Re: [PATCH v2] mm/gup: disallow GUP writing to file-backed mappings by default

[Lorenzo Stoakes]

On Mon, Apr 24, 2023 at 10:39:25AM -0300, Jason Gunthorpe wrote:
> On Mon, Apr 24, 2023 at 01:38:49PM +0100, Lorenzo Stoakes wrote:
>
> > I was being fairly conservative in that list, though we certainly need to
> > set the flag for /proc/$pid/mem and ptrace to avoid breaking this
> > functionality (I observed breakpoints breaking without it which obviously
> > is a no go :). I'm not sure if there's a more general way we could check
> > for this though?
>
> More broadly we should make sure these usages of GUP safe somehow so
> that it can reliably write to those types of pages without breaking
> the current FS contract..
>
> I forget exactly, but IIRC, don't you have to hold some kind of page
> spinlock while writing to the page memory?
>

I think perhaps you're thinking of the mm->mmap_lock? Which will be held
for the FOLL_GET cases and simply prevent the VMA from disappearing below
us but not do much else.

> So, users that do this, or can be fixed to do this, can get file
> backed pages. It suggests that a flag name is more like
> FOLL_CALLER_USES_FILE_WRITE_LOCKING
>

As stated above, I'm not sure what locking you're referring to, but seems
to me that FOLL_GET already implies what you're thinking?

I wonder whether we should do this check purely for FOLL_PIN to be honest?
As this indicates medium to long-term access without mmap_lock held. This
would exclude the /proc/$pid/mem and ptrace paths which use gup_remote().

That and a very specific use of uprobes are the only places that use
FOLL_GET in this instance and each of them are careful in any case to
handle setting the dirty page flag.

All PUP cases that do not specify FOLL_LONGTERM also do this, so we could
atually go so far as to reduce the patch to simply performing the
vma_wants_writenotify() check if (FOLL_PIN | FOLL_LONGTERM) is specified,
which covers the io_uring case.

Alternatively if we wanted to be safer, we could add a FOLL_ALLOW_FILE_PIN
that is checked on FOLL_PIN and ignored on FOLL_LONGTERM?

> > I wouldn't be totally opposed to dropping it for RDMA too, because I
> > suspect accessing file-backed mappings for that is pretty iffy.
> >
> > Do you have a sense of which in the list you feel could be pared back?
>
> Anything using FOLL_LONGTERM should not set the flag, GUP should even
> block the combination.

OK

>
> And we need to have in mind that the flag indicates the code is
> buggy, so if you set it then we should understand how is that caller
> expected to be fixed.
>
> Jason

I think we are working towards a much simpler solution in any case!