RE: [PATCH v2 2/2] optee: allow plain TMEM buffers with NULL address

[Paul Durrant <xadimgnik@gmail.com>]

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> -----Original Message-----
> From: Julien Grall <julien@xen.org>
> Sent: 26 June 2020 18:54
> To: Stefano Stabellini <sstabellini@kernel.org>; paul(a)xen.org
> Cc: Volodymyr Babchuk <Volodymyr_Babchuk@epam.com>; xen-devel(a)lists.xenproject.org; op-
> tee(a)lists.trustedfirmware.org
> Subject: Re: [PATCH v2 2/2] optee: allow plain TMEM buffers with NULL address
> 
> (using paul xen.org's email)
> 

Thanks. Avoids annoying warning banners :-)

> Hi,
> 
> Apologies for the late answer.
> 
> On 23/06/2020 22:09, Stefano Stabellini wrote:
> > On Tue, 23 Jun 2020, Julien Grall wrote:
> >> On 23/06/2020 03:49, Volodymyr Babchuk wrote:
> >>>
> >>> Hi Stefano,
> >>>
> >>> Stefano Stabellini writes:
> >>>
> >>>> On Fri, 19 Jun 2020, Volodymyr Babchuk wrote:
> >>>>> Trusted Applications use popular approach to determine required size
> >>>>> of buffer: client provides a memory reference with the NULL pointer to
> >>>>> a buffer. This is so called "Null memory reference". TA updates the
> >>>>> reference with the required size and returns it back to the
> >>>>> client. Then client allocates buffer of needed size and repeats the
> >>>>> operation.
> >>>>>
> >>>>> This behavior is described in TEE Client API Specification, paragraph
> >>>>> 3.2.5. Memory References.
> >>>>>
> >>>>> OP-TEE represents this null memory reference as a TMEM parameter with
> >>>>> buf_ptr = 0x0. This is the only case when we should allow TMEM
> >>>>> buffer without the OPTEE_MSG_ATTR_NONCONTIG flag. This also the
> >>>>> special case for a buffer with OPTEE_MSG_ATTR_NONCONTIG flag.
> >>>>>
> >>>>> This could lead to a potential issue, because IPA 0x0 is a valid
> >>>>> address, but OP-TEE will treat it as a special case. So, care should
> >>>>> be taken when construction OP-TEE enabled guest to make sure that such
> >>>>> guest have no memory at IPA 0x0 and none of its memory is mapped at PA
> >>>>> 0x0.
> >>>>>
> >>>>> Signed-off-by: Volodymyr Babchuk <volodymyr_babchuk@epam.com>
> >>>>> ---
> >>>>>
> >>>>> Changes from v1:
> >>>>>    - Added comment with TODO about possible PA/IPA 0x0 issue
> >>>>>    - The same is described in the commit message
> >>>>>    - Added check in translate_noncontig() for the NULL ptr buffer
> >>>>>
> >>>>> ---
> >>>>>    xen/arch/arm/tee/optee.c | 27 ++++++++++++++++++++++++---
> >>>>>    1 file changed, 24 insertions(+), 3 deletions(-)
> >>>>>
> >>>>> diff --git a/xen/arch/arm/tee/optee.c b/xen/arch/arm/tee/optee.c
> >>>>> index 6963238056..70bfef7e5f 100644
> >>>>> --- a/xen/arch/arm/tee/optee.c
> >>>>> +++ b/xen/arch/arm/tee/optee.c
> >>>>> @@ -215,6 +215,15 @@ static bool optee_probe(void)
> >>>>>        return true;
> >>>>>    }
> >>>>>    +/*
> >>>>> + * TODO: There is a potential issue with guests that either have RAM
> >>>>> + * at IPA of 0x0 or some of theirs memory is mapped at PA 0x0. This is
> >>>>                                  ^ their
> >>>>
> >>>>> + * because PA of 0x0 is considered as NULL pointer by OP-TEE. It will
> >>>>> + * not be able to map buffer with such pointer to TA address space, or
> >>>>> + * use such buffer for communication with the guest. We either need to
> >>>>> + * check that guest have no such mappings or ensure that OP-TEE
> >>>>> + * enabled guest will not be created with such mappings.
> >>>>> + */
> >>>>>    static int optee_domain_init(struct domain *d)
> >>>>>    {
> >>>>>        struct arm_smccc_res resp;
> >>>>> @@ -725,6 +734,15 @@ static int translate_noncontig(struct optee_domain
> >>>>> *ctx,
> >>>>>            uint64_t next_page_data;
> >>>>>        } *guest_data, *xen_data;
> >>>>>    +    /*
> >>>>> +     * Special case: buffer with buf_ptr == 0x0 is considered as NULL
> >>>>> +     * pointer by OP-TEE. No translation is needed. This can lead to
> >>>>> +     * an issue as IPA 0x0 is a valid address for Xen. See the comment
> >>>>> +     * near optee_domain_init()
> >>>>> +     */
> >>>>> +    if ( !param->u.tmem.buf_ptr )
> >>>>> +        return 0;
> >>>>
> >>>> Given that today it is not possible for this to happen, it could even be
> >>>> an ASSERT. But I think I would just return an error, maybe -EINVAL?
> >>>
> >>> Hmm, looks like my comment is somewhat misleading :(
> >>
> >> How about the following comment:
> >>
> >> We don't want to translate NULL (0) as it can be used by the guest to fetch
> >> the size of the buffer to allocate. This behavior depends on TA, but there is
> >> a guarantee that OP-TEE will not try to map it (see more details on top of
> >> optee_domain_init()).
> >>
> >>>
> >>> What I mean, is that param->u.tmem.buf_ptr == 0 is the normal situation.
> >>> This is the special case, when OP-TEE treats this buffer as a NULL. So
> >>> we are doing nothing there. Thus, "return 0".
> >>>
> >>> But, as Julien pointed out, we can have machine where 0x0 is the valid
> >>> memory address and there is a chance, that some guest will use it as a
> >>> pointer to buffer.
> >>>
> >>>> Aside from this, and the small grammar issue, everything else looks fine
> >>>> to me.
> >>>>
> >>>> Let's wait for Julien's reply, but if this is the only thing I could fix
> >>>> on commit.
> >>
> >> I agree with Volodymyr, this is the normal case here. There are more work to
> >> prevent MFN 0 to be mapped in the guest but this shouldn't be an issue today.
> >
> > Let's put the MFN 0 issue aside for a moment.
> >
> >  From the commit message I thought that if the guest wanted to pass a
> > NULL buffer ("Null memory reference") then it would also *not* set
> > OPTEE_MSG_ATTR_NONCONTIG, which would be handled by the "else" statement
> > also modified by this patch. Thus, I thought that reaching
> > translate_noncontig with buf_ptr == NULL would always be an error.
> >
> > But re-reading the commit message and from both your answers it is not
> > the case: a "Null memory reference" is allowed with
> > OPTEE_MSG_ATTR_NONCONTIG set too.
> >
> > Thus, I have no further comments and the improvements on the in-code
> > comment could be done on commit.
> 
> Good :). IIRC Paul gave a provisional RaB for this series. @Paul, now
> that we are settled, could we get a formal one?

Sure.

Release-acked-by: Paul Durrant <paul@xen.org>

> 
> Cheers,
> 
> --
> Julien Grall