Re: [PATCH v2] mm/MADV_COLLAPSE: catch !none !huge !bad pmd lookups

[Yang Shi]

On Wed, Jan 25, 2023 at 2:54 PM Zach O'Keefe <zokeefe@xxxxxxxxxx> wrote:
>
> In commit 34488399fa08 ("mm/madvise: add file and shmem support to
> MADV_COLLAPSE") we make the following change to find_pmd_or_thp_or_none():
>
>         -       if (!pmd_present(pmde))
>         -               return SCAN_PMD_NULL;
>         +       if (pmd_none(pmde))
>         +               return SCAN_PMD_NONE;
>
> This was for-use by MADV_COLLAPSE file/shmem codepaths, where MADV_COLLAPSE
> might identify a pte-mapped hugepage, only to have khugepaged race-in, free
> the pte table, and clear the pmd.  Such codepaths include:
>
> A) If we find a suitably-aligned compound page of order HPAGE_PMD_ORDER
>    already in the pagecache.
> B) In retract_page_tables(), if we fail to grab mmap_lock for the target
>    mm/address.
>
> In these cases, collapse_pte_mapped_thp() really does expect a none (not
> just !present) pmd, and we want to suitably identify that case separate
> from the case where no pmd is found, or it's a bad-pmd (of course, many
> things could happen once we drop mmap_lock, and the pmd could plausibly
> undergo multiple transitions due to intervening fault, split, etc).
> Regardless, the code is prepared install a huge-pmd only when the existing
> pmd entry is either a genuine pte-table-mapping-pmd, or the none-pmd.
>
> However, the commit introduces a logical hole; namely, that we've allowed
> !none- && !huge- && !bad-pmds to be classified as genuine
> pte-table-mapping-pmds.  One such example that could leak through are swap
> entries.  The pmd values aren't checked again before use in
> pte_offset_map_lock(), which is expecting nothing less than a genuine
> pte-table-mapping-pmd.
>
> We want to put back the !pmd_present() check (below the pmd_none() check),
> but need to be careful to deal with subtleties in pmd transitions and
> treatments by various arch.
>
> The issue is that __split_huge_pmd_locked() temporarily clears the present
> bit (or otherwise marks the entry as invalid), but pmd_present()
> and pmd_trans_huge() still need to return true while the pmd is in this
> transitory state.  For example, x86's pmd_present() also checks the
> _PAGE_PSE , riscv's version also checks the _PAGE_LEAF bit, and arm64 also
> checks a PMD_PRESENT_INVALID bit.
>
> Covering all 4 cases for x86 (all checks done on the same pmd value):
>
> 1) pmd_present() && pmd_trans_huge()
>    All we actually know here is that the PSE bit is set. Either:
>    a) We aren't racing with __split_huge_page(), and PRESENT or PROTNONE
>       is set.
>       => huge-pmd
>    b) We are currently racing with __split_huge_page().  The danger here
>       is that we proceed as-if we have a huge-pmd, but really we are
>       looking at a pte-mapping-pmd.  So, what is the risk of this
>       danger?
>
>       The only relevant path is:
>
>         madvise_collapse() -> collapse_pte_mapped_thp()
>
>       Where we might just incorrectly report back "success", when really
>       the memory isn't pmd-backed.  This is fine, since split could
>       happen immediately after (actually) successful madvise_collapse().
>       So, it should be safe to just assume huge-pmd here.
>
> 2) pmd_present() && !pmd_trans_huge()
>    Either:
>    a) PSE not set and either PRESENT or PROTNONE is.
>       => pte-table-mapping pmd (or PROT_NONE)
>    b) devmap.  This routine can be called immediately after
>       unlocking/locking mmap_lock -- or called with no locks held (see
>       khugepaged_scan_mm_slot()), so previous VMA checks have since been
>       invalidated.
>
> 3) !pmd_present() && pmd_trans_huge()
>   Not possible.
>
> 4) !pmd_present() && !pmd_trans_huge()
>   Neither PRESENT nor PROTNONE set
>   => not present
>
> I've checked all archs that implement pmd_trans_huge() (arm64, riscv,
> powerpc, longarch, x86, mips, s390) and this logic roughly translates
> (though devmap treatment is unique to x86 and powerpc, and (3) doesn't
> necessarily hold in general -- but that doesn't matter since !pmd_present()
> always takes failure path).
>
> Also, add a comment above find_pmd_or_thp_or_none() to help future
> travelers reason about the validity of the code; namely, the possible
> mutations that might happen out from under us, depending on how
> mmap_lock is held (if at all).
>
> Fixes: 34488399fa08 ("mm/madvise: add file and shmem support to MADV_COLLAPSE")
> Reported-by: Hugh Dickins <hughd@xxxxxxxxxx>
> Signed-off-by: Zach O'Keefe <zokeefe@xxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx

Reviewed-by: Yang Shi <shy828301@xxxxxxxxx>

>
> ---
> Request that this be pulled into stable since it's theoretically
> possible (though I have no reproducer) that while mmap_lock is dropped,
> racing thp migration installs a pmd migration entry which then has a path to
> be consumed, unchecked, by pte_offset_map().
>
> v1 -> v2: Fix typo
> ---
>  mm/khugepaged.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
>
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index 9548644bdb56..1face2ae5877 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -943,6 +943,10 @@ static int hugepage_vma_revalidate(struct mm_struct *mm, unsigned long address,
>         return SCAN_SUCCEED;
>  }
>
> +/*
> + * See pmd_trans_unstable() for how the result may change out from
> + * underneath us, even if we hold mmap_lock in read.
> + */
>  static int find_pmd_or_thp_or_none(struct mm_struct *mm,
>                                    unsigned long address,
>                                    pmd_t **pmd)
> @@ -961,8 +965,12 @@ static int find_pmd_or_thp_or_none(struct mm_struct *mm,
>  #endif
>         if (pmd_none(pmde))
>                 return SCAN_PMD_NONE;
> +       if (!pmd_present(pmde))
> +               return SCAN_PMD_NULL;
>         if (pmd_trans_huge(pmde))
>                 return SCAN_PMD_MAPPED;
> +       if (pmd_devmap(pmde))
> +               return SCAN_PMD_NULL;
>         if (pmd_bad(pmde))
>                 return SCAN_PMD_NULL;
>         return SCAN_SUCCEED;
> --
> 2.39.1.456.gfc5497dd1b-goog
>