Re: [PATCH v1 2/4] mm/gup: Make follow_page() succeed again on PROT_NONE PTEs/PMDs

[John Hubbard]

On 7/27/23 14:28, David Hildenbrand wrote:
> We accidentally enforced PROT_NONE PTE/PMD permission checks for
> follow_page() like we do for get_user_pages() and friends. That was
> undesired, because follow_page() is usually only used to lookup a currently
> mapped page, not to actually access it. Further, follow_page() does not
> actually trigger fault handling, but instead simply fails.

I see that follow_page() is also completely undocumented. And that
reduces us to deducing how it should be used...these things that
change follow_page()'s behavior maybe should have a go at documenting
it too, perhaps.

> Let's restore that behavior by conditionally setting FOLL_FORCE if
> FOLL_WRITE is not set. This way, for example KSM and migration code will
> no longer fail on PROT_NONE mapped PTEs/PMDS.
>
> Handling this internally doesn't require us to add any new FOLL_FORCE
> usage outside of GUP code.
>
> While at it, refuse to accept FOLL_FORCE: we don't even perform VMA
> permission checks like in check_vma_flags(), so especially
> FOLL_FORCE|FOLL_WRITE would be dodgy.
>
> This issue was identified by code inspection. We'll add some
> documentation regarding FOLL_FORCE next.
>
> Reported-by: Peter Xu <peterx@xxxxxxxxxx>
> Fixes: 474098edac26 ("mm/gup: replace FOLL_NUMA by gup_can_follow_protnone()")
> Cc: <stable@xxxxxxxxxxxxxxx>
> Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
> ---
>   mm/gup.c | 10 +++++++++-
>   1 file changed, 9 insertions(+), 1 deletion(-)
>
> diff --git a/mm/gup.c b/mm/gup.c
> index 2493ffa10f4b..da9a5cc096ac 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -841,9 +841,17 @@ struct page *follow_page(struct vm_area_struct *vma, unsigned long address,
>   	if (vma_is_secretmem(vma))
>   		return NULL;
>   
> -	if (WARN_ON_ONCE(foll_flags & FOLL_PIN))
> +	if (WARN_ON_ONCE(foll_flags & (FOLL_PIN | FOLL_FORCE)))
>   		return NULL;

This is not a super happy situation: follow_page() is now prohibited
(see above: we should document that interface) from passing in
FOLL_FORCE...

>   
> +	/*
> +	 * Traditionally, follow_page() succeeded on PROT_NONE-mapped pages
> +	 * but failed follow_page(FOLL_WRITE) on R/O-mapped pages. Let's
> +	 * keep these semantics by setting FOLL_FORCE if FOLL_WRITE is not set.
> +	 */
> +	if (!(foll_flags & FOLL_WRITE))
> +		foll_flags |= FOLL_FORCE;
> +

...but then we set it anyway, for special cases. It's awkward because
FOLL_FORCE is not an "internal to gup" flag (yet?).

I don't yet have suggestions, other than:

1) Yes, the FOLL_NUMA made things bad.

2) And they are still very confusing, especially the new use of
   FOLL_FORCE.

...I'll try to let this soak in and maybe recommend something
in a more productive way. :)

thanks,
--
John Hubbard
NVIDIA