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

[Julien Grall <julien.grall@xxxxxxx>]

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"? If so, why 512 pages (i.e 1MB) is 
plenty enough for most of the use cases? What does "xtest" consist on?
> 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.
>           page = get_domain_ram_page(gaddr_to_gfn(guest_data->pages_list[idx]));
>           if ( !page )
Cheers,

--
Julien Grall

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