linux-kernel - Re: [PATCH] pci/pciehp: bail on bogus pcie reads from removed devices

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Date:	Tue, 4 Aug 2015 16:11:49 -0500
From:	Bjorn Helgaas <bhelgaas@...gle.com>
To:	Jarod Wilson <jarod@...hat.com>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
	"linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>
Subject: Re: [PATCH] pci/pciehp: bail on bogus pcie reads from removed devices

On Tue, Aug 4, 2015 at 3:22 PM, Jarod Wilson <jarod@...hat.com> wrote:
> On 8/4/2015 3:27 PM, Bjorn Helgaas wrote:
>>
>> On Tue, Aug 4, 2015 at 1:46 PM, Jarod Wilson <jarod@...hat.com> wrote:
>>>
>>> On 8/4/2015 1:51 PM, Bjorn Helgaas wrote:
>
> ...
>
>>>>>> Can you try the version I posted, with the additional tests in
>>>>>> pcie_poll_cmd() and pcie_do_write_cmd()?  We should try to read from
>>>>>> the device there, even before we free the IRQ, so we might see several
>>>>>> messages.  Maybe there's a way we can be smarter about bailing out
>>>>>> there.
>>>>>
>>>>>
>>>>>
>>>>> The above was with your additions munged in with the older patch, I
>>>>> actually do see "pcie_do_write_cmd: no response from device" just
>>>>> two lines ahead of each "Device has gone away" message from
>>>>> pcie_isr().
>>>>>
>>>>> pciehp 0000:06:00.0:pcie24: pcie_do_write_cmd: no response from device
>>>>> pciehp 0000:06:00.0:pcie24: pcie_disable_notification: SLOTCTRL d8
>>>>> write cmd 0
>>>>> pciehp 0000:06:00.0:pcie24: Device has gone away <- from pcie_isr()
>>>>
>>>>
>>>>
>>>> Oh, sorry!  I should have noticed that.  I just wanted to make sure I
>>>> didn't cause a flood of extra messages.
>>>>
>>>> I think I'll merge this version (with all three checks).  We still have
>>>> a
>>>> slot lifetime issue, but that's a separate problem.
>>>
>>>
>>>
>>> Sounds good to me, thanks much for your help on this.
>>>
>>> Do we really still have a slot lifetime issue though? It looks to be the
>>> path from pciehp_release_ctrl that leads to free_irq and __free_irq
>>> calling
>>> pcie_isr one last time, and there's a ctrl_info("Latch %s on Slot(%s)",
>>> open
>>> ? ..., slot_name(slot)); in pcie_isr *if* we aren't bailing when we read
>>> all
>>> 1's from PCI_EXP_SLTSTA. I think when we bail early, we should never see
>>> the
>>> subsequent attempt to read the freed slot.
>>
>>
>> It's possible that we avoid referencing the freed data, but I don't
>> have warm fuzzies because it's hard to prove that by analyzing the
>> source code.  It's hard to even know what to look for -- there's no
>> clue in the code that says "don't reference slot->hotplug_slot after
>> this point."  And it feels like a poor design to hang on to that
>> pointer after the slot has been freed.
>>
>> IIRC, your initial report mentioned possible memory corruption, and I
>> don't even have a theory about where that happened.  The
>> slot->hotplug_slot references I saw were all reads where we printed
>> junk but shouldn't have actually corrupted anything.
>
>
> Looking at the output I was seeing, it looks like one of the ~0 reads is
> interpreted as a switch interrupt received, data link layer state change,
> etc., followed by "Enabling domain:bus:device=0000:0x:00" from
> pciehp_power_thread. Subsequently, we're calling pciehp_enable_slot, which
> calls board_added, and in the output I've got, its tripping over
> board_added's call to pciehp_check_link_status ("Failed to check link
> status"), which means going to err_exit and calling set_slot_off.
>
> Next up, set_slot_off is calling pciehp_power_off_slot, which does a
> pcie_write_cmd(). Is it possible that write might lead to memory corruption?

I doubt it; pcie_write_cmd() by itself just writes to a bridge
register.  Even if the device is gone, that shouldn't corrupt memory.
But I don't know what really happened, and I don't remember what led
to the corruption hypothesis in the first place.  I think the
corrupted-looking slot name strings are just a consequence of reading
memory that had already been freed.  With some work, we might be able
to confirm that by matching it with a poison pattern or something.

Bjorn
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