:120
> [inline] at addr ffff88006bab1410
> BUG: KASAN: use-after-free in timerfd_release+0x28e/0x310
> fs/timerfd.c:209 at addr ffff88006bab1410
> Write of size 8 by task a.out/2897
>
[..]
> Seems that ctx->might_cancel is racy.
>

Indeed it is. Can you try the patch below please. If it works I'll send
it in a nicer form.

diff --git a/fs/timerfd.c b/fs/timerfd.c
index c173cc1..63f91c3 100644
--- a/fs/timerfd.c
+++ b/fs/timerfd.c
@@ -112,14 +112,30 @@ void timerfd_clock_was_set(void)
rcu_read_unlock();
}

+static void timerfd_set_cancel(struct timerfd_ctx *ctx)
+{
+ if (ctx->might_cancel)
+ return;
+
+ spin_lock(&cancel_lock);
+ if (!ctx->might_cancel) {
+ ctx->might_cancel = true;
+ list_add_rcu(&ctx->clist, &cancel_list);
+ }
+ spin_unlock(&cancel_lock);
+}
+
static void timerfd_remove_cancel(struct timerfd_ctx *ctx)
{
+ if (!ctx->might_cancel)
+ return;
+
+ spin_lock(&cancel_lock);
if (ctx->might_cancel) {
ctx->might_cancel = false;
- spin_lock(&cancel_lock);
list_del_rcu(&ctx->clist);
- spin_unlock(&cancel_lock);
}
+ spin_unlock(&cancel_lock);
}

static bool timerfd_canceled(struct timerfd_ctx *ctx)
@@ -134,16 +150,10 @@ static void timerfd_setup_cancel(struct timerfd_ctx *ctx, int flags)
{
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;
- spin_lock(&cancel_lock);
- list_add_rcu(&ctx->clist, &cancel_list);
- spin_unlock(&cancel_lock);
- }
- } else if (ctx->might_cancel) {
+ (flags & TFD_TIMER_ABSTIME) && (flags & TFD_TIMER_CANCEL_ON_SET))
+ timerfd_set_cancel(ctx);
+ else
timerfd_remove_cancel(ctx);
- }
}

static ktime_t timerfd_get_remaining(struct timerfd_ctx *ctx)
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