Re: [XEN PATCH v8 02/22] xen/arm: tee: add a primitive FF-A mediator

[Bertrand Marquis <Bertrand.Marquis@xxxxxxx>]

Hi Andrew,

> On 14 Apr 2023, at 10:58, Jens Wiklander <jens.wiklander@xxxxxxxxxx> wrote:
> 
> Hi,
> 
> On Thu, Apr 13, 2023 at 3:27 PM Andrew Cooper <andrew.cooper3@xxxxxxxxxx> 
> wrote:
>> 
>> On 13/04/2023 1:26 pm, Julien Grall wrote:
>>>> +static int ffa_domain_init(struct domain *d)
>>>> +{
>>>> +    struct ffa_ctx *ctx;
>>>> +
>>>> +    if ( !ffa_version )
>>>> +        return -ENODEV;
>>>> +
>>>> +    ctx = xzalloc(struct ffa_ctx);
>>>> +    if ( !ctx )
>>>> +        return -ENOMEM;
>>>> +
>>>> +    d->arch.tee = ctx;
>>>> +
>>>> +    return 0;
>>>> +}
>>>> +
>>>> +/* This function is supposed to undo what ffa_domain_init() has done */
>>> 
>>> I think there is a problem in the TEE framework. The callback
>>> .relinquish_resources() will not be called if domain_create() failed.
>>> So this will result to a memory leak.
>>> 
>>> We also can't call .relinquish_resources() on early domain creation
>>> failure because relinquishing resources can take time and therefore
>>> needs to be preemptible.
>>> 
>>> So I think we need to introduce a new callback domain_free() that will
>>> be called arch_domain_destroy(). Is this something you can look at?
>> 
>> 
>> Cleanup of an early domain creation failure, however you do it, is at
>> most "the same amount of time again".  It cannot (absent of development
>> errors) take the same indefinite time periods of time that a full
>> domain_destroy() can.
>> 
>> The error path in domain_create() explicitly does call domain_teardown()
>> so we can (eventually) purge these duplicate cleanup paths.  There are
>> far too many easy errors to be made which occur from having split
>> cleanup, and we have had to issue XSAs in the past to address some of
>> them.  (Hence the effort to try and specifically change things, and
>> remove the ability to introduce the errors in the first place.)
>> 
>> 
>> Right now, it is specifically awkward to do this nicely because
>> domain_teardown() doesn't call into a suitable arch hook.
>> 
>> IMO the best option here is extend domain_teardown() with an
>> arch_domain_teardown() state/hook, and wire in the TEE cleanup path into
>> this too.
>> 
>> Anything else is explicitly adding to technical debt that I (or someone
>> else) is going to have to revert further down the line.
>> 
>> If you want, I am happy to prototype the arch_domain_teardown() bit of
>> the fix, but I will have to defer wiring in the TEE part to someone
>> capable of testing it.
> 
> You're more than welcome to prototype the fix, I can test it and add
> it to the next version of the patch set when we're happy with the
> result.


Could you tell us if you are still happy to work on the prototype for
arch_domain_teardown and when you would be able to give a first prototype ?

Regards
Bertrand