Re: [PATCH v8 3/3] mm/gup: disallow FOLL_LONGTERM GUP-fast writing to file-backed mappings

[Lorenzo Stoakes <lstoakes@gmail.com>]

On Tue, May 02, 2023 at 07:18:21PM -0700, Andrew Morton wrote:
> On Tue,  2 May 2023 23:51:35 +0100 Lorenzo Stoakes <lstoakes@gmail.com> wrote:
>
> > Writing to file-backed dirty-tracked mappings via GUP is inherently broken
> > as we cannot rule out folios being cleaned and then a GUP user writing to
> > them again and possibly marking them dirty unexpectedly.
> >
> > This is especially egregious for long-term mappings (as indicated by the
> > use of the FOLL_LONGTERM flag), so we disallow this case in GUP-fast as
> > we have already done in the slow path.
> >
> > We have access to less information in the fast path as we cannot examine
> > the VMA containing the mapping, however we can determine whether the folio
> > is anonymous or belonging to a whitelisted filesystem - specifically
> > hugetlb and shmem mappings.
> >
> > We take special care to ensure that both the folio and mapping are safe to
> > access when performing these checks and document folio_fast_pin_allowed()
> > accordingly.
> >
> > It's important to note that there are no APIs allowing users to specify
> > FOLL_FAST_ONLY for a PUP-fast let alone with FOLL_LONGTERM, so we can
> > always rely on the fact that if we fail to pin on the fast path, the code
> > will fall back to the slow path which can perform the more thorough check.
>
> arm allnoconfig said
>
> mm/gup.c:115:13: warning: 'folio_fast_pin_allowed' defined but not used [-Wunused-function]
>   115 | static bool folio_fast_pin_allowed(struct folio *folio, unsigned int flags)
>       |             ^~~~~~~~~~~~~~~~~~~~~~
>
> so I moved the definition inside CONFIG_ARCH_HAS_PTE_SPECIAL.
>
>
>
>  mm/gup.c |  154 ++++++++++++++++++++++++++---------------------------
>  1 file changed, 77 insertions(+), 77 deletions(-)
>
> --- a/mm/gup.c~mm-gup-disallow-foll_longterm-gup-fast-writing-to-file-backed-mappings-fix
> +++ a/mm/gup.c
> @@ -96,83 +96,6 @@ retry:
>  	return folio;
>  }
>
> -/*
> - * Used in the GUP-fast path to determine whether a pin is permitted for a
> - * specific folio.
> - *
> - * This call assumes the caller has pinned the folio, that the lowest page table
> - * level still points to this folio, and that interrupts have been disabled.
> - *
> - * Writing to pinned file-backed dirty tracked folios is inherently problematic
> - * (see comment describing the writable_file_mapping_allowed() function). We
> - * therefore try to avoid the most egregious case of a long-term mapping doing
> - * so.
> - *
> - * This function cannot be as thorough as that one as the VMA is not available
> - * in the fast path, so instead we whitelist known good cases and if in doubt,
> - * fall back to the slow path.
> - */
> -static bool folio_fast_pin_allowed(struct folio *folio, unsigned int flags)
> -{
> -	struct address_space *mapping;
> -	unsigned long mapping_flags;
> -
> -	/*
> -	 * If we aren't pinning then no problematic write can occur. A long term
> -	 * pin is the most egregious case so this is the one we disallow.
> -	 */
> -	if ((flags & (FOLL_PIN | FOLL_LONGTERM | FOLL_WRITE)) !=
> -	    (FOLL_PIN | FOLL_LONGTERM | FOLL_WRITE))
> -		return true;
> -
> -	/* The folio is pinned, so we can safely access folio fields. */
> -
> -	/* Neither of these should be possible, but check to be sure. */
> -	if (unlikely(folio_test_slab(folio) || folio_test_swapcache(folio)))
> -		return false;
> -
> -	/* hugetlb mappings do not require dirty-tracking. */
> -	if (folio_test_hugetlb(folio))
> -		return true;
> -
> -	/*
> -	 * GUP-fast disables IRQs. When IRQS are disabled, RCU grace periods
> -	 * cannot proceed, which means no actions performed under RCU can
> -	 * proceed either.
> -	 *
> -	 * inodes and thus their mappings are freed under RCU, which means the
> -	 * mapping cannot be freed beneath us and thus we can safely dereference
> -	 * it.
> -	 */
> -	lockdep_assert_irqs_disabled();
> -
> -	/*
> -	 * However, there may be operations which _alter_ the mapping, so ensure
> -	 * we read it once and only once.
> -	 */
> -	mapping = READ_ONCE(folio->mapping);
> -
> -	/*
> -	 * The mapping may have been truncated, in any case we cannot determine
> -	 * if this mapping is safe - fall back to slow path to determine how to
> -	 * proceed.
> -	 */
> -	if (!mapping)
> -		return false;
> -
> -	/* Anonymous folios are fine, other non-file backed cases are not. */
> -	mapping_flags = (unsigned long)mapping & PAGE_MAPPING_FLAGS;
> -	if (mapping_flags)
> -		return mapping_flags == PAGE_MAPPING_ANON;
> -
> -	/*
> -	 * At this point, we know the mapping is non-null and points to an
> -	 * address_space object. The only remaining whitelisted file system is
> -	 * shmem.
> -	 */
> -	return shmem_mapping(mapping);
> -}
> -
>  /**
>   * try_grab_folio() - Attempt to get or pin a folio.
>   * @page:  pointer to page to be grabbed
> @@ -2474,6 +2397,83 @@ static void __maybe_unused undo_dev_page
>
>  #ifdef CONFIG_ARCH_HAS_PTE_SPECIAL
>  /*
> + * Used in the GUP-fast path to determine whether a pin is permitted for a
> + * specific folio.
> + *
> + * This call assumes the caller has pinned the folio, that the lowest page table
> + * level still points to this folio, and that interrupts have been disabled.
> + *
> + * Writing to pinned file-backed dirty tracked folios is inherently problematic
> + * (see comment describing the writable_file_mapping_allowed() function). We
> + * therefore try to avoid the most egregious case of a long-term mapping doing
> + * so.
> + *
> + * This function cannot be as thorough as that one as the VMA is not available
> + * in the fast path, so instead we whitelist known good cases and if in doubt,
> + * fall back to the slow path.
> + */
> +static bool folio_fast_pin_allowed(struct folio *folio, unsigned int flags)
> +{
> +	struct address_space *mapping;
> +	unsigned long mapping_flags;
> +
> +	/*
> +	 * If we aren't pinning then no problematic write can occur. A long term
> +	 * pin is the most egregious case so this is the one we disallow.
> +	 */
> +	if ((flags & (FOLL_PIN | FOLL_LONGTERM | FOLL_WRITE)) !=
> +	    (FOLL_PIN | FOLL_LONGTERM | FOLL_WRITE))
> +		return true;
> +
> +	/* The folio is pinned, so we can safely access folio fields. */
> +
> +	/* Neither of these should be possible, but check to be sure. */
> +	if (unlikely(folio_test_slab(folio) || folio_test_swapcache(folio)))
> +		return false;
> +
> +	/* hugetlb mappings do not require dirty-tracking. */
> +	if (folio_test_hugetlb(folio))
> +		return true;
> +
> +	/*
> +	 * GUP-fast disables IRQs. When IRQS are disabled, RCU grace periods
> +	 * cannot proceed, which means no actions performed under RCU can
> +	 * proceed either.
> +	 *
> +	 * inodes and thus their mappings are freed under RCU, which means the
> +	 * mapping cannot be freed beneath us and thus we can safely dereference
> +	 * it.
> +	 */
> +	lockdep_assert_irqs_disabled();
> +
> +	/*
> +	 * However, there may be operations which _alter_ the mapping, so ensure
> +	 * we read it once and only once.
> +	 */
> +	mapping = READ_ONCE(folio->mapping);
> +
> +	/*
> +	 * The mapping may have been truncated, in any case we cannot determine
> +	 * if this mapping is safe - fall back to slow path to determine how to
> +	 * proceed.
> +	 */
> +	if (!mapping)
> +		return false;
> +
> +	/* Anonymous folios are fine, other non-file backed cases are not. */
> +	mapping_flags = (unsigned long)mapping & PAGE_MAPPING_FLAGS;
> +	if (mapping_flags)
> +		return mapping_flags == PAGE_MAPPING_ANON;
> +
> +	/*
> +	 * At this point, we know the mapping is non-null and points to an
> +	 * address_space object. The only remaining whitelisted file system is
> +	 * shmem.
> +	 */
> +	return shmem_mapping(mapping);
> +}
> +
> +/*
>   * Fast-gup relies on pte change detection to avoid concurrent pgtable
>   * operations.
>   *
> _
>

Ack thanks for this, I think it doesn't quite cover all cases (kernel bot
moaning on -next for mips), needs some more fiddly #ifdef stuff, I will
spin a v9 shortly anyway to fix this up and address a few other minor
things.