Re: [Xen-devel] [PATCH 1/5] xen/arm: optee: impose limit on shared buffer size

[Volodymyr Babchuk <Volodymyr_Babchuk@xxxxxxxx>]

Julien Grall writes:

> Hi Volodymyr,
>
> On 8/23/19 7:48 PM, Volodymyr Babchuk wrote:
>> We want to limit number of calls to lookup_and_pin_guest_ram_addr()
>> per one request. There are two ways to do this: either preempt
>> translate_noncontig() or to limit size of one shared buffer size.
>>
>> It is quite hard to preempt translate_noncontig(), because it is deep
>> nested. So we chose second option. We will allow 512 pages per one
>> shared buffer. This does not interfere with GP standard, as it
>> requires that size limit for shared buffer should be at lest 512kB.
>
> Do you mean "least" instead of "lest"?
Yes

> If so, why 512 pages (i.e 1MB)
> is plenty enough for most of the use cases? What does "xtest" consist
> on?
Bigger buffer xtest tries to allocate is mere 32KB. I believe that 1MB
is enough for the most cases, because OP-TEE itself have a very limited
resources. But this value is chosen arbitrary.

>
>> Also, with this limitation OP-TEE still passes own "xtest" test suite,
>> so this is okay for now.
>>
>> Signed-off-by: Volodymyr Babchuk <volodymyr_babchuk@xxxxxxxx>
>> ---
>>   xen/arch/arm/tee/optee.c | 30 ++++++++++++++++++------------
>>   1 file changed, 18 insertions(+), 12 deletions(-)
>>
>> diff --git a/xen/arch/arm/tee/optee.c b/xen/arch/arm/tee/optee.c
>> index ec5402e89b..f4fa8a7758 100644
>> --- a/xen/arch/arm/tee/optee.c
>> +++ b/xen/arch/arm/tee/optee.c
>> @@ -72,6 +72,17 @@
>>    */
>>   #define MAX_TOTAL_SMH_BUF_PG    16384
>>   +/*
>> + * Arbitrary value that limits maximum shared buffer size. It is
>> + * merely coincidence that it equals to both default OP-TEE SHM buffer
>> + * size limit and to (1 << CONFIG_DOMU_MAX_ORDER). Please note that
>> + * this define limits number of pages. But user buffer can be not
>> + * aligned to a page boundary. So it is possible that user would not
>> + * be able to share exactly MAX_SHM_BUFFER_PG * PAGE_SIZE bytes with
>> + * OP-TEE.
>> + */
>> +#define MAX_SHM_BUFFER_PG       512
>> +
>>   #define OPTEE_KNOWN_NSEC_CAPS OPTEE_SMC_NSEC_CAP_UNIPROCESSOR
>>   #define OPTEE_KNOWN_SEC_CAPS (OPTEE_SMC_SEC_CAP_HAVE_RESERVED_SHM | \
>>                                 OPTEE_SMC_SEC_CAP_UNREGISTERED_SHM | \
>> @@ -697,15 +708,17 @@ static int translate_noncontig(struct optee_domain 
>> *ctx,
>>       size = ROUNDUP(param->u.tmem.size + offset, 
>> OPTEE_MSG_NONCONTIG_PAGE_SIZE);
>>         pg_count = DIV_ROUND_UP(size,
>> OPTEE_MSG_NONCONTIG_PAGE_SIZE);
>> +    if ( pg_count > MAX_SHM_BUFFER_PG )
>> +        return -ENOMEM;
>> +
>>       order = get_order_from_bytes(get_pages_list_size(pg_count));
>>         /*
>> -     * In the worst case we will want to allocate 33 pages, which is
>> -     * MAX_TOTAL_SMH_BUF_PG/511 rounded up. This gives order 6 or at
>> -     * most 64 pages allocated. This buffer will be freed right after
>> -     * the end of the call and there can be no more than
>> +     * In the worst case we will want to allocate 2 pages, which is
>> +     * MAX_SHM_BUFFER_PG/511 rounded up. This buffer will be freed
>> +     * right after the end of the call and there can be no more than
>>        * max_optee_threads calls simultaneously. So in the worst case
>> -     * guest can trick us to allocate 64 * max_optee_threads pages in
>> +     * guest can trick us to allocate 2 * max_optee_threads pages in
>>        * total.
>>        */
>>       xen_pgs = alloc_domheap_pages(current->domain, order, 0);
>> @@ -747,13 +760,6 @@ static int translate_noncontig(struct optee_domain *ctx,
>>               xen_data = __map_domain_page(xen_pgs);
>>           }
>>   -        /*
>> -         * TODO: That function can pin up to 64MB of guest memory by
>> -         * calling lookup_and_pin_guest_ram_addr() 16384 times
>> -         * (assuming that PAGE_SIZE equals to 4096).
>> -         * This should be addressed before declaring OP-TEE security
>> -         * supported.
>> -         */
>>           BUILD_BUG_ON(PAGE_SIZE != 4096);
>
> Without the comment, the BUILD_BUG_ON() looks random. So either you
> want to have a different version of the comment or you want to move
> the BUILD_BUG_ON() to somewhere else.

It is still before get_domain_ram_page() call. But for clarity I can add
comment like "Only 4k pages are supported right now".
>>           page = 
>> get_domain_ram_page(gaddr_to_gfn(guest_data->pages_list[idx]));
>>           if ( !page )
>>
>
> Cheers,


-- 
Volodymyr Babchuk at EPAM
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