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

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[CVE-2017-1000405 zesty] 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]
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.

---
 mm/huge_memory.c | 19 +++++++------------
 1 file changed, 7 insertions(+), 12 deletions(-)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 49cb70b5993d..65f19bb6ae58 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -756,20 +756,15 @@ int vmf_insert_pfn_pmd(struct vm_area_struct *vma, unsigned long addr,
 EXPORT_SYMBOL_GPL(vmf_insert_pfn_pmd);
 
 static void touch_pmd(struct vm_area_struct *vma, unsigned long addr,
- pmd_t *pmd)
+ pmd_t *pmd, int flags)
 {
  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);
 }
 
@@ -798,7 +793,7 @@ struct page *follow_devmap_pmd(struct vm_area_struct *vma, unsigned long addr,
  return NULL;
 
  if (flags & FOLL_TOUCH)
- touch_pmd(vma, addr, pmd);
+ touch_pmd(vma, addr, pmd, flags);
 
  /*
  * device mapped pages can only be returned if the
@@ -1168,7 +1163,7 @@ struct page *follow_trans_huge_pmd(struct vm_area_struct *vma,
  page = pmd_page(*pmd);
  VM_BUG_ON_PAGE(!PageHead(page) && !is_zone_device_page(page), page);
  if (flags & FOLL_TOUCH)
- touch_pmd(vma, addr, pmd);
+ touch_pmd(vma, addr, pmd, flags);
  if ((flags & FOLL_MLOCK) && (vma->vm_flags & VM_LOCKED)) {
  /*
  * We don't mlock() pte-mapped THPs. This way we can avoid
--
2.14.1


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

Stefan Bader-2
On 01.12.2017 21:11, 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]
> 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.

PUD level transparent huge pages only introduced with 4.11, so backport looks
good to me. And tested.

-stefan

>
> ---
>  mm/huge_memory.c | 19 +++++++------------
>  1 file changed, 7 insertions(+), 12 deletions(-)
>
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 49cb70b5993d..65f19bb6ae58 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -756,20 +756,15 @@ int vmf_insert_pfn_pmd(struct vm_area_struct *vma, unsigned long addr,
>  EXPORT_SYMBOL_GPL(vmf_insert_pfn_pmd);
>  
>  static void touch_pmd(struct vm_area_struct *vma, unsigned long addr,
> - pmd_t *pmd)
> + pmd_t *pmd, int flags)
>  {
>   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);
>  }
>  
> @@ -798,7 +793,7 @@ struct page *follow_devmap_pmd(struct vm_area_struct *vma, unsigned long addr,
>   return NULL;
>  
>   if (flags & FOLL_TOUCH)
> - touch_pmd(vma, addr, pmd);
> + touch_pmd(vma, addr, pmd, flags);
>  
>   /*
>   * device mapped pages can only be returned if the
> @@ -1168,7 +1163,7 @@ struct page *follow_trans_huge_pmd(struct vm_area_struct *vma,
>   page = pmd_page(*pmd);
>   VM_BUG_ON_PAGE(!PageHead(page) && !is_zone_device_page(page), page);
>   if (flags & FOLL_TOUCH)
> - touch_pmd(vma, addr, pmd);
> + touch_pmd(vma, addr, pmd, flags);
>   if ((flags & FOLL_MLOCK) && (vma->vm_flags & VM_LOCKED)) {
>   /*
>   * We don't mlock() pte-mapped THPs. This way we can avoid
>


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Re: [CVE-2017-1000405 zesty] 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 20:11, 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]
> 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.
>
> ---
>  mm/huge_memory.c | 19 +++++++------------
>  1 file changed, 7 insertions(+), 12 deletions(-)
>
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 49cb70b5993d..65f19bb6ae58 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -756,20 +756,15 @@ int vmf_insert_pfn_pmd(struct vm_area_struct *vma, unsigned long addr,
>  EXPORT_SYMBOL_GPL(vmf_insert_pfn_pmd);
>  
>  static void touch_pmd(struct vm_area_struct *vma, unsigned long addr,
> - pmd_t *pmd)
> + pmd_t *pmd, int flags)
>  {
>   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);
>  }
>  
> @@ -798,7 +793,7 @@ struct page *follow_devmap_pmd(struct vm_area_struct *vma, unsigned long addr,
>   return NULL;
>  
>   if (flags & FOLL_TOUCH)
> - touch_pmd(vma, addr, pmd);
> + touch_pmd(vma, addr, pmd, flags);
>  
>   /*
>   * device mapped pages can only be returned if the
> @@ -1168,7 +1163,7 @@ struct page *follow_trans_huge_pmd(struct vm_area_struct *vma,
>   page = pmd_page(*pmd);
>   VM_BUG_ON_PAGE(!PageHead(page) && !is_zone_device_page(page), page);
>   if (flags & FOLL_TOUCH)
> - touch_pmd(vma, addr, pmd);
> + touch_pmd(vma, addr, pmd, flags);
>   if ((flags & FOLL_MLOCK) && (vma->vm_flags & VM_LOCKED)) {
>   /*
>   * We don't mlock() pte-mapped THPs. This way we can avoid
>
Backport looks OK, there is no pud handling for THP required. Positive
test results.

Acked-by: Colin Ian King <[hidden email]>

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APPLIED: [CVE-2017-1000405 zesty] 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 zesty master-next branch.

Thanks.
Cascardo.

Applied-to: zesty/master-next

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