Re: [PATCH v5] mm/gup: disallow GUP writing to file-backed mappings by default

Lorenzo Stoakes
2023-04-28T15:50:22Z

On Fri, Apr 28, 2023 at 05:33:17PM +0200, David Hildenbrand wrote:
> On 28.04.23 17:24, Lorenzo Stoakes wrote:
> > On Fri, Apr 28, 2023 at 05:13:07PM +0200, David Hildenbrand wrote:
> > > [...]
> > >
> > > > > This change has the potential to break existing setups. Simple example:
> > > > > libvirt domains configured for file-backed VM memory that also has a vfio
> > > > > device configured. It can easily be configured by users (evolving VM
> > > > > configuration, copy-paste etc.). And it works from a VM perspective, because
> > > > > the guest memory is essentially stale once the VM is shutdown and the pages
> > > > > were unpinned. At least we're not concerned about stale data on disk.
> > > > >
> > > > > With your changes, such VMs would no longer start, breaking existing user
> > > > > setups with a kernel update.
> > > >
> > > > Which vfio vm_ops are we talking about? vfio_pci_mmap_ops for example
> > > > doesn't specify page_mkwrite or pfn_mkwrite. Unless you mean some arbitrary
> > > > file system in the guest?
> > >
> > > Sorry, you define a VM to have its memory backed by VM memory and, at the
> > > same time, define a vfio-pci device for your VM, which will end up long-term
> > > pinning the VM memory.
> >
>
> "memory backed by file memory", I guess you figured that out :)

Ack yeah I vaguely assumed this was what you meant :) as in a virtualised 'file'
system that ultimately actually is in reality memory backed but not from guest's
persective.

>
> > Ah ack. Jason seemed concerned that this was already a broken case, I guess
> > that's one for you two to hash out...
> >
> > >
> > > >
> > > > I may well be missing context on this so forgive me if I'm being a little
> > > > dumb here, but it'd be good to get a specific example.
> > >
> > > I was giving to little details ;)
> > >
> > > [...]
> > >
> > > > >
> > > > > I know, Jason und John will disagree, but I don't think we want to be very
> > > > > careful with changing the default.
> > > > >
> > > > > Sure, we could warn, or convert individual users using a flag (io_uring).
> > > > > But maybe we should invest more energy on a fix?
> > > >
> > > > This is proactively blocking a cleanup (eliminating vmas) that I believe
> > > > will be useful in moving things forward. I am not against an opt-in option
> > > > (I have been responding to community feedback in adapting my approach),
> > > > which is the way I implemented it all the way back then :)
> > >
> > > There are alternatives: just use a flag as Jason initially suggested and use
> > > that in io_uring code. Then, you can also bail out on the GUP-fast path as
> > > "cannot support it right now, never do GUP-fast".
> >
> > I already implemented the alternatives (look back through revisions to see :)
> > and there were objections for various reasons.
> >
> > Personally my preference is to provide a FOLL_SAFE_FILE_WRITE flag or such and
> > replace the FOLL_LONGTERM check with this (that'll automatically get rejected
> > for GUP-fast so the GUP-fast conundrum wouldn't be a thing either).
> >
> > GUP-fast is a problem as you say,, but it feels like a fundamental issue with
> > GUP-fast as a whole since you don't get to look at a VMA since you can't take
> > the mmap_lock. You could just look at the folio->mapping once you've walked the
> > page tables and say 'I'm out' if FOLL_WRITE and it's non-anon if that's what
> > you're suggesting?
>
> See my other reply, kind-of yes. Like we do with gup_must_unshare(). I'm
> only concerned about how to keep GUP-fast working on hugetlb and shmem.
>
> >
> > I'm not against that change but this being incremental, I think that would be a
> > further increment.
>
> If we want to fix a security issue, as Jason said, incremental is IMHO the
> wrong approach.
>
> It's often too tempting to ignore the hard part and fix the easy part,
> making the hard part an increment for the future that nobody will really
> implement ... because it's hard.
> [...]
>
> > > >
> > > > But given we know this is both entirely broken and a potential security
> > > > issue, and FOLL_LONGTERM is about as egregious as you can get (user
> > > > explicitly saying they'll hold write access indefinitely) I feel it is an
> > > > important improvement and makes clear that this is not an acceptable usage.
> > > >
> > > > I see Jason has said more on this also :)
> > > >
> > > > >
> > > > >
> > > > >
> > > > >
> > > > > > Suggested-by: Jason Gunthorpe <jgg@nvidia.com>
> > > > > > Signed-off-by: Lorenzo Stoakes <lstoakes@gmail.com>
> > > > > > ---
> > > > > > include/linux/mm.h | 1 +
> > > > > > mm/gup.c | 41 ++++++++++++++++++++++++++++++++++++++++-
> > > > > > mm/mmap.c | 36 +++++++++++++++++++++++++++---------
> > > > > > 3 files changed, 68 insertions(+), 10 deletions(-)
> > > > > >
> > > > > > diff --git a/include/linux/mm.h b/include/linux/mm.h
> > > > > > index 37554b08bb28..f7da02fc89c6 100644
> > > > > > --- a/include/linux/mm.h
> > > > > > +++ b/include/linux/mm.h
> > > > > > @@ -2433,6 +2433,7 @@ extern unsigned long move_page_tables(struct vm_area_struct *vma,
> > > > > > #define MM_CP_UFFD_WP_ALL (MM_CP_UFFD_WP | \
> > > > > > MM_CP_UFFD_WP_RESOLVE)
> > > > > >
> > > > > > +bool vma_needs_dirty_tracking(struct vm_area_struct *vma);
> > > > > > int vma_wants_writenotify(struct vm_area_struct *vma, pgprot_t vm_page_prot);
> > > > > > static inline bool vma_wants_manual_pte_write_upgrade(struct vm_area_struct *vma)
> > > > > > {
> > > > > > diff --git a/mm/gup.c b/mm/gup.c
> > > > > > index 1f72a717232b..d36a5db9feb1 100644
> > > > > > --- a/mm/gup.c
> > > > > > +++ b/mm/gup.c
> > > > > > @@ -959,16 +959,51 @@ static int faultin_page(struct vm_area_struct *vma,
> > > > > > return 0;
> > > > > > }
> > > > > >
> > > > > > +/*
> > > > > > + * Writing to file-backed mappings which require folio dirty tracking using GUP
> > > > > > + * is a fundamentally broken operation, as kernel write access to GUP mappings
> > > > > > + * do not adhere to the semantics expected by a file system.
> > > > > > + *
> > > > > > + * Consider the following scenario:-
> > > > > > + *
> > > > > > + * 1. A folio is written to via GUP which write-faults the memory, notifying
> > > > > > + * the file system and dirtying the folio.
> > > > > > + * 2. Later, writeback is triggered, resulting in the folio being cleaned and
> > > > > > + * the PTE being marked read-only.
> > > > > > + * 3. The GUP caller writes to the folio, as it is mapped read/write via the
> > > > > > + * direct mapping.
> > > > > > + * 4. The GUP caller, now done with the page, unpins it and sets it dirty
> > > > > > + * (though it does not have to).
> > > > > > + *
> > > > > > + * This results in both data being written to a folio without writenotify, and
> > > > > > + * the folio being dirtied unexpectedly (if the caller decides to do so).
> > > > > > + */
> > > > > > +static bool writeable_file_mapping_allowed(struct vm_area_struct *vma,
> > > > > > + unsigned long gup_flags)
> > > > > > +{
> > > > > > + /* If we aren't pinning then no problematic write can occur. */
> > > > > > + if (!(gup_flags & (FOLL_GET | FOLL_PIN)))
> > > > > > + return true;
> > > > >
> > > > > FOLL_LONGTERM only applies to FOLL_PIN. This check can be dropped.
> > > >
> > > > I understand that of course (well maybe not of course, but I mean I do, I
> > > > have oodles of diagrams referencing this int he book :) This is intended to
> > > > document the fact that the check isn't relevant if we don't pin at all,
> > > > e.g. reading this you see:-
> > > >
> > > > - (implicit) if not writing or anon we're good
> > > > - if not pin we're good
> > > > - ok we are only currently checking one especially egregious case
> > > > - finally, perform the dirty tracking check.
> > > >
> > > > So this is intentional.
> > > >
> > > > >
> > > > > > +
> > > > > > + /* We limit this check to the most egregious case - a long term pin. */
> > > > > > + if (!(gup_flags & FOLL_LONGTERM))
> > > > > > + return true;
> > > > > > +
> > > > > > + /* If the VMA requires dirty tracking then GUP will be problematic. */
> > > > > > + return vma_needs_dirty_tracking(vma);
> > > > > > +}
> > > > > > +
> > > > > > static int check_vma_flags(struct vm_area_struct *vma, unsigned long gup_flags)
> > > > > > {
> > > > > > vm_flags_t vm_flags = vma->vm_flags;
> > > > > > int write = (gup_flags & FOLL_WRITE);
> > > > > > int foreign = (gup_flags & FOLL_REMOTE);
> > > > > > + bool vma_anon = vma_is_anonymous(vma);
> > > > > >
> > > > > > if (vm_flags & (VM_IO | VM_PFNMAP))
> > > > > > return -EFAULT;
> > > > > >
> > > > > > - if (gup_flags & FOLL_ANON && !vma_is_anonymous(vma))
> > > > > > + if ((gup_flags & FOLL_ANON) && !vma_anon)
> > > > > > return -EFAULT;
> > > > > >
> > > > > > if ((gup_flags & FOLL_LONGTERM) && vma_is_fsdax(vma))
> > > > > > @@ -978,6 +1013,10 @@ static int check_vma_flags(struct vm_area_struct *vma, unsigned long gup_flags)
> > > > > > return -EFAULT;
> > > > > >
> > > > > > if (write) {
> > > > > > + if (!vma_anon &&
> > > > > > + !writeable_file_mapping_allowed(vma, gup_flags))
> > > > > > + return -EFAULT;
> > > > > > +
> > > > > > if (!(vm_flags & VM_WRITE)) {
> > > > > > if (!(gup_flags & FOLL_FORCE))
> > > > > > return -EFAULT;
> > > > > > diff --git a/mm/mmap.c b/mm/mmap.c
> > > > > > index 536bbb8fa0ae..7b6344d1832a 100644
> > > > > > --- a/mm/mmap.c
> > > > >
> > > > >
> > > > > I'm probably missing something, why don't we have to handle GUP-fast (having
> > > > > said that, it's hard to handle ;) )? The sequence you describe above should
> > > > > apply to GUP-fast as well, no?
> > > > >
> > > > > 1) Pin writable mapped page using GUP-fast
> > > > > 2) Trigger writeback
> > > > > 3) Write to page via pin
> > > > > 4) Unpin and set dirty
> > > >
> > > > You're right, and this is an excellent point. I worry about other GUP use
> > > > cases too, but we're a bit out of luck there because we don't get to check
> > > > the VMA _at all_ (which opens yet another Pandora's box about how safe it
> > > > is to do unlocked pinning :)
> > > >
> > > > But again, this comes down to the fact we're trying to make things
> > > > _incrementally__ better rather than throwing our hands up and saying one
> > > > day my ship will come in...
> > >
> > > That's not how security fixes are supposed to work IMHO, sorry.
> >
> > Sure, but I don't think the 'let things continue to be terribly broken for X
> > more years' is also a great approach.
>
> Not at all, people (including me) were simply not aware that it is that much
> of an (security) issue because I never saw any real bug reports (or CVE
> numbers) and only herd John talk about possible fixes a year ago :)
>
> So I'm saying we either try to block it completely or finally look into
> fixing it for good. I'm not a friend of anything in between.
>
> You don't gain a lot of security by locking the front door but knowingly
> leaving the back door unlocked.
>
> >
> > 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.

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?

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 :]

>
> --
> Thanks,
>
> David / dhildenb
>