Re: [PATCH v2 1/1] fs/splice: add missing callback for inaccessible pages

[Christian Borntraeger]

On 01.05.20 00:06, Dave Hansen wrote:
> I was also wondering if Claudio was right about the debug patch having
> races.  I went to go look how the s390 code avoids races when pages go
> from accessible->inaccessible.
> 
> Because, if if all of the traps are in place to transform pages from
> inaccessible->accessible, the code *after* those traps is still
> vulnerable.  What *keeps* pages accessible?
> 
> The race avoidance is this, basically:
> 
> 	down_read(&gmap->mm->mmap_sem);
> 	lock_page(page);
>         ptep = get_locked_pte(gmap->mm, uaddr, &ptelock);
> ...
>>         expected = expected_page_refs(page);
>>         if (!page_ref_freeze(page, expected))
>>                 return -EBUSY;
>>         set_bit(PG_arch_1, &page->flags);
>>         rc = uv_call(0, (u64)uvcb);
>>         page_ref_unfreeze(page, expected);
> 
> ... up_read(mmap_sem) / unlock_page() / unlock pte
> 
> I'm assuming that after the uv_call(), the page is inaccessible and I/O
> devices will go boom if they touch the page.
> 
> The page_ref_freeze() ensures that references come between the
> freeze/unfreeze are noticed, but it doesn't actually *stop* new ones for
> users that hold references already.  For the page cache, especially,
> someone could do:
> 
> 	page = find_get_page();
> 	arch_make_page_accessible();
> 					lock_page();
> 	...				make_secure_pte();

Not sure if I got your point here, but this make_secure_pte should bail
out because we actually do check for a calculated refcount value and return
-EBUSY. The find_get_page should have raised this refcount to a value that
would go beyond the expected value, No? 


> 					unlock_page();
> 	get_page();
> 	// ^ OK because I have a ref
> 	// do DMA on inaccessible page
> 
> Because the make_secure_pte() code isn't looking for a *specific*
> 'expected' value, it has no way of noticing that the extra ref snuck in
> there.

I think the expected calcution is actually doing that,giving back the minimum
value when no one else has any references that are valid for I/O.

But I might not have understood what you are trying to tell me?

> 
> I _think_ expected actually needs to be checked for having a specific
> (low) value so that if there's a *possibility* of a reference holder
> acquiring additional references, the page is known to be off-limits.
> mm/migrate.c has a few examples of this, but I'm not quite sure how
> bulletproof they are.  Some of it appears to just be optimizations.
> 
> 
> 
b