Re: [PATCH] isdn: hisax: isac: fix a possible concurrency use-after-free bug in ISAC_l1hw()

[David Miller]

From: Young Xiao <92siuyang@xxxxxxxxx>
Date: Wed, 29 May 2019 16:48:59 +0800

> In drivers/isdn/hisax/isac.c, the function isac_interrupt() and
> ISAC_l1hw() may be concurrently executed.
> 
> ISAC_l1hw()
>     line 499: if (!cs->tx_skb)
> 
> isac_interrupt()
>     line 250: dev_kfree_skb_irq(cs->tx_skb);
> 
> Thus, a possible concurrency use-after-free bug may occur in ISAC_l1hw().
 ...
> +		spin_lock_irqsave(&cs->lock, flags);
>  		if (!cs->tx_skb) {
>  			test_and_clear_bit(FLG_L1_PULL_REQ, &st->l1.Flags);
>  			st->l1.l1l2(st, PH_PULL | CONFIRM, NULL);
>  		} else
>  			test_and_set_bit(FLG_L1_PULL_REQ, &st->l1.Flags);
> +		spin_unlock_irqrestore(&cs->lock, flags);

Nothing in this code accesses the cs->tx_skb object.  It is just a logic test
upon whether it is NULL or not.

I'm not applying stuff like this, sorry.

You have to show how something can actually go wrong when fixing a bug.