Messages in this thread | | | Date | Thu, 23 Dec 2021 13:54:38 +0100 | From | Jan Kara <> | Subject | Re: [PATCH v1 06/11] mm: support GUP-triggered unsharing via FAULT_FLAG_UNSHARE (!hugetlb) |
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On Wed 22-12-21 10:40:18, Linus Torvalds wrote: > On Wed, Dec 22, 2021 at 8:08 AM Jan Kara <jack@suse.cz> wrote: > > > > Well, but O_DIRECT reads must use FOLL_PIN in any case because they modify > > page data (and so we need to detect them both for COW and filesystem needs). > > Well, O_DIRECT reads do, but not necessarily writes.
I agree.
> And hey, even reads have been dodgy in the past when we didn't really > have the pinning logic - there's been a lot of users that just wanted > it to work for their particular use-case rather than in general and in > all situations..
Yes, but currently a malicious user can take the system down (BUG_ON) or cause DIF/DIX failures if he is nasty and tries hard enough with O_DIRECT reads (practically, the window is small so I haven't really seen a report that I could trace to O_DIRECT reads but in principle the problem is the same as with pinning & dirtying done e.g. by video capture drivers and there we've seen these problem happen). So forcing pinning for O_DIRECT reads is IMO mandatory.
Honza
-- Jan Kara <jack@suse.com> SUSE Labs, CR
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