Re: [PATCH] xfrm: iptfs: fix deadlock in iptfs_destroy_state

[Simon Horman <horms@kernel.org>]

From: 'Simon Horman' <horms@kernel.org>

This is an AI-generated review of your patch. The human sending this
email has considered the AI review valid, or at least plausible.
Full review at: https://sashiko.dev
---
xfrm: iptfs: fix deadlock in iptfs_destroy_state

This commit resolves an ABBA deadlock in iptfs_destroy_state() by moving
the cancellation of iptfs_timer and drop_timer outside of their
respective spinlocks.

> diff --git a/net/xfrm/xfrm_iptfs.c b/net/xfrm/xfrm_iptfs.c
> index 97bc979e55baf..11291b87158cb 100644
> --- a/net/xfrm/xfrm_iptfs.c
> +++ b/net/xfrm/xfrm_iptfs.c
> @@ -2708,8 +2708,10 @@ static void iptfs_destroy_state(struct xfrm_state *x)
>  	if (!xtfs)
>  		return;
>  
> -	spin_lock_bh(&xtfs->x->lock);
>  	hrtimer_cancel(&xtfs->iptfs_timer);
> +	hrtimer_cancel(&xtfs->drop_timer);
> +
> +	spin_lock_bh(&xtfs->x->lock);
>  	__skb_queue_head_init(&list);
>  	skb_queue_splice_init(&xtfs->queue, &list);
>  	spin_unlock_bh(&xtfs->x->lock);

Is it safe to call hrtimer_cancel() and skb_queue_splice_init() on xtfs
here if it was created via iptfs_clone_state() but failed initialization?

If iptfs_clone_state() uses kmemdup() to shallow-copy the xtfs structure
from the original state, the copy includes xtfs->queue and the hrtimer
structs (iptfs_timer and drop_timer). Reinitialization only happens later
in iptfs_init_state() -> __iptfs_init_state().

If x->type->init_state() fails during __xfrm_init_state(), the state is
destroyed via xfrm_state_put() before iptfs_init_state() is called.

On this destruction path, does calling hrtimer_cancel() result in a
regression where it operates on the copied timers? If the original timer
was queued, the copied timer has the HRTIMER_STATE_ENQUEUED bit set, but
its rb_node is not natively in the rbtree. Calling hrtimer_cancel() might
call rb_erase_cached() using the cloned node's pointers (which point to
the original node's parent/children).

Additionally, since xtfs->queue is a shallow copy, its next/prev pointers
would point to the original state's queue. Could splicing it here lead to
a regression by modifying the original state's skb queue? If the original
queue was empty, it points to itself, and this code might dequeue it and
call kfree_skb() on an address inside orig->mode_data.