On Fri, Jun 11, 2021 at 06:15:45PM +0200, Jann Horn 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).
>
> Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
> Cc: Kirill A. Shutemov <kirill@xxxxxxxxxxxxx>
> Cc: John Hubbard <jhubbard@xxxxxxxxxx>
> Cc: Jan Kara <jack@xxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx
> Fixes: 7aef4172c795 ("mm: handle PTE-mapped tail pages in gerneric fast gup implementaiton")
> Signed-off-by: Jann Horn <jannh@xxxxxxxxxx>
Looks good to me:
Acked-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
--
Kirill A. Shutemov
