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Subject: Re: [PATCH] virt: acrn: fix invalid check past list iterator
From:   Jakob Koschel <jakobkoschel@gmail.com>
In-Reply-To: <20220401090545.GA32168@louislifei-OptiPlex-7050>
Date:   Fri, 1 Apr 2022 11:08:02 +0200
Cc:     Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        rppt@kernel.org, bjohannesmeyer@gmail.com, c.giuffrida@vu.nl,
        h.j.bos@vu.nl
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Message-Id: <0607BE1D-B2A6-4B93-9B4C-C04715B41E8C@gmail.com>
References: <20220319203819.2559993-1-jakobkoschel@gmail.com>
 <20220330075742.GA22544@louislifei-OptiPlex-7050>
 <6E68C33F-9CBB-418C-A11D-2AD863C0B19A@gmail.com>
 <20220401011533.GA29696@louislifei-OptiPlex-7050>
 <F54F358F-FFBC-4D6D-AB5C-9BC4A617C9BF@gmail.com>
 <20220401035742.GA31162@louislifei-OptiPlex-7050>
 <CE06BFFA-93CA-4E25-AB63-524C3F293556@gmail.com>
 <20220401075711.GA31912@louislifei-OptiPlex-7050>
 <65660B0C-8888-4283-80E4-D1A334EEF166@gmail.com>
 <20220401090545.GA32168@louislifei-OptiPlex-7050>
To:     Li Fei1 <fei1.li@intel.com>
Precedence: bulk


> On 1. Apr 2022, at 11:05, Li Fei1 <fei1.li@intel.com> wrote:
>=20
> On Fri, Apr 01, 2022 at 10:50:51AM +0200, Jakob Koschel wrote:
>>=20
>>=20
>>> On 1. Apr 2022, at 09:57, Li Fei1 <fei1.li@intel.com> wrote:
>>>=20
>>> On Fri, Apr 01, 2022 at 09:16:48AM +0200, Jakob Koschel wrote:
>>>>=20
>>>>=20
>>>>> On 1. Apr 2022, at 05:57, Li Fei1 <fei1.li@intel.com> wrote:
>>>>>=20
>>>>> On Fri, Apr 01, 2022 at 05:22:36AM +0200, Jakob Koschel wrote:
>>>>>>=20
>>>>>>> On 1. Apr 2022, at 03:15, Li Fei1 <fei1.li@intel.com> wrote:
>>>>>>>=20
>>>>>>> On Thu, Mar 31, 2022 at 01:20:50PM +0200, Jakob Koschel wrote:
>>>>>>>>=20
>>>>>>>>> On 30. Mar 2022, at 09:57, Li Fei1 <fei1.li@intel.com> wrote:
>>>>>>>>>=20
>>>>>>>>> On Sat, Mar 19, 2022 at 09:38:19PM +0100, Jakob Koschel wrote:
>>>>>>>>>> The condition retry =3D=3D 0 is theoretically possible even =
if 'client'
>>>>>>>>>> does not point to a valid element because no break was hit.
>>>>>>>>>>=20
>>>>>>>>>> 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.
>>>>>>>>>>=20
>>>>>>>>> Hi Koschel
>>>>>>>>>=20
>>>>>>>>> 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 =3D true =
&& retry =3D=3D 0
>>>>>>>>=20
>>>>>>>> Maybe I don't understand but looking isolated at this function =
I could see a way to call
>>>>>>>> the dev_warn() with has_pending =3D false && retry =3D=3D 0.
>>>>>>> Yes, even has_pending =3D true && retry =3D=3D 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.
>>>>>>=20
>>>>>> I'm sorry, I don't quite understand what you mean by that.
>>>>>> Do you agree that has_pending =3D false && retry =3D=3D 0 is =
possible when calling the dev_warn()
>>>>>> or not?
>>>>>>=20
>>>>> Yes, so what ? It just a hint there may have pending request.
>>>>=20
>>>> if has_pending =3D=3D false && retry =3D=3D 0 when calling =
dev_warn() there are very clear
>>>> dependencies met:
>>>>=20
>>>> * has_pending =3D=3D 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 ?
>>=20
>> I'm afraid I don't know exactly what you mean here.
>>=20
>> More specifically:
>> to check what? and I'm also not sure what you mean with "when client =
in ioreq_clients would been destroyed".
>>=20
>> Actually thinking about it more the check should be
>>=20
>> 	if (client && retry =3D=3D 0)
>>=20
>> 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 =3D=3D false =
&& retry =3D=3D 0?
Then the 'client' used in the dev_msg() cannot be valid.

>>=20
>> if has_pending =3D=3D false I read the code as if no client was found =
that has a pending request=20
>> so I don't follow how:
>>=20
>> 	"%s cannot flush pending request!\n", client->name);
>>=20
>> can be valid since no client has a pending request.
>>=20
>>>=20
>>>>=20
>>>> I'm *only* talking about the case where has_pending =3D=3D false, =
in case that's not clear.
>>>>=20
>>>>=20
>>>>> Even retry =3D=3D 0 && has_pending =3D true,
>>>>> When we call dev_warn, the clent may not is pending.
>>>>>=20
>>>>>=20
>>>>>>>>=20
>>>>>>>>> 		list_for_each_entry(client, &vm->ioreq_clients, =
list) {
>>>>>>>>> 			has_pending =3D =
has_pending_request(client);
>>>>>>>>> 			if (has_pending)
>>>>>>>>> 		}
>>>>>>>>> 		spin_unlock_bh(&vm->ioreq_clients_lock);
>>>>>>>>=20
>>>>>>>> imagine has_pending =3D=3D false && retry =3D=3D 1 here, then =
client will not hold a valid list entry.
>>>>>>> What do you mean "client will not hold a valid list entry" ?=20
>>>>>>=20
>>>>>> Imagine a very simple example:
>>>>>>=20
>>>>>> 	struct acrn_ioreq_client *client;
>>>>>> 	list_for_each_entry(client, &vm->ioreq_clients, list) {
>>>>>> 		continue;
>>>>>> 	}
>>>>>>=20
>>>>>> 	dev_warn(acrn_dev.this_device,
>>>>>> 		 "%s cannot flush pending request!\n", client->name); /* =
NOT GOOD */
>>>>>>=20
>>>>> 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.
>>>>>=20
>>>>> 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.
>>>>>=20
>>>>>=20
>>>>>>=20
>>>>>> 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:
>>>>>>=20
>>>>>> 	list_entry(&vm->ioreq_clients, struct acrn_ioreq_client *, =
list);
>>>>>>=20
>>>>>> resulting in a:
>>>>>>=20
>>>>>> 	container_of(&vm->ioreq_clients, struct acrn_ioreq_client *, =
list);
>>>>>>=20
>>>>>> &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'.
>>>>>>=20
>>>>>> This would be the case with has_pending =3D false && retry =3D=3D =
0.
>>>>>>=20
>>>>>> I hope this makes sense.
>>>>>>=20
>>>>>>>=20
>>>>>>>>=20
>>>>>>>>>=20
>>>>>>>>> 		if (has_pending)
>>>>>>>>> 			schedule_timeout_interruptible(HZ / =
100);
>>>>>>>>> 	} while (has_pending && --retry > 0);
>>>>>>>>=20
>>>>>>>> since has_pending && --retry > 0 is no longer true the loop =
stops.
>>>>>>>>=20
>>>>>>>>> 	if (retry =3D=3D 0)
>>>>>>>>> 		dev_warn(acrn_dev.this_device,
>>>>>>>>> 			 "%s cannot flush pending request!\n", =
client->name);
>>>>>>>> client->name is accessed since retry =3D=3D 0 now, but client =
is not a valid struct ending up
>>>>>>>> in a type confusion.
>>>>>>>>=20
>>>>>>>>>=20
>>>>>>>>> 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.
>>>>>>>>>=20
>>>>>>>>> Thanks.
>>>>>>>>=20
>>>>>>>> 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.
>>>>>>=20
>>>>>> 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.
>>>>>>=20
>>>>>> The only thing we know after the do while loop is that: =
!has_pending || retry =3D=3D 0
>>>>>> And iff has_pending && retry =3D=3D 0, then we shouldn't call the =
dev_warn().
>>>>>>=20
>>>>>>>> at compile time instead of relying on certain (difficult to =
maintain) conditions to be met
>>>>>>>> to avoid the type confusion.
>>>>>>>>=20
>>>>>>>> Thanks,
>>>>>>>> Jakob
>>>>>>=20
>>>>>> 	Jakob
>>>>=20
>>>> 	Jakob