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

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

Thadeu Lima de Souza Cascardo-3
From: "Kirill A. Shutemov" <[hidden email]>

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 <[hidden email]>
Cc: Michal Hocko <[hidden email]>
Cc: Hugh Dickins <[hidden email]>
Signed-off-by: Linus Torvalds <[hidden email]>
(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 <[hidden email]>
---

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)) {
--
2.14.1


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

Stefan Bader-2
On 01.12.2017 22:19, Thadeu Lima de Souza Cascardo wrote:

> From: "Kirill A. Shutemov" <[hidden email]>
>
> 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 <[hidden email]>
> Cc: Michal Hocko <[hidden email]>
> Cc: Hugh Dickins <[hidden email]>
> Signed-off-by: Linus Torvalds <[hidden email]>
> (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 <[hidden email]>
Acked-by: Stefan Bader <[hidden email]>

> ---
>
> 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.

Sounds good with explanation. And tested.

-Stefan

>
> 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)) {
>


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

Colin King
In reply to this post by Thadeu Lima de Souza Cascardo-3
On 01/12/17 21:19, Thadeu Lima de Souza Cascardo wrote:

> From: "Kirill A. Shutemov" <[hidden email]>
>
> 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 <[hidden email]>
> Cc: Michal Hocko <[hidden email]>
> Cc: Hugh Dickins <[hidden email]>
> Signed-off-by: Linus Torvalds <[hidden email]>
> (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 <[hidden email]>
> ---
>
> 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 <[hidden email]>

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

Thadeu Lima de Souza Cascardo-3
In reply to this post by Thadeu Lima de Souza Cascardo-3
Applied to xenial master-next branch.

Thanks.
Cascardo.

Applied-to: xenial/master-next

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