linux-kernel - Re: [PATCH v4 2/2] mm: prevent gup_fast from racing with COW during fork

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Date:   Wed, 11 Nov 2020 14:59:20 -0500
From:   Peter Xu <peterx@...hat.com>
To:     Jason Gunthorpe <jgg@...dia.com>
Cc:     linux-kernel@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        "Ahmed S. Darwish" <a.darwish@...utronix.de>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>,
        Christoph Hellwig <hch@....de>,
        Hugh Dickins <hughd@...gle.com>, Jan Kara <jack@...e.cz>,
        Jann Horn <jannh@...gle.com>,
        John Hubbard <jhubbard@...dia.com>,
        Kirill Shutemov <kirill@...temov.name>,
        Kirill Tkhai <ktkhai@...tuozzo.com>,
        Leon Romanovsky <leonro@...dia.com>,
        Linux-MM <linux-mm@...ck.org>, Michal Hocko <mhocko@...e.com>,
        Oleg Nesterov <oleg@...hat.com>
Subject: Re: [PATCH v4 2/2] mm: prevent gup_fast from racing with COW during
 fork

On Tue, Nov 10, 2020 at 07:44:09PM -0400, Jason Gunthorpe wrote:
> Since commit 70e806e4e645 ("mm: Do early cow for pinned pages during
> fork() for ptes") pages under a FOLL_PIN will not be write protected
> during COW for fork. This means that pages returned from
> pin_user_pages(FOLL_WRITE) should not become write protected while the pin
> is active.
> 
> However, there is a small race where get_user_pages_fast(FOLL_PIN) can
> establish a FOLL_PIN at the same time copy_present_page() is write
> protecting it:
> 
>         CPU 0                             CPU 1
>    get_user_pages_fast()
>     internal_get_user_pages_fast()
>                                        copy_page_range()
>                                          pte_alloc_map_lock()
>                                            copy_present_page()
>                                              atomic_read(has_pinned) == 0
> 					     page_maybe_dma_pinned() == false
>      atomic_set(has_pinned, 1);
>      gup_pgd_range()
>       gup_pte_range()
>        pte_t pte = gup_get_pte(ptep)
>        pte_access_permitted(pte)
>        try_grab_compound_head()
>                                              pte = pte_wrprotect(pte)
> 	                                     set_pte_at();
>                                          pte_unmap_unlock()
>       // GUP now returns with a write protected page
> 
> The first attempt to resolve this by using the write protect caused
> problems (and was missing a barrrier), see commit f3c64eda3e50 ("mm: avoid
> early COW write protect games during fork()")
> 
> Instead wrap copy_p4d_range() with the write side of a seqcount and check
> the read side around gup_pgd_range(). If there is a collision then
> get_user_pages_fast() fails and falls back to slow GUP.
> 
> Slow GUP is safe against this race because copy_page_range() is only
> called while holding the exclusive side of the mmap_lock on the src
> mm_struct.
> 
> Fixes: f3c64eda3e50 ("mm: avoid early COW write protect games during fork()")
> Suggested-by: Linus Torvalds <torvalds@...ux-foundation.org>
> Link: https://lore.kernel.org/r/CAHk-=wi=iCnYCARbPGjkVJu9eyYeZ13N64tZYLdOB8CP5Q_PLw@mail.gmail.com
> Reviewed-by: John Hubbard <jhubbard@...dia.com>
> Reviewed-by: Jan Kara <jack@...e.cz>
> Signed-off-by: Jason Gunthorpe <jgg@...dia.com>

Reviewed-by: Peter Xu <peterx@...hat.com>

-- 
Peter Xu