Re: [Intel-wired-lan] [PATCH iwl-net v1] i40e: fix races in PTP external timestamp work handling

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---
--- Patch [1]: [PATCH iwl-net v1] i40e: fix races in PTP external timestamp work handling ---
commit 4359cf206cb2602e153fdeac3151f8d4bacf9195
Author: Xuanqiang Luo <[email protected]>

i40e: fix races in PTP external timestamp work handling

This commit disables external timestamp events and drains the work
queue before unregistering the PHC to fix a use-after-free, and
moves the work initialization to device init instead of reset. It also
excludes reset recovery during teardown to avoid concurrent rebuilds.

> diff --git a/drivers/net/ethernet/intel/i40e/i40e_main.c b/drivers/net/ethernet/intel/i40e/i40e_main.c
> index a04683004a567..2909a58d57562 100644
> --- a/drivers/net/ethernet/intel/i40e/i40e_main.c
> +++ b/drivers/net/ethernet/intel/i40e/i40e_main.c
[ ... ]
> @@ -16157,14 +16158,6 @@ static void i40e_remove(struct pci_dev *pdev)
>  
>  	i40e_devlink_unregister(pf);
>  
> -	i40e_dbg_pf_exit(pf);
> -
> -	i40e_ptp_stop(pf);
> -
> -	/* Disable RSS in hw */
> -	i40e_write_rx_ctl(hw, I40E_PFQF_HENA(0), 0);
> -	i40e_write_rx_ctl(hw, I40E_PFQF_HENA(1), 0);
> -
>  	/* Grab __I40E_RESET_RECOVERY_PENDING and set __I40E_IN_REMOVE
>  	 * flags, once they are set, i40e_rebuild should not be called as
>  	 * i40e_prep_for_reset always returns early.
> @@ -16173,6 +16166,14 @@ static void i40e_remove(struct pci_dev *pdev)
>  		usleep_range(1000, 2000);

Simon says: I think the issue flagged below is somewhat orthogonal to
            this patch and can be considered in the context of possible
	    follow-up.

[Severity: High]
This is a pre-existing issue, but does this ad-hoc polling loop violate core
networking synchronization rules?

The subsystem guidelines state that using retry/poll loops on flags to guard
code sections is a bug, as it evades lockdep deadlock detection and lacks
proper memory ordering guarantees. 

i40e_remove() spins on __I40E_RESET_RECOVERY_PENDING with a sleep instead of
using a standard lock or workqueue synchronization, which exposes the teardown
path to races during hotplug or PCIe AER unbinds.

>  	set_bit(__I40E_IN_REMOVE, pf->state);
>  
> +	i40e_dbg_pf_exit(pf);
> +
> +	i40e_ptp_stop(pf);

[ ... ]
> diff --git a/drivers/net/ethernet/intel/i40e/i40e_ptp.c b/drivers/net/ethernet/intel/i40e/i40e_ptp.c
> index ff62b5f2c8150..fd51ab8c10c20 100644
> --- a/drivers/net/ethernet/intel/i40e/i40e_ptp.c
> +++ b/drivers/net/ethernet/intel/i40e/i40e_ptp.c
[ ... ]
> @@ -1541,6 +1550,13 @@ void i40e_ptp_stop(struct i40e_pf *pf)
>  	struct i40e_hw *hw = &pf->hw;
>  	u32 regval;
>  
> +	/* Stop external timestamp events before unregistering the clock. */
> +	regval = rd32(hw, I40E_PRTTSYN_CTL0);
> +	regval &= ~I40E_PRTTSYN_CTL0_EVENT_INT_ENA_MASK;
> +	wr32(hw, I40E_PRTTSYN_CTL0, regval);
> +
> +	disable_work_sync(&pf->ptp_extts0_work);
> +

[Severity: Medium]
Does this reordering introduce a TOCTOU race condition with the PTP ioctl?

The flag I40E_FLAG_PTP_ENA is still set here while we disable the workqueue.
Because i40e_remove() does not hold the rtnl_lock(), a concurrent
SIOCSHWTSTAMP ioctl could execute i40e_ptp_hwtstamp_set().

Since the ioctl checks I40E_FLAG_PTP_ENA, it might observe it as still set
and call i40e_ptp_set_timestamp_mode(). This would overwrite I40E_PRTTSYN_CTL0
and re-enable hardware interrupts immediately after i40e_ptp_stop() just
disabled them, leaving the device generating events for a disabled workqueue.

>  	clear_bit(I40E_FLAG_PTP_ENA, pf->flags);
>  	pf->ptp_tx = false;
>  	pf->ptp_rx = false;
>  
>  	if (pf->ptp_tx_skb) {
>  		struct sk_buff *skb = pf->ptp_tx_skb;
>  
>  		pf->ptp_tx_skb = NULL;
>  		clear_bit_unlock(__I40E_PTP_TX_IN_PROGRESS, pf->state);
>  		dev_kfree_skb_any(skb);
>  	}

Simon says: Although the issue flagged below is flagged as pre-existing
            I am concerned that it is exacerbated by this patch.

[Severity: High]
This is a pre-existing issue, but could this freeing pattern result in a
double-free or use-after-free?

A user configuring PTP and transmitting a packet sets pf->ptp_tx_skb.
The watchdog task in i40e_ptp_tx_hang() checks the timeout and reads
pf->ptp_tx_skb without an atomic test-and-set claim.

If a hardware interrupt arrives at this exact time, i40e_ptp_tx_hwtstamp()
can preempt the watchdog, read the exact same pointer, set it to NULL,
call skb_tstamp_tx() and then dev_kfree_skb_any().

When the interrupt returns, the watchdog task resumes and frees the exact
same SKB pointer again. This teardown path in i40e_ptp_stop() also forcibly
frees ptp_tx_skb without atomic protections against the interrupt handler,
compounding the race.

[ ... ]