Re: [PATCH] tee: add FOLL_LONGTERM for CMA case when alloc shm

[David Hildenbrand]

On 18.05.23 08:40, Xiaoming Ding (丁晓明) wrote:
>  From 35fd062d5cbc4d182eee0183843cd6350d126788 Mon Sep 17 00:00:00 2001
> From: Xiaoming Ding <xiaoming.ding@xxxxxxxxxxxx>
> Date: Wed, 10 May 2023 10:15:23 +0800
> Subject: [PATCH v2] tee: add FOLL_LONGTERM for CMA case when alloc shm
>
> CMA is widely used on insufficient memory platform for
> secure media playback case, and FOLL_LONGTERM will
> avoid tee_shm alloc pages from CMA region.
> without FOLL_LONGTERM, CMA region may alloc failed since
> tee_shm has a chance to use it in advance.
>
> modify is verified on OPTEE XTEST and kinds of secure + clear playback
>
>
> Fixes: 033ddf12bcf5 ("tee: add register user memory")
> Signed-off-by: Xiaoming Ding <xiaoming.ding@xxxxxxxxxxxx>
> ---
> v1 -> v2: take off the ifdef and apply FOLL_LONGTERM by default
>
>   drivers/tee/tee_shm.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/drivers/tee/tee_shm.c b/drivers/tee/tee_shm.c
> index 673cf0359494..38878e549ca4 100644
> --- a/drivers/tee/tee_shm.c
> +++ b/drivers/tee/tee_shm.c
> @@ -257,7 +257,7 @@ register_shm_helper(struct tee_context *ctx,
> unsigned long addr,
>   	}
>   
>   	if (flags & TEE_SHM_USER_MAPPED)
> -		rc = pin_user_pages_fast(start, num_pages, FOLL_WRITE,
> +		rc = pin_user_pages_fast(start, num_pages, FOLL_WRITE |
> FOLL_LONGTERM,
>   					 shm->pages);
>   	else
>   		rc = shm_get_kernel_pages(start, num_pages, shm-
> > pages);

I didn't dive deeply into that code, but I can spot that we can end up 
long-term pinning multiple pages -- possibly unbound or is there any 
sane limit on the number of pages?

Take a look at io_uring/rsrc.c and how we account long-term pinned pages 
against user->locked_vm/ctx->mm_account->pinned_vm in io_account_mem().

If user space could only end up pinning one or two pages via that 
interface, ok. But it looks like this interface could be abused to 
create real real trouble by unprivileged users that should be able to 
long-term pin that many pages.

Am I missing something important (i.e., interface is only accessible by 
privileged users) or should there be proper accounting and 
RLIMIT_MEMLOCK checks?

--
Thanks,

David / dhildenb