Re: [PATCH 1/1] gup: document and work around "COW can break either way" issue

Greg KH
2021-10-12T08:42:51Z

On Tue, Oct 12, 2021 at 10:14:27AM +0200, Vlastimil Babka wrote:
> On 10/12/21 10:06, Jan Kara wrote:
> > On Mon 11-10-21 18:52:44, Suren Baghdasaryan wrote:
> >> From: Linus Torvalds <torvalds@linux-foundation.org>
> >>
> >> From: Linus Torvalds <torvalds@linux-foundation.org>
> >>
> >> commit 17839856fd588f4ab6b789f482ed3ffd7c403e1f upstream.
> >>
> >> Doing a "get_user_pages()" on a copy-on-write page for reading can be
> >> ambiguous: the page can be COW'ed at any time afterwards, and the
> >> direction of a COW event isn't defined.
> >>
> >> Yes, whoever writes to it will generally do the COW, but if the thread
> >> that did the get_user_pages() unmapped the page before the write (and
> >> that could happen due to memory pressure in addition to any outright
> >> action), the writer could also just take over the old page instead.
> >>
> >> End result: the get_user_pages() call might result in a page pointer
> >> that is no longer associated with the original VM, and is associated
> >> with - and controlled by - another VM having taken it over instead.
> >>
> >> So when doing a get_user_pages() on a COW mapping, the only really safe
> >> thing to do would be to break the COW when getting the page, even when
> >> only getting it for reading.
> >>
> >> At the same time, some users simply don't even care.
> >>
> >> For example, the perf code wants to look up the page not because it
> >> cares about the page, but because the code simply wants to look up the
> >> physical address of the access for informational purposes, and doesn't
> >> really care about races when a page might be unmapped and remapped
> >> elsewhere.
> >>
> >> This adds logic to force a COW event by setting FOLL_WRITE on any
> >> copy-on-write mapping when FOLL_GET (or FOLL_PIN) is used to get a page
> >> pointer as a result.
> >>
> >> The current semantics end up being:
> >>
> >> - __get_user_pages_fast(): no change. If you don't ask for a write,
> >> you won't break COW. You'd better know what you're doing.
> >>
> >> - get_user_pages_fast(): the fast-case "look it up in the page tables
> >> without anything getting mmap_sem" now refuses to follow a read-only
> >> page, since it might need COW breaking. Which happens in the slow
> >> path - the fast path doesn't know if the memory might be COW or not.
> >>
> >> - get_user_pages() (including the slow-path fallback for gup_fast()):
> >> for a COW mapping, turn on FOLL_WRITE for FOLL_GET/FOLL_PIN, with
> >> very similar semantics to FOLL_FORCE.
> >>
> >> If it turns out that we want finer granularity (ie "only break COW when
> >> it might actually matter" - things like the zero page are special and
> >> don't need to be broken) we might need to push these semantics deeper
> >> into the lookup fault path. So if people care enough, it's possible
> >> that we might end up adding a new internal FOLL_BREAK_COW flag to go
> >> with the internal FOLL_COW flag we already have for tracking "I had a
> >> COW".
> >>
> >> Alternatively, if it turns out that different callers might want to
> >> explicitly control the forced COW break behavior, we might even want to
> >> make such a flag visible to the users of get_user_pages() instead of
> >> using the above default semantics.
> >>
> >> But for now, this is mostly commentary on the issue (this commit message
> >> being a lot bigger than the patch, and that patch in turn is almost all
> >> comments), with that minimal "enable COW breaking early" logic using the
> >> existing FOLL_WRITE behavior.
> >>
> >> [ It might be worth noting that we've always had this ambiguity, and it
> >> could arguably be seen as a user-space issue.
> >>
> >> You only get private COW mappings that could break either way in
> >> situations where user space is doing cooperative things (ie fork()
> >> before an execve() etc), but it _is_ surprising and very subtle, and
> >> fork() is supposed to give you independent address spaces.
> >>
> >> So let's treat this as a kernel issue and make the semantics of
> >> get_user_pages() easier to understand. Note that obviously a true
> >> shared mapping will still get a page that can change under us, so this
> >> does _not_ mean that get_user_pages() somehow returns any "stable"
> >> page ]
> >>
> >> [surenb: backport notes
> >> Since gup_pgd_range does not exist, made appropriate changes on
> >> the the gup_huge_pgd, gup_huge_pd and gup_pud_range calls instead.
> >> Replaced (gup_flags | FOLL_WRITE) with write=1 in gup_huge_pgd,
> >> gup_huge_pd and gup_pud_range.
> >> Removed FOLL_PIN usage in should_force_cow_break since it's missing in
> >> the earlier kernels.]
> >
> > I'd be really careful with backporting this to stable. There was a lot of
> > userspace breakage caused by this change if I remember right which needed
> > to be fixed up later. There is a nice summary at
> > https://lwn.net/Articles/849638/ and https://lwn.net/Articles/849876/ and
> > some problems are still being found...
>
> Yeah that was my initial reaction. But looks like back in April we agreed
> that backporting only this commit could be feasible - the relevant subthread
> starts around here [1]. The known breakage for just this commit was uffd
> functionality introduced only in 5.7, and strace on dax on pmem (that was
> never properly root caused). 5.4 stable already has the backport since year
> ago, Suren posted 4.14 and 4.19 in April after [1]. Looks like nobody
> reported issues? Continuing with 4.4 and 4.9 makes this consistent at least,
> although the risk of breaking something is always there and the CVE probably
> not worth it, but whatever...

I have had people "complain" that the issue was not fixed on these older
kernels, now if that is just because those groups have a "it has a CVE
so it must be fixed!" policy or not, it is hard to tell.

But this seems to be exploitable, and we have a reproducer somewhere
around here, so it would be nice to get resolved for the reason of it
being a bug that we should fix if possible.

So I would err on the side of "lets merge this" as fixing a known issue
is ALWAYS better than the fear of "maybe something might break". We can
always revert if the latter happens in testing.

thanks,

greg k-h