get_user_pages() and EXEC_ONLY mapping.

[Jason Gunthorpe]

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.

Jason