linux-kernel - Re: [PATCH mm-unstable v1 05/20] mm: add early FAULT_FLAG

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Date:   Fri, 18 Nov 2022 18:03:00 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     David Hildenbrand <david@...hat.com>, linux-kernel@...r.kernel.org
Cc:     x86@...nel.org, linux-alpha@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-ia64@...r.kernel.org,
        linux-mips@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        sparclinux@...r.kernel.org, linux-um@...ts.infradead.org,
        etnaviv@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org,
        linux-samsung-soc@...r.kernel.org, linux-rdma@...r.kernel.org,
        linux-media@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-mm@...ck.org, linux-perf-users@...r.kernel.org,
        linux-security-module@...r.kernel.org,
        linux-kselftest@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jason Gunthorpe <jgg@...pe.ca>,
        John Hubbard <jhubbard@...dia.com>,
        Peter Xu <peterx@...hat.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Hugh Dickins <hughd@...gle.com>, Nadav Amit <namit@...are.com>,
        Matthew Wilcox <willy@...radead.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Muchun Song <songmuchun@...edance.com>,
        Shuah Khan <shuah@...nel.org>,
        Lucas Stach <l.stach@...gutronix.de>,
        David Airlie <airlied@...il.com>,
        Oded Gabbay <ogabbay@...nel.org>,
        Arnd Bergmann <arnd@...db.de>,
        Christoph Hellwig <hch@...radead.org>,
        Alex Williamson <alex.williamson@...hat.com>
Subject: Re: [PATCH mm-unstable v1 05/20] mm: add early FAULT_FLAG_WRITE
 consistency checks

On 11/16/22 11:26, David Hildenbrand wrote:
> 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>

Reviewed-by: Vlastimil Babka <vbabka@...e.cz>

> ---
>  mm/memory.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/mm/memory.c b/mm/memory.c
> index e014435a87db..c4fa378ec2a0 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -5170,6 +5170,14 @@ 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;
>  	}
>  	return 0;
>  }