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

[David Hildenbrand <david@redhat.com>]

On 28.07.23 11:08, David Hildenbrand wrote:
> On 28.07.23 04:30, John Hubbard wrote:
>> 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.
> 
> I can certainly be motivated to do that. :)
> 
>>
>>>
>>> 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@redhat.com>
>>> Fixes: 474098edac26 ("mm/gup: replace FOLL_NUMA by gup_can_follow_protnone()")
>>> Cc: <stable@vger.kernel.org>
>>> Signed-off-by: David Hildenbrand <david@redhat.com>
>>> ---
>>>     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...
> 
> I guess you saw my patch #4.
> 
> If you take a look at the existing callers (that are fortunately very
> limited), you'll see that nobody cares.
> 
> Most of the FOLL flags don't make any sense for follow_page(), and
> limiting further (ab)use is at least to me very appealing.
> 
>>
>>>     
>>> +	/*
>>> +	 * 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. :)
> 
> What I can offer that might be very appealing is the following:
> 
> Get rid of the flags parameter for follow_page() *completely*. Yes, then
> we can even rename FOLL_ to something reasonable in the context where it
> is nowadays used ;)
> 
> 
> Internally, we'll then set
> 
> FOLL_GET | FOLL_DUMP | FOLL_FORCE
> 
> and document exactly what this functions does. Any user that needs
> something different should just look into using get_user_pages() instead.
> 
> I can prototype that on top of this work easily.

The end result looks something like:

/**
  * follow_page - look up and reference a page descriptor from a user-virtual
  * 		 address
  * @vma: vm_area_struct mapping @address
  * @address: virtual address to look up
  *
  * follow_page() will look up the page mapped at the given address and
  * take a reference on the page. The returned page has to be released using
  * put_page().
  *
  * follow_page() will not return special (like zero) pages and does not check
  * PTE protection: the returned page might be mapped PROT_NONE, R/O or R/W.
  * Consequently, follow_page() will not trigger NUMA hinting faults.
  *
  * follow_page() does not trigger page faults. If no page is mapped, or
  * a special (like zero) page is mapped, it returns %NULL or an error pointer.
  *
  * Note: new users with different requirements are probably better off using
  *       one of the get_user_pages() variants or one of the walk_page_range()
  *       variants.
  *
  * Return: the mapped (struct page *), %NULL if no mapping exists, or
  * an error pointer if there is a mapping to something not represented
  * by a page descriptor (see also vm_normal_page()) or the zero page.
  */
struct page *follow_page(struct vm_area_struct *vma, unsigned long address)
{
	struct follow_page_context ctx = { NULL };
	unsigned long gup_flags;
	struct page *page;

	if (vma_is_secretmem(vma))
		return NULL;

	/*
	 * FOLL_GET: We always want a reference on the returned page.
	 * FOL_DUMP: Ignore special (like zero) pages.
	 * FOLL_FORCE: Succeeded on PROT_NONE-mapped pages.
	 */
	gup_flags = FOLL_GET | FOLL_DUMP | FOLL_FORCE;

	page = follow_page_mask(vma, address, gup_flags, &ctx);
	if (ctx.pgmap)
		put_dev_pagemap(ctx.pgmap);
	return page;
}

-- 
Cheers,

David / dhildenb