linux-kernel - Re: [PATCH] mm/khugepaged: Fix ->anon

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Date:   Wed, 11 Jan 2023 17:06:22 -0800
From:   Yang Shi <shy828301@...il.com>
To:     Jann Horn <jannh@...gle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        "Zach O'Keefe" <zokeefe@...gle.com>, linux-kernel@...r.kernel.org,
        David Hildenbrand <david@...hat.com>
Subject: Re: [PATCH] mm/khugepaged: Fix ->anon_vma race

On Wed, Jan 11, 2023 at 5:33 AM Jann Horn <jannh@...gle.com> wrote:
>
> If an ->anon_vma is attached to the VMA, collapse_and_free_pmd() requires
> it to be locked. retract_page_tables() bails out if an ->anon_vma is
> attached, but does this check before holding the mmap lock (as the comment
> above the check explains).
>
> If we racily merge an existing ->anon_vma (shared with a child process)
> from a neighboring VMA, subsequent rmap traversals on pages belonging to
> the child will be able to see the page tables that we are concurrently
> removing while assuming that nothing else can access them.
>
> Repeat the ->anon_vma check once we hold the mmap lock to ensure that there
> really is no concurrent page table access.
>
> Reported-by: Zach O'Keefe <zokeefe@...gle.com>
> Fixes: f3f0e1d2150b ("khugepaged: add support of collapse for tmpfs/shmem pages")
> Cc: stable@...r.kernel.org
> Signed-off-by: Jann Horn <jannh@...gle.com>
> ---
> zokeefe@ pointed out to me that the current code (after my last round of patches)
> can hit a lockdep assert by racing, and after staring at it a bit I've

I'm supposed the lockdep is the one in collapse_and_free_pmd(). It is
better to have the splat included in the commit log.

> convinced myself that this is a real, preexisting bug.
> (I haven't written a reproducer for it though. One way to hit it might be
> something along the lines of:
>
>  - set up a process A with a private-file-mapping VMA V1
>  - let A fork() to create process B, thereby copying V1 in A to V1' in B
>  - let B extend the end of V1'
>  - let B put some anon pages into the extended part of V1'

I don't quite get why we need this step. A cow fault on A's V1 isn't
enough to have anon_vma for V1? This should not prevent V1 and V2 from
sharing anon_vma. Did I miss something?

>  - let A map a new private-file-mapping VMA V2 directly behind V1, without
>    an anon_vma
> [race begins here]
>   - in A's thread 1: begin retract_page_tables() on V2, run through first
>     ->anon_vma check
>   - in A's thread 2: run __anon_vma_prepare() on V2 and ensure that it
>     merges the anon_vma of V1 (which implies V1 and V2 must be mapping the
>     same file at compatible offsets)
>   - in B: trigger rmap traversal on anon page in V1'
>
>  mm/khugepaged.c | 14 +++++++++++++-
>  1 file changed, 13 insertions(+), 1 deletion(-)
>
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index 5cb401aa2b9d..0bfed37f3a3b 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -1644,7 +1644,7 @@ static int retract_page_tables(struct address_space *mapping, pgoff_t pgoff,
>                  * has higher cost too. It would also probably require locking
>                  * the anon_vma.
>                  */
> -               if (vma->anon_vma) {
> +               if (READ_ONCE(vma->anon_vma)) {
>                         result = SCAN_PAGE_ANON;
>                         goto next;
>                 }
> @@ -1672,6 +1672,18 @@ static int retract_page_tables(struct address_space *mapping, pgoff_t pgoff,
>                 result = SCAN_PTE_MAPPED_HUGEPAGE;
>                 if ((cc->is_khugepaged || is_target) &&
>                     mmap_write_trylock(mm)) {
> +                       /*
> +                        * Re-check whether we have an ->anon_vma, because
> +                        * collapse_and_free_pmd() requires that either no
> +                        * ->anon_vma exists or the anon_vma is locked.
> +                        * We already checked ->anon_vma above, but that check
> +                        * is racy because ->anon_vma can be populated under the
> +                        * mmap lock in read mode.
> +                        */
> +                       if (vma->anon_vma) {
> +                               result = SCAN_PAGE_ANON;
> +                               goto unlock_next;
> +                       }
>                         /*
>                          * When a vma is registered with uffd-wp, we can't
>                          * recycle the pmd pgtable because there can be pte
>
> base-commit: 7dd4b804e08041ff56c88bdd8da742d14b17ed25
> --
> 2.39.0.314.g84b9a713c41-goog
>