Re: [PATCH v8 24/26] mm/gup: track FOLL_PIN pages

Jan Kara
2019-12-10T13:39:46Z

On Mon 09-12-19 14:53:42, John Hubbard wrote:
> Add tracking of pages that were pinned via FOLL_PIN.
>
> As mentioned in the FOLL_PIN documentation, callers who effectively set
> FOLL_PIN are required to ultimately free such pages via unpin_user_page().
> The effect is similar to FOLL_GET, and may be thought of as "FOLL_GET
> for DIO and/or RDMA use".
>
> Pages that have been pinned via FOLL_PIN are identifiable via a
> new function call:
>
> bool page_dma_pinned(struct page *page);
>
> What to do in response to encountering such a page, is left to later
> patchsets. There is discussion about this in [1], [2], and [3].
>
> This also changes a BUG_ON(), to a WARN_ON(), in follow_page_mask().
>
> [1] Some slow progress on get_user_pages() (Apr 2, 2019):
> https://lwn.net/Articles/784574/
> [2] DMA and get_user_pages() (LPC: Dec 12, 2018):
> https://lwn.net/Articles/774411/
> [3] The trouble with get_user_pages() (Apr 30, 2018):
> https://lwn.net/Articles/753027/
>
> Suggested-by: Jan Kara <jack@suse.cz>
> Suggested-by: Jérôme Glisse <jglisse@redhat.com>
> Signed-off-by: John Hubbard <jhubbard@nvidia.com>

Looks nice, some comments below...

> +/*
> + * try_grab_compound_head() - attempt to elevate a page's refcount, by a
> + * flags-dependent amount.
> + *
> + * This has a default assumption of "use FOLL_GET behavior, if FOLL_PIN is not
> + * set".
> + *
> + * "grab" names in this file mean, "look at flags to decide with to use FOLL_PIN
> + * or FOLL_GET behavior, when incrementing the page's refcount.
> + */
> +static struct page *try_grab_compound_head(struct page *page, int refs,
> + unsigned int flags)
> +{
> + if (flags & FOLL_PIN)
> + return try_pin_compound_head(page, refs);
> +
> + return try_get_compound_head(page, refs);
> +}
> +
> +/**
> + * grab_page() - elevate a page's refcount by a flag-dependent amount
> + *
> + * This might not do anything at all, depending on the flags argument.
> + *
> + * "grab" names in this file mean, "look at flags to decide with to use FOLL_PIN
^^^ whether

> + * or FOLL_GET behavior, when incrementing the page's refcount.
> + *
> + * @page: pointer to page to be grabbed
> + * @flags: gup flags: these are the FOLL_* flag values.
> + *
> + * Either FOLL_PIN or FOLL_GET (or neither) may be set, but not both at the same
> + * time. (That's true throughout the get_user_pages*() and pin_user_pages*()
> + * APIs.) Cases:
> + *
> + * FOLL_GET: page's refcount will be incremented by 1.
> + * FOLL_PIN: page's refcount will be incremented by GUP_PIN_COUNTING_BIAS.
> + */
> +void grab_page(struct page *page, unsigned int flags)
> +{
> + if (flags & FOLL_GET)
> + get_page(page);
> + else if (flags & FOLL_PIN) {
> + get_page(page);
> + WARN_ON_ONCE(flags & FOLL_GET);
> + /*
> + * Use get_page(), above, to do the refcount error
> + * checking. Then just add in the remaining references:
> + */
> + page_ref_add(page, GUP_PIN_COUNTING_BIAS - 1);

This is wrong for two reasons:

1) You miss compound_head() indirection from get_page() for this
page_ref_add().

2) page_ref_add() could overflow the counter without noticing.

Especially with GUP_PIN_COUNTING_BIAS being non-trivial, it is realistic
that an attacker might try to overflow the page refcount and we have to
protect the kernel against that. So I think that all the places that would
use grab_page() actually need to use try_grab_page() and then gracefully
deal with the failure.

> @@ -278,11 +425,23 @@ static struct page *follow_page_pte(struct vm_area_struct *vma,
> goto retry;
> }
>
> - if (flags & FOLL_GET) {
> + if (flags & (FOLL_PIN | FOLL_GET)) {
> + /*
> + * Allow try_get_page() to take care of error handling, for
> + * both cases: FOLL_GET or FOLL_PIN:
> + */
> if (unlikely(!try_get_page(page))) {
> page = ERR_PTR(-ENOMEM);
> goto out;
> }
> +
> + if (flags & FOLL_PIN) {
> + WARN_ON_ONCE(flags & FOLL_GET);
> +
> + /* We got a +1 refcount from try_get_page(), above. */
> + page_ref_add(page, GUP_PIN_COUNTING_BIAS - 1);
> + __update_proc_vmstat(page, NR_FOLL_PIN_REQUESTED, 1);
> + }
> }

The same problem here as above, plus this place should use the same
try_grab..() helper, shouldn't it?

> @@ -544,8 +703,8 @@ static struct page *follow_page_mask(struct vm_area_struct *vma,
> /* make this handle hugepd */
> page = follow_huge_addr(mm, address, flags & FOLL_WRITE);
> if (!IS_ERR(page)) {
> - BUG_ON(flags & FOLL_GET);
> - return page;
> + WARN_ON_ONCE(flags & (FOLL_GET | FOLL_PIN));
> + return NULL;

I agree with the change to WARN_ON_ONCE but why is correct the change of
the return value? Note that this is actually a "success branch".

Honza
--
Jan Kara <jack@suse.com>
SUSE Labs, CR