linux-kernel - Re: [PATCH v5 5/7] mm: shmem: fix missing cache flush in shmem_mfill_atomic

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Date:   Tue, 15 Feb 2022 11:11:51 -0800
From:   Mike Kravetz <mike.kravetz@...cle.com>
To:     Muchun Song <songmuchun@...edance.com>, akpm@...ux-foundation.org,
        zi.yan@...rutgers.edu, kirill.shutemov@...ux.intel.com,
        rientjes@...gle.com, lars.persson@...s.com, ziy@...dia.com
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        duanxiongchun@...edance.com, fam.zheng@...edance.com
Subject: Re: [PATCH v5 5/7] mm: shmem: fix missing cache flush in
 shmem_mfill_atomic_pte()

On 2/10/22 04:30, Muchun Song wrote:
> The userfaultfd calls shmem_mfill_atomic_pte() which does not do any
> cache flushing for the target page.  Then the target page will be mapped
> to the user space with a different address (user address), which might
> have an alias issue with the kernel address used to copy the data from the
> user to.  Insert flush_dcache_page() in non-zero-page case.  And replace
> clear_highpage() with clear_user_highpage() which already considers
> the cache maintenance.
> 
> Fixes: 8d1039634206 ("userfaultfd: shmem: add shmem_mfill_zeropage_pte for userfaultfd support")
> Fixes: 4c27fe4c4c84 ("userfaultfd: shmem: add shmem_mcopy_atomic_pte for userfaultfd support")
> Signed-off-by: Muchun Song <songmuchun@...edance.com>
> ---
>  mm/shmem.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)

Thanks,

It might have been better to combine this and the next patch.  When looking
at this, I noted the 'fallback to copy_from_user outside mmap_lock' case needs
to be addressed as well.  It is in the next patch.  No need to change.

Reviewed-by: Mike Kravetz <mike.kravetz@...cle.com>

-- 
Mike Kravetz

> 
> diff --git a/mm/shmem.c b/mm/shmem.c
> index eb0fd9001130..2e17ec9231a2 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -2371,8 +2371,10 @@ int shmem_mfill_atomic_pte(struct mm_struct *dst_mm,
>  				/* don't free the page */
>  				goto out_unacct_blocks;
>  			}
> +
> +			flush_dcache_page(page);
>  		} else {		/* ZEROPAGE */
> -			clear_highpage(page);
> +			clear_user_highpage(page, dst_addr);
>  		}
>  	} else {
>  		page = *pagep;


-- 
Mike Kravetz