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 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.

-- 
Cheers,

David / dhildenb