linux-kernel - Re: [PATCH resend] mm/gup: fix try_grab_compound_head() race with split_huge

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Date:   Mon, 14 Jun 2021 16:10:56 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Jann Horn <jannh@...gle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Matthew Wilcox <willy@...radead.org>,
        John Hubbard <jhubbard@...dia.com>, Jan Kara <jack@...e.cz>,
        stable@...r.kernel.org
Subject: Re: [PATCH resend] mm/gup: fix try_grab_compound_head() race with
 split_huge_page()

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@...radead.org>
> Cc: Kirill A. Shutemov <kirill@...temov.name>
> Cc: John Hubbard <jhubbard@...dia.com>
> Cc: Jan Kara <jack@...e.cz>
> Cc: stable@...r.kernel.org
> Fixes: 7aef4172c795 ("mm: handle PTE-mapped tail pages in gerneric fast gup implementaiton")
> Signed-off-by: Jann Horn <jannh@...gle.com>

Looks good to me:

Acked-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>


-- 
 Kirill A. Shutemov