Messages in this thread | | | Subject | Re: [PATCH] virt: acrn: fix invalid check past list iterator | From | Jakob Koschel <> | Date | Fri, 1 Apr 2022 10:50:51 +0200 |
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> 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.
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
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