Re: [PATCH resend] mm/gup: fix try_grab_compound_head() race with split_huge_page()

[Andrew Morton <akpm@linux-foundation.org>]

On Fri, 11 Jun 2021 18:15:45 +0200 Jann Horn <jannh@google.com> wrote:

> try_grab_compound_head() is used to grab a reference to a page from
> get_user_pages_fast(), which is only protected against concurrent
> freeing of page tables (via local_irq_save()), but not against
> concurrent TLB flushes, freeing of data pages, or splitting of compound
> pages.
> 
> Because no reference is held to the page when try_grab_compound_head()
> is called, the page may have been freed and reallocated by the time its
> refcount has been elevated; therefore, once we're holding a stable
> reference to the page, the caller re-checks whether the PTE still points
> to the same page (with the same access rights).
> 
> The problem is that try_grab_compound_head() has to grab a reference on
> the head page; but between the time we look up what the head page is and
> the time we actually grab a reference on the head page, the compound
> page may have been split up (either explicitly through split_huge_page()
> or by freeing the compound page to the buddy allocator and then
> allocating its individual order-0 pages).
> If that happens, get_user_pages_fast() may end up returning the right
> page but lifting the refcount on a now-unrelated page, leading to
> use-after-free of pages.
> 
> To fix it:
> Re-check whether the pages still belong together after lifting the
> refcount on the head page.
> Move anything else that checks compound_head(page) below the refcount
> increment.
> 
> This can't actually happen on bare-metal x86 (because there, disabling
> IRQs locks out remote TLB flushes), but it can happen on virtualized x86
> (e.g. under KVM) and probably also on arm64. The race window is pretty
> narrow, and constantly allocating and shattering hugepages isn't exactly
> fast; for now I've only managed to reproduce this in an x86 KVM guest with
> an artificially widened timing window (by adding a loop that repeatedly
> calls `inl(0x3f8 + 5)` in `try_get_compound_head()` to force VM exits,
> so that PV TLB flushes are used instead of IPIs).
> 
> ...
>
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -43,8 +43,21 @@ static void hpage_pincount_sub(struct page *page, int refs)
>  
>  	atomic_sub(refs, compound_pincount_ptr(page));
>  }
>  
> +/* Equivalent to calling put_page() @refs times. */
> +static void put_page_refs(struct page *page, int refs)
> +{
> +	VM_BUG_ON_PAGE(page_ref_count(page) < refs, page);

I don't think there's a need to nuke the whole kernel in this case. 
Can we warn then simply leak the page?  That way we have a much better
chance of getting a good bug report.

> +	/*
> +	 * Calling put_page() for each ref is unnecessarily slow. Only the last
> +	 * ref needs a put_page().
> +	 */
> +	if (refs > 1)
> +		page_ref_sub(page, refs - 1);
> +	put_page(page);
> +}