Date:   Fri, 1 Apr 2022 15:57:11 +0800
From:   Li Fei1 <fei1.li@...el.com>
To:     Jakob Koschel <jakobkoschel@...il.com>
Cc:     linux-kernel@...r.kernel.org, rppt@...nel.org,
        bjohannesmeyer@...il.com, c.giuffrida@...nl, h.j.bos@...nl,
        fei1.li@...el.com
Subject: Re: [PATCH] virt: acrn: fix invalid check past list iterator

On Fri, Apr 01, 2022 at 09:16:48AM +0200, Jakob Koschel wrote:
> 
> 
> > On 1. Apr 2022, at 05:57, Li Fei1 <fei1.li@...el.com> wrote:
> > 
> > On Fri, Apr 01, 2022 at 05:22:36AM +0200, Jakob Koschel wrote:
> >> 
> >>> On 1. Apr 2022, at 03:15, Li Fei1 <fei1.li@...el.com> wrote:
> >>> 
> >>> On Thu, Mar 31, 2022 at 01:20:50PM +0200, Jakob Koschel wrote:
> >>>> 
> >>>>> On 30. Mar 2022, at 09:57, Li Fei1 <fei1.li@...el.com> wrote:
> >>>>> 
> >>>>> On Sat, Mar 19, 2022 at 09:38:19PM +0100, Jakob Koschel wrote:
> >>>>>> The condition retry == 0 is theoretically possible even if 'client'
> >>>>>> does not point to a valid element because no break was hit.
> >>>>>> 
> >>>>>> To only execute the dev_warn if actually a break within the loop was
> >>>>>> hit, a separate variable is used that is only set if it is ensured to
> >>>>>> point to a valid client struct.
> >>>>>> 
> >>>>> Hi Koschel
> >>>>> 
> >>>>> Thanks for you to help us to try to improve the code. Maybe you don't get the point.
> >>>>> The dev_warn should only been called when has_pending = true && retry == 0
> >>>> 
> >>>> Maybe I don't understand but looking isolated at this function I could see a way to call
> >>>> the dev_warn() with has_pending = false && retry == 0.
> >>> Yes, even has_pending = true && retry == 0 at the beginning, we could not make sure
> >>> has_pending is true after schedule_timeout_interruptible and we even didn't check
> >>> there're other pending client on the ioreq_clients (because we can't make sure
> >>> when we dev_warn this clent is still pending). So we just use dev_warn not higher log level.
> >> 
> >> I'm sorry, I don't quite understand what you mean by that.
> >> Do you agree that has_pending = false && retry == 0 is possible when calling the dev_warn()
> >> or not?
> >> 
> > Yes, so what ? It just a hint there may have pending request.
> 
> if has_pending == false && retry == 0 when calling dev_warn() there are very clear
> dependencies met:
> 
> * has_pending == false means that the list_for_each_entry() macro it *not* exit early.
> * since list_for_each_entry() did *not* exit early, client will not hold a valid list entry
> * using client->name is not safe and will not point to a valid string (perhaps not even an address)
So you'd better to check when the client in ioreq_clients would been destroyed, right ?

> 
> I'm *only* talking about the case where has_pending == false, in case that's not clear.
> 
> 
> > Even retry == 0 && has_pending = true,
> > When we call dev_warn, the clent may not is pending.
> > 
> > 
> >>>> 
> >>>>> 		list_for_each_entry(client, &vm->ioreq_clients, list) {
> >>>>> 			has_pending = has_pending_request(client);
> >>>>> 			if (has_pending)
> >>>>> 		}
> >>>>> 		spin_unlock_bh(&vm->ioreq_clients_lock);
> >>>> 
> >>>> imagine has_pending == false && retry == 1 here, then client will not hold a valid list entry.
> >>> What do you mean "client will not hold a valid list entry" ? 
> >> 
> >> Imagine a very simple example:
> >> 
> >> 	struct acrn_ioreq_client *client;
> >> 	list_for_each_entry(client, &vm->ioreq_clients, list) {
> >> 		continue;
> >> 	}
> >> 
> >> 	dev_warn(acrn_dev.this_device,
> >> 		 "%s cannot flush pending request!\n", client->name); /* NOT GOOD */
> >> 
> > If there're pending request, we would call schedule to schedule out then schedule back
> > to check the list from the beginning. If there's no pending client, we point to the last
> > client and break out the while loop.
> > 
> > The code doesn't want to find the pending client and break out the while loop and call
> > dev_warn. Please see the function comment.
> > 
> > 
> >> 
> >> Since there is no break for the list_for_each_entry() iterator to return early,
> >> client *cannot* be a valid entry of the list. In fact if you look at the list_for_each_entry()
> >> macro, it will be an 'bogus' pointer, pointing somewhere into 'vm'.
> >> Essentially before the terminating condition of the list traversal the following code is called:
> >> 
> >> 	list_entry(&vm->ioreq_clients, struct acrn_ioreq_client *, list);
> >> 
> >> resulting in a:
> >> 
> >> 	container_of(&vm->ioreq_clients, struct acrn_ioreq_client *, list);
> >> 
> >> &vm->ioreq_clients however is not contained in a struct acrn_ioreq_client, making
> >> this call compute an invalid pointer.
> >> Therefore using 'client' as in the example above (e.g. client->name) after the loop is
> >> not safe. Since the loop can never return early in the simple example above it will
> >> always break. On other cases (the one we are discussing here) there might be a chance that
> >> there is one code path (in theory) where the loop did not exit early and 'client'
> >> holds that 'invalid entry'.
> >> 
> >> This would be the case with has_pending = false && retry == 0.
> >> 
> >> I hope this makes sense.
> >> 
> >>> 
> >>>> 
> >>>>> 
> >>>>> 		if (has_pending)
> >>>>> 			schedule_timeout_interruptible(HZ / 100);
> >>>>> 	} while (has_pending && --retry > 0);
> >>>> 
> >>>> since has_pending && --retry > 0 is no longer true the loop stops.
> >>>> 
> >>>>> 	if (retry == 0)
> >>>>> 		dev_warn(acrn_dev.this_device,
> >>>>> 			 "%s cannot flush pending request!\n", client->name);
> >>>> client->name is accessed since retry == 0 now, but client is not a valid struct ending up
> >>>> in a type confusion.
> >>>> 
> >>>>> 
> >>>>> If retry > 0 and has_pending is true, we would call schedule_timeout_interruptible
> >>>>> to schedule out to wait all the pending I/O requests would been completed.
> >>>>> 
> >>>>> Thanks.
> >>>> 
> >>>> Again, I'm not sure if this is realistically possible. I'm trying to remove
> >>>> any use of the list iterator after the loop to make such potentially issues detectable
> >>> You may think we still in the loop (could we ?), even that we didn't write the list iterator then.
> >> 
> >> I'm not exactly sure which loop you are referring to but no, I don't think we are still in
> >> the do while loop.
> >> 
> >> The only thing we know after the do while loop is that: !has_pending || retry == 0
> >> And iff has_pending && retry == 0, then we shouldn't call the dev_warn().
> >> 
> >>>> at compile time instead of relying on certain (difficult to maintain) conditions to be met
> >>>> to avoid the type confusion.
> >>>> 
> >>>> Thanks,
> >>>> Jakob
> >> 
> >> 	Jakob
> 
> 	Jakob
> 

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