Re: timerfd: use-after-free in timerfd_remove_cancel

[Mateusz Guzik]

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)