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Subject: Re: [PATCH] khugepaged: Fix null-pointer dereference due to race
To:     "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Andrew Morton <akpm@linux-foundation.org>
Cc:     linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        syzbot+ed318e8b790ca72c5ad0@syzkaller.appspotmail.com
References: <20200722121439.44328-1-kirill.shutemov@linux.intel.com>
From:   David Hildenbrand <david@redhat.com>
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Organization: Red Hat GmbH
Message-ID: <c97addfd-6536-7611-d00e-323688234555@redhat.com>
Date:   Wed, 22 Jul 2020 15:30:47 +0200
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On 22.07.20 14:14, Kirill A. Shutemov wrote:
> khugepaged has to drop mmap lock several times while collapsing a page.
> The situation can change while the lock is dropped and we need to
> re-validate that the VMA is still in place and the PMD is still subject
> for collapse.
> 
> But we miss one corner case: while collapsing an anonymous pages the VMA
> could be replaced with file VMA. If the file VMA doesn't have any
> private pages we get NULL pointer dereference:
> 
> 	general protection fault, probably for non-canonical address 0xdffffc0000000000: 0000 [#1] PREEMPT SMP KASAN
> 	KASAN: null-ptr-deref in range [0x0000000000000000-0x0000000000000007]
> 	anon_vma_lock_write include/linux/rmap.h:120 [inline]
> 	collapse_huge_page mm/khugepaged.c:1110 [inline]
> 	khugepaged_scan_pmd mm/khugepaged.c:1349 [inline]
> 	khugepaged_scan_mm_slot mm/khugepaged.c:2110 [inline]
> 	khugepaged_do_scan mm/khugepaged.c:2193 [inline]
> 	khugepaged+0x3bba/0x5a10 mm/khugepaged.c:2238
> 
> The fix is to make sure that the VMA is anonymous in
> hugepage_vma_revalidate(). The helper is only used for collapsing
> anonymous pages.
> 
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> Fixes: 99cb0dbd47a1 ("mm,thp: add read-only THP support for (non-shmem) FS")
> Reported-by: syzbot+ed318e8b790ca72c5ad0@syzkaller.appspotmail.com
> ---
>  mm/khugepaged.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index b043c40a21d4..700f5160f3e4 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -958,6 +958,9 @@ static int hugepage_vma_revalidate(struct mm_struct *mm, unsigned long address,
>  		return SCAN_ADDRESS_RANGE;
>  	if (!hugepage_vma_check(vma, vma->vm_flags))
>  		return SCAN_VMA_CHECK;
> +	/* Anon VMA expected */
> +	if (!vma->anon_vma || vma->vm_ops)
> +		return SCAN_VMA_CHECK;
>  	return 0;
>  }
>  
> 

LGTM

Reviewed-by: David Hildenbrand <david@redhat.com>

-- 
Thanks,

David / dhildenb