Re: get_user_pages() and EXEC_ONLY mapping.

[Aneesh Kumar K V <aneesh.kumar@linux.ibm.com>]

On 11/10/23 8:23 PM, Jason Gunthorpe wrote:
> On Fri, Nov 10, 2023 at 08:19:23PM +0530, Aneesh Kumar K.V wrote:
>>
>> Hello,
>>
>> Some architectures can now support EXEC_ONLY mappings and I am wondering
>> what get_user_pages() on those addresses should return. 
> 
> -EPERM
> 
>> Earlier PROT_EXEC implied PROT_READ and pte_access_permitted()
>> returned true for that. But arm64 does have this explicit comment
>> that says
>>
>>  /*
>>  * p??_access_permitted() is true for valid user mappings (PTE_USER
>>  * bit set, subject to the write permission check). For execute-only
>>  * mappings, like PROT_EXEC with EPAN (both PTE_USER and PTE_UXN bits
>>  * not set) must return false. PROT_NONE mappings do not have the
>>  * PTE_VALID bit set.
>>  */
>>
>> Is that correct? We should be able to get struct page for PROT_EXEC
>> mappings?
> 
> If the memory is unreadable then providing a back door through
> O_DIRECT and everthing else to read it sounds wrong to me.
> 
> If there is some case where a get_user_pages caller is exec-only
> compatible then a new FOLL_EXEC flag to permit it would make sense.
> 

I was expecting pin_user_pages() to return -EPERM and get_user_pages()
return struct page. This was based on Documentation/core-api/pin_user_pages.rst  

"Another way of thinking about these flags is as a progression of restrictions:
FOLL_GET is for struct page manipulation, without affecting the data that the
struct page refers to. FOLL_PIN is a *replacement* for FOLL_GET, and is for
short term pins on pages whose data *will* get accessed. "

May be we can clarify PROT_EXEC details in the documentation? 

-aneesh