Messages in this thread | | | Subject | Re: [PATCH] virt: acrn: fix invalid check past list iterator | From | Jakob Koschel <> | Date | Fri, 1 Apr 2022 09:16:48 +0200 |
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> On 1. Apr 2022, at 05:57, Li Fei1 <fei1.li@intel.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@intel.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@intel.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)
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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