Messages in this thread | | | Date | Fri, 28 Apr 2023 18:00:22 +0200 | From | David Hildenbrand <> | Subject | Re: [PATCH v5] mm/gup: disallow GUP writing to file-backed mappings by default |
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>>> >>> Personally I come at this from the 'I just want my vmas patch series' unblocked >>> perspective :) and feel there's a functional aspect here too. >> >> I know, it always gets messy when touching such sensible topics :P > > I feel that several people owe me drinks at LSF/MM :P > > To cut a long story short to your other points, I'm _really_ leaning > towards an opt-in variant of this change that we just hand to io_uring to > make everything simple with minimum risk (if Jens was also open to this > idea, it'd simply be deleting the open coded vma checks there and adding > FOLL_SAFE_FILE_WRITE). > > That way we can save the delightful back and forth for another time while > adding a useful feature and documenting the issue.
Just for the records: I'm not opposed to disabling it system-wide, especially once this is an actual security issue and can bring down the machine easily (thanks to Jason for raising the security aspect). I just wanted to raise awareness that there might be users affected ...
Sure, we could glue this to some system knob like Jason said, if we want to play safe.
> > Altneratively I could try to adapt this to also do the GUP-fast check, > hoping that no FOLL_FAST_ONLY users would get nixed (I'd have to check who > uses that). The others should just get degraded to a standard GUP right?
Yes. When you need the VMA to make a decision, fallback to standard GUP.
The only problematic part is something like get_user_pages_fast_only(), that would observe a change. But KVM never passes FOLL_LONGTERM, so at least in that context the change should be fine I guess.
The performance concern is the most problematic thing (how to identify shmem pages).
> > I feel these various series have really helped beat out some details about > GUP, so as to your point on another thread (trying to reduce noise here > :P), I think discussion at LSF/MM is also a sensible idea, also you know, > if beers were bought too it could all work out nicely :]
The issue is, that GUP is so complicated, that each and every MM developer familiar with GUP has something to add :P
What stood out to me is that we disallow something for ordinary GUP but disallow it for GUP-fast, which looks very odd.
So sorry again for jumping in late ...
-- Thanks,
David / dhildenb
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