ACK: [CVE-2017-1000405 xenial] mm, thp: Do not make page table dirty unconditionally in touch_p[mu]d()

[Colin Ian King]

On 01/12/17 21:19, Thadeu Lima de Souza Cascardo wrote:
> From: "Kirill A. Shutemov" <kirill.shutemov at linux.intel.com>
> 
> Currently, we unconditionally make page table dirty in touch_pmd().
> It may result in false-positive can_follow_write_pmd().
> 
> We may avoid the situation, if we would only make the page table entry
> dirty if caller asks for write access -- FOLL_WRITE.
> 
> The patch also changes touch_pud() in the same way.
> 
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov at linux.intel.com>
> Cc: Michal Hocko <mhocko at suse.com>
> Cc: Hugh Dickins <hughd at google.com>
> Signed-off-by: Linus Torvalds <torvalds at linux-foundation.org>
> (backported from commit a8f97366452ed491d13cf1e44241bc0b5740b1f0)
> [cascardo: dropped touch_pud parts]
> [cascardo: fix in-place of where touch_pmd would be called]
> CVE-2017-1000405
> Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo at canonical.com>
> ---
> 
> Reproducer has been tested. It "exploits" without the patch. With the patch, it
> fails.
> 
> touch_pmd was introduced later as a helper. Now, we have only one call site as
> follow_devmap_pmd was not on xenial or trusty yet. So, fixing it in place is
> just as fine.
> 
> Cascardo.
> 
> ---
>  mm/huge_memory.c | 14 ++++----------
>  1 file changed, 4 insertions(+), 10 deletions(-)
> 
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 6c6f5ccfcda1..8f3769ec8575 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -1304,17 +1304,11 @@ struct page *follow_trans_huge_pmd(struct vm_area_struct *vma,
>  	VM_BUG_ON_PAGE(!PageHead(page), page);
>  	if (flags & FOLL_TOUCH) {
>  		pmd_t _pmd;
> -		/*
> -		 * We should set the dirty bit only for FOLL_WRITE but
> -		 * for now the dirty bit in the pmd is meaningless.
> -		 * And if the dirty bit will become meaningful and
> -		 * we'll only set it with FOLL_WRITE, an atomic
> -		 * set_bit will be required on the pmd to set the
> -		 * young bit, instead of the current set_pmd_at.
> -		 */
> -		_pmd = pmd_mkyoung(pmd_mkdirty(*pmd));
> +		_pmd = pmd_mkyoung(*pmd);
> +		if (flags & FOLL_WRITE)
> +			_pmd = pmd_mkdirty(_pmd);
>  		if (pmdp_set_access_flags(vma, addr & HPAGE_PMD_MASK,
> -					  pmd, _pmd,  1))
> +					  pmd, _pmd, flags & FOLL_WRITE))
>  			update_mmu_cache_pmd(vma, addr, pmd);
>  	}
>  	if ((flags & FOLL_MLOCK) && (vma->vm_flags & VM_LOCKED)) {
> 
Backport looks OK, there is no pud handling for THP in this older
kernel. Positive test results.

Acked-by: Colin Ian King <colin.king at canonical.com>