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Date:   Fri, 28 Apr 2023 17:23:29 +0200
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From:   David Hildenbrand <david@redhat.com>
To:     Jason Gunthorpe <jgg@nvidia.com>
Cc:     Lorenzo Stoakes <lstoakes@gmail.com>, linux-mm@kvack.org,
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 <afcc124e-7a9b-879c-dfdf-200426b84e24@redhat.com>
 <ZEvZtIb2EDb/WudP@nvidia.com>
 <094d2074-5b69-5d61-07f7-9f962014fa68@redhat.com>
Organization: Red Hat
Subject: Re: [PATCH v5] mm/gup: disallow GUP writing to file-backed mappings
 by default
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>>
>> Security is the primary case where we have historically closed uAPI
>> items.
> 
> As this patch
> 
> 1) Does not tackle GUP-fast
> 2) Does not take care of !FOLL_LONGTERM
> 
> I am not convinced by the security argument in regard to this patch.
> 
> 
> If we want to sells this as a security thing, we have to block it
> *completely* and then CC stable.

Regarding GUP-fast, to fix the issue there as well, I guess we could do 
something similar as I did in gup_must_unshare():

If we're in GUP-fast (no VMA), and want to pin a !anon page writable, 
fallback to ordinary GUP. IOW, if we don't know, better be safe.

Of course, this would prevent hugetlb/shmem from getting pinned writable 
during gup-fast. Unless we're able to whitelist them somehow in there.


For FOLL_LONGTERM it might fairly uncontroversial. For everything else 
I'm not sure if there could be undesired side-effects.

-- 
Thanks,

David / dhildenb