Message-ID: <YWXar5/JJD4NYcHa@mussarela>
Date:   Tue, 12 Oct 2021 15:57:51 -0300
From:   Thadeu Lima de Souza Cascardo <cascardo@...onical.com>
To:     Greg KH <gregkh@...uxfoundation.org>
Cc:     Vlastimil Babka <vbabka@...e.cz>, Jan Kara <jack@...e.cz>,
        Suren Baghdasaryan <surenb@...gle.com>, stable@...r.kernel.org,
        jannh@...gle.com, torvalds@...ux-foundation.org, peterx@...hat.com,
        aarcange@...hat.com, david@...hat.com, jgg@...pe.ca,
        ktkhai@...tuozzo.com, shli@...com, namit@...are.com, hch@....de,
        oleg@...hat.com, kirill@...temov.name, willy@...radead.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        kernel-team@...roid.com
Subject: Re: [PATCH 1/1] gup: document and work around "COW can break either
 way" issue

On Tue, Oct 12, 2021 at 10:42:40AM +0200, Greg KH wrote:
> 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@...ux-foundation.org>
> > >> 
> > >> From: Linus Torvalds <torvalds@...ux-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

When we backported this to the Ubuntu kernel based on 4.4, we found a
regression that required commit 38e088546522e1e86d2b8f401a1354ad3a9b3303
("mm: check VMA flags to avoid invalid PROT_NONE NUMA balancing") as a fix.

I tested that this was also the case with the 4.4.y stable-rc tree and I am
providing our backport below, which I also tested. The reproducer that
regresses reads from /proc/self/mem. Writing to /proc/self/mem seems to
have been a bug on 4.4 for a while and is also fixed by this backport, so
should be considered in any case.

Cascardo.

View attachment "0001-mm-check-VMA-flags-to-avoid-invalid-PROT_NONE-NUMA-b.patch" of type "text/x-diff" (4549 bytes)

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