Messages in this thread | | | Subject | Re: [PATCH] virt: acrn: fix invalid check past list iterator | From | Jakob Koschel <> | Date | Fri, 1 Apr 2022 11:08:02 +0200 |
| |
> On 1. Apr 2022, at 11:05, Li Fei1 <fei1.li@intel.com> wrote: > > On Fri, Apr 01, 2022 at 10:50:51AM +0200, Jakob Koschel wrote: >> >> >>> On 1. Apr 2022, at 09:57, Li Fei1 <fei1.li@intel.com> wrote: >>> >>> On Fri, Apr 01, 2022 at 09:16:48AM +0200, Jakob Koschel wrote: >>>> >>>> >>>>> 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) >>> So you'd better to check when the client in ioreq_clients would been destroyed, right ? >> >> I'm afraid I don't know exactly what you mean here. >> >> More specifically: >> to check what? and I'm also not sure what you mean with "when client in ioreq_clients would been destroyed". >> >> Actually thinking about it more the check should be >> >> if (client && retry == 0) >> >> to be correct. > The client in ioreq_clients would always been "valid" (here valid means the client struct would not > been destroyed) when this function been called. That's guaranteed by the code logic.
Now I'm very confused. Didn't you say the dev_msg() can be called with has_pending == false && retry == 0? Then the 'client' used in the dev_msg() cannot be valid.
>> >> if has_pending == false I read the code as if no client was found that has a pending request >> so I don't follow how: >> >> "%s cannot flush pending request!\n", client->name); >> >> can be valid since no client has a pending request. >> >>> >>>> >>>> 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
| |