linux-kernel - Re: [PATCH RFC 05/19] mm: add early FAULT_FLAG

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Message-ID: <E1E8C21A-EAEB-4FA3-A9B9-1DFF81FCDA70@vmware.com>
Date:   Mon, 7 Nov 2022 19:03:48 +0000
From:   Nadav Amit <namit@...are.com>
To:     David Hildenbrand <david@...hat.com>
CC:     kernel list <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>,
        "etnaviv@...ts.freedesktop.org" <etnaviv@...ts.freedesktop.org>,
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        Linus Torvalds <torvalds@...ux-foundation.org>,
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        David Airlie <airlied@...il.com>,
        Oded Gabbay <ogabbay@...nel.org>, Arnd Bergmann <arnd@...db.de>
Subject: Re: [PATCH RFC 05/19] mm: add early FAULT_FLAG_WRITE consistency
 checks

On Nov 7, 2022, at 8:17 AM, David Hildenbrand <david@...hat.com> wrote:

> !! External Email
> 
> Let's catch abuse of FAULT_FLAG_WRITE early, such that we don't have to
> care in all other handlers and might get "surprises" if we forget to do
> so.
> 
> Write faults without VM_MAYWRITE don't make any sense, and our
> maybe_mkwrite() logic could have hidden such abuse for now.
> 
> Write faults without VM_WRITE on something that is not a COW mapping is
> similarly broken, and e.g., do_wp_page() could end up placing an
> anonymous page into a shared mapping, which would be bad.
> 
> This is a preparation for reliable R/O long-term pinning of pages in
> private mappings, whereby we want to make sure that we will never break
> COW in a read-only private mapping.
> 
> Signed-off-by: David Hildenbrand <david@...hat.com>
> ---
> mm/memory.c | 8 ++++++++
> 1 file changed, 8 insertions(+)
> 
> diff --git a/mm/memory.c b/mm/memory.c
> index fe131273217a..826353da7b23 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -5159,6 +5159,14 @@ static vm_fault_t sanitize_fault_flags(struct vm_area_struct *vma,
>                 */
>                if (!is_cow_mapping(vma->vm_flags))
>                        *flags &= ~FAULT_FLAG_UNSHARE;
> +       } else if (*flags & FAULT_FLAG_WRITE) {
> +               /* Write faults on read-only mappings are impossible ... */
> +               if (WARN_ON_ONCE(!(vma->vm_flags & VM_MAYWRITE)))
> +                       return VM_FAULT_SIGSEGV;
> +               /* ... and FOLL_FORCE only applies to COW mappings. */
> +               if (WARN_ON_ONCE(!(vma->vm_flags & VM_WRITE) &&
> +                                !is_cow_mapping(vma->vm_flags)))
> +                       return VM_FAULT_SIGSEGV;

Not sure about the WARN_*(). Seems as if it might trigger in benign even if
rare scenarios, e.g., mprotect() racing with page-fault.