On Tue 02-05-23 23:51:35, Lorenzo Stoakes 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.
>
> Suggested-by: David Hildenbrand <david@xxxxxxxxxx>
> Suggested-by: Kirill A . Shutemov <kirill@xxxxxxxxxxxxx>
> Suggested-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Signed-off-by: Lorenzo Stoakes <lstoakes@xxxxxxxxx>
The patch looks good to me now. Feel free to add:
Reviewed-by: Jan Kara <jack@xxxxxxx>
Honza
> ---
> mm/gup.c | 102 +++++++++++++++++++++++++++++++++++++++++++++++++++++++
> 1 file changed, 102 insertions(+)
>
> diff --git a/mm/gup.c b/mm/gup.c
> index 0ea9ebec9547..1ab369b5d889 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -18,6 +18,7 @@
> #include <linux/migrate.h>
> #include <linux/mm_inline.h>
> #include <linux/sched/mm.h>
> +#include <linux/shmem_fs.h>
>
> #include <asm/mmu_context.h>
> #include <asm/tlbflush.h>
> @@ -95,6 +96,83 @@ static inline struct folio *try_get_folio(struct page *page, int refs)
> 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
> @@ -2464,6 +2542,11 @@ static int gup_pte_range(pmd_t pmd, pmd_t *pmdp, unsigned long addr,
> goto pte_unmap;
> }
>
> + if (!folio_fast_pin_allowed(folio, flags)) {
> + gup_put_folio(folio, 1, flags);
> + goto pte_unmap;
> + }
> +
> if (!pte_write(pte) && gup_must_unshare(NULL, flags, page)) {
> gup_put_folio(folio, 1, flags);
> goto pte_unmap;
> @@ -2656,6 +2739,11 @@ static int gup_hugepte(pte_t *ptep, unsigned long sz, unsigned long addr,
> return 0;
> }
>
> + if (!folio_fast_pin_allowed(folio, flags)) {
> + gup_put_folio(folio, refs, flags);
> + return 0;
> + }
> +
> if (!pte_write(pte) && gup_must_unshare(NULL, flags, &folio->page)) {
> gup_put_folio(folio, refs, flags);
> return 0;
> @@ -2722,6 +2810,10 @@ static int gup_huge_pmd(pmd_t orig, pmd_t *pmdp, unsigned long addr,
> return 0;
> }
>
> + if (!folio_fast_pin_allowed(folio, flags)) {
> + gup_put_folio(folio, refs, flags);
> + return 0;
> + }
> if (!pmd_write(orig) && gup_must_unshare(NULL, flags, &folio->page)) {
> gup_put_folio(folio, refs, flags);
> return 0;
> @@ -2762,6 +2854,11 @@ static int gup_huge_pud(pud_t orig, pud_t *pudp, unsigned long addr,
> return 0;
> }
>
> + if (!folio_fast_pin_allowed(folio, flags)) {
> + gup_put_folio(folio, refs, flags);
> + return 0;
> + }
> +
> if (!pud_write(orig) && gup_must_unshare(NULL, flags, &folio->page)) {
> gup_put_folio(folio, refs, flags);
> return 0;
> @@ -2797,6 +2894,11 @@ static int gup_huge_pgd(pgd_t orig, pgd_t *pgdp, unsigned long addr,
> return 0;
> }
>
> + if (!folio_fast_pin_allowed(folio, flags)) {
> + gup_put_folio(folio, refs, flags);
> + return 0;
> + }
> +
> *nr += refs;
> folio_set_referenced(folio);
> return 1;
> --
> 2.40.1
>
--
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR
