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

On 29/06/2020 08:42, Paul Durrant wrote:
-----Original Message-----
From: Julien Grall <julien@xxxxxxx>
Sent: 26 June 2020 18:54
To: Stefano Stabellini <sstabellini@xxxxxxxxxx>; paul@xxxxxxx
Cc: Volodymyr Babchuk <Volodymyr_Babchuk@xxxxxxxx>; 
xen-devel@xxxxxxxxxxxxxxxxxxxx; op-
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 :-)


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

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

Signed-off-by: Volodymyr Babchuk <volodymyr_babchuk@xxxxxxxx>

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
            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

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

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?


Release-acked-by: Paul Durrant <paul@xxxxxxx>


It is not clear to me what Stefano had in mind for the "in-code comment". So I will leave him committing the series.


Julien Grall



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