[PATCH] mm: Fix potential NULL pointer dereference

[PATCH] mm: Fix potential NULL pointer dereference

[Libin]

find_vma may return NULL, thus check the return
value to avoid NULL pointer dereference.

Signed-off-by: Libin <huawei.libin@huawei.com>
---
 mm/huge_memory.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 243e710..d4423f4 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -2294,6 +2294,8 @@ static void collapse_huge_page(struct mm_struct *mm,
 		goto out;
 
 	vma = find_vma(mm, address);
+	if (!vma)
+		goto out;
 	hstart = (vma->vm_start + ~HPAGE_PMD_MASK) & HPAGE_PMD_MASK;
 	hend = vma->vm_end & HPAGE_PMD_MASK;
 	if (address < hstart || address + HPAGE_PMD_SIZE > hend)
-- 
1.8.2.1


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Re: [PATCH] mm: Fix potential NULL pointer dereference

[Wanpeng Li]

On Wed, Jul 24, 2013 at 11:48:19AM +0800, Libin wrote:
>find_vma may return NULL, thus check the return
>value to avoid NULL pointer dereference.
>

When can this happen since down_read(&mm->mmap_sem) is held?

>Signed-off-by: Libin <huawei.libin@huawei.com>
>---
> mm/huge_memory.c | 2 ++
> 1 file changed, 2 insertions(+)
>
>diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>index 243e710..d4423f4 100644
>--- a/mm/huge_memory.c
>+++ b/mm/huge_memory.c
>@@ -2294,6 +2294,8 @@ static void collapse_huge_page(struct mm_struct *mm,
> 		goto out;
>
> 	vma = find_vma(mm, address);
>+	if (!vma)
>+		goto out;
> 	hstart = (vma->vm_start + ~HPAGE_PMD_MASK) & HPAGE_PMD_MASK;
> 	hend = vma->vm_end & HPAGE_PMD_MASK;
> 	if (address < hstart || address + HPAGE_PMD_SIZE > hend)
>-- 
>1.8.2.1
>
>
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Re: [PATCH] mm: Fix potential NULL pointer dereference

[Wanpeng Li]

On Wed, Jul 24, 2013 at 11:48:19AM +0800, Libin wrote:
>find_vma may return NULL, thus check the return
>value to avoid NULL pointer dereference.
>

When can this happen since down_read(&mm->mmap_sem) is held?

>Signed-off-by: Libin <huawei.libin@huawei.com>
>---
> mm/huge_memory.c | 2 ++
> 1 file changed, 2 insertions(+)
>
>diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>index 243e710..d4423f4 100644
>--- a/mm/huge_memory.c
>+++ b/mm/huge_memory.c
>@@ -2294,6 +2294,8 @@ static void collapse_huge_page(struct mm_struct *mm,
> 		goto out;
>
> 	vma = find_vma(mm, address);
>+	if (!vma)
>+		goto out;
> 	hstart = (vma->vm_start + ~HPAGE_PMD_MASK) & HPAGE_PMD_MASK;
> 	hend = vma->vm_end & HPAGE_PMD_MASK;
> 	if (address < hstart || address + HPAGE_PMD_SIZE > hend)
>-- 
>1.8.2.1
>
>
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Re: [PATCH] mm: Fix potential NULL pointer dereference

[Wanpeng Li]

On Wed, Jul 24, 2013 at 12:22:08PM +0800, Wanpeng Li wrote:
>On Wed, Jul 24, 2013 at 11:48:19AM +0800, Libin wrote:
>>find_vma may return NULL, thus check the return
>>value to avoid NULL pointer dereference.
>>
>
>When can this happen since down_read(&mm->mmap_sem) is held?
>

Between mmap_sem read lock released and write lock held I think.

>>Signed-off-by: Libin <huawei.libin@huawei.com>
>>---
>> mm/huge_memory.c | 2 ++
>> 1 file changed, 2 insertions(+)
>>
>>diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>>index 243e710..d4423f4 100644
>>--- a/mm/huge_memory.c
>>+++ b/mm/huge_memory.c
>>@@ -2294,6 +2294,8 @@ static void collapse_huge_page(struct mm_struct *mm,
>> 		goto out;
>>
>> 	vma = find_vma(mm, address);
>>+	if (!vma)
>>+		goto out;
>> 	hstart = (vma->vm_start + ~HPAGE_PMD_MASK) & HPAGE_PMD_MASK;
>> 	hend = vma->vm_end & HPAGE_PMD_MASK;
>> 	if (address < hstart || address + HPAGE_PMD_SIZE > hend)
>>-- 
>>1.8.2.1
>>
>>
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Re: [PATCH] mm: Fix potential NULL pointer dereference

[Wanpeng Li]

On Wed, Jul 24, 2013 at 12:22:08PM +0800, Wanpeng Li wrote:
>On Wed, Jul 24, 2013 at 11:48:19AM +0800, Libin wrote:
>>find_vma may return NULL, thus check the return
>>value to avoid NULL pointer dereference.
>>
>
>When can this happen since down_read(&mm->mmap_sem) is held?
>

Between mmap_sem read lock released and write lock held I think.

>>Signed-off-by: Libin <huawei.libin@huawei.com>
>>---
>> mm/huge_memory.c | 2 ++
>> 1 file changed, 2 insertions(+)
>>
>>diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>>index 243e710..d4423f4 100644
>>--- a/mm/huge_memory.c
>>+++ b/mm/huge_memory.c
>>@@ -2294,6 +2294,8 @@ static void collapse_huge_page(struct mm_struct *mm,
>> 		goto out;
>>
>> 	vma = find_vma(mm, address);
>>+	if (!vma)
>>+		goto out;
>> 	hstart = (vma->vm_start + ~HPAGE_PMD_MASK) & HPAGE_PMD_MASK;
>> 	hend = vma->vm_end & HPAGE_PMD_MASK;
>> 	if (address < hstart || address + HPAGE_PMD_SIZE > hend)
>>-- 
>>1.8.2.1
>>
>>
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RE: [PATCH] mm: Fix potential NULL pointer dereference

[Kirill A. Shutemov]

Libin wrote:
> find_vma may return NULL, thus check the return
> value to avoid NULL pointer dereference.
> 
> Signed-off-by: Libin <huawei.libin@huawei.com>

Looks correct to me.

Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>

Have you ever triggered the race or just found it by reading the code?
I wounder if it's subject for stable@.

> ---
>  mm/huge_memory.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 243e710..d4423f4 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -2294,6 +2294,8 @@ static void collapse_huge_page(struct mm_struct *mm,
>  		goto out;
>  
>  	vma = find_vma(mm, address);
> +	if (!vma)
> +		goto out;
>  	hstart = (vma->vm_start + ~HPAGE_PMD_MASK) & HPAGE_PMD_MASK;
>  	hend = vma->vm_end & HPAGE_PMD_MASK;
>  	if (address < hstart || address + HPAGE_PMD_SIZE > hend)
> -- 
> 1.8.2.1

-- 
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Re: [PATCH] mm: Fix potential NULL pointer dereference

[Wanpeng Li]

On Wed, Jul 24, 2013 at 11:48:19AM +0800, Libin wrote:
>find_vma may return NULL, thus check the return
>value to avoid NULL pointer dereference.
>

Reviewed-by: Wanpeng Li <liwanp@linux.vnet.ibm.com>

>Signed-off-by: Libin <huawei.libin@huawei.com>
>---
> mm/huge_memory.c | 2 ++
> 1 file changed, 2 insertions(+)
>
>diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>index 243e710..d4423f4 100644
>--- a/mm/huge_memory.c
>+++ b/mm/huge_memory.c
>@@ -2294,6 +2294,8 @@ static void collapse_huge_page(struct mm_struct *mm,
> 		goto out;
>
> 	vma = find_vma(mm, address);
>+	if (!vma)
>+		goto out;
> 	hstart = (vma->vm_start + ~HPAGE_PMD_MASK) & HPAGE_PMD_MASK;
> 	hend = vma->vm_end & HPAGE_PMD_MASK;
> 	if (address < hstart || address + HPAGE_PMD_SIZE > hend)
>-- 
>1.8.2.1
>
>
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Re: [PATCH] mm: Fix potential NULL pointer dereference

[Wanpeng Li]

On Wed, Jul 24, 2013 at 11:48:19AM +0800, Libin wrote:
>find_vma may return NULL, thus check the return
>value to avoid NULL pointer dereference.
>

Reviewed-by: Wanpeng Li <liwanp@linux.vnet.ibm.com>

>Signed-off-by: Libin <huawei.libin@huawei.com>
>---
> mm/huge_memory.c | 2 ++
> 1 file changed, 2 insertions(+)
>
>diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>index 243e710..d4423f4 100644
>--- a/mm/huge_memory.c
>+++ b/mm/huge_memory.c
>@@ -2294,6 +2294,8 @@ static void collapse_huge_page(struct mm_struct *mm,
> 		goto out;
>
> 	vma = find_vma(mm, address);
>+	if (!vma)
>+		goto out;
> 	hstart = (vma->vm_start + ~HPAGE_PMD_MASK) & HPAGE_PMD_MASK;
> 	hend = vma->vm_end & HPAGE_PMD_MASK;
> 	if (address < hstart || address + HPAGE_PMD_SIZE > hend)
>-- 
>1.8.2.1
>
>
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Re: [PATCH] mm: Fix potential NULL pointer dereference

[Libin]

On 2013/7/24 19:48, Kirill A. Shutemov wrote:
> Libin wrote:
>> find_vma may return NULL, thus check the return
>> value to avoid NULL pointer dereference.
>>
>> Signed-off-by: Libin <huawei.libin@huawei.com>
> 
> Looks correct to me.
> 
> Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> 
> Have you ever triggered the race or just found it by reading the code?
> I wounder if it's subject for stable@.
> 

I had not triggered the bug, although it is a small probability but it looks
likely to occur.
Thanks!
Libin.

>> ---
>>  mm/huge_memory.c | 2 ++
>>  1 file changed, 2 insertions(+)
>>
>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>> index 243e710..d4423f4 100644
>> --- a/mm/huge_memory.c
>> +++ b/mm/huge_memory.c
>> @@ -2294,6 +2294,8 @@ static void collapse_huge_page(struct mm_struct *mm,
>>  		goto out;
>>  
>>  	vma = find_vma(mm, address);
>> +	if (!vma)
>> +		goto out;
>>  	hstart = (vma->vm_start + ~HPAGE_PMD_MASK) & HPAGE_PMD_MASK;
>>  	hend = vma->vm_end & HPAGE_PMD_MASK;
>>  	if (address < hstart || address + HPAGE_PMD_SIZE > hend)
>> -- 
>> 1.8.2.1
> 


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Re: [PATCH] mm: Fix potential NULL pointer dereference

[Michal Hocko]

On Wed 24-07-13 11:48:19, Libin wrote:
> find_vma may return NULL, thus check the return
> value to avoid NULL pointer dereference.

Please add a note that the check matters only because
khugepaged_alloc_page drops mmap_sem.

> Signed-off-by: Libin <huawei.libin@huawei.com>

Other than that
Reviewed-by: Michal Hocko <mhocko@suse.cz>

+ I guess this is worth backporting to the stable trees. This goes back
to when khugepaged was introduced AFAICS.

> ---
>  mm/huge_memory.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 243e710..d4423f4 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -2294,6 +2294,8 @@ static void collapse_huge_page(struct mm_struct *mm,
>  		goto out;
>  
>  	vma = find_vma(mm, address);
> +	if (!vma)
> +		goto out;
>  	hstart = (vma->vm_start + ~HPAGE_PMD_MASK) & HPAGE_PMD_MASK;
>  	hend = vma->vm_end & HPAGE_PMD_MASK;
>  	if (address < hstart || address + HPAGE_PMD_SIZE > hend)
> -- 
> 1.8.2.1
> 
> 
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-- 
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SUSE Labs

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Re: [PATCH] mm: Fix potential NULL pointer dereference

[Libin]

On 2013/7/24 12:35, Wanpeng Li wrote:
> On Wed, Jul 24, 2013 at 12:22:08PM +0800, Wanpeng Li wrote:
>> On Wed, Jul 24, 2013 at 11:48:19AM +0800, Libin wrote:
>>> find_vma may return NULL, thus check the return
>>> value to avoid NULL pointer dereference.
>>>
>>
>> When can this happen since down_read(&mm->mmap_sem) is held?
>>
> 
> Between mmap_sem read lock released and write lock held I think.
> 

Yes, In khugepaged_alloc_page mmap_sem read lock was released.
Thanks for your reply.
Libin.

>>> Signed-off-by: Libin <huawei.libin@huawei.com>
>>> ---
>>> mm/huge_memory.c | 2 ++
>>> 1 file changed, 2 insertions(+)
>>>
>>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>>> index 243e710..d4423f4 100644
>>> --- a/mm/huge_memory.c
>>> +++ b/mm/huge_memory.c
>>> @@ -2294,6 +2294,8 @@ static void collapse_huge_page(struct mm_struct *mm,
>>> 		goto out;
>>>
>>> 	vma = find_vma(mm, address);
>>> +	if (!vma)
>>> +		goto out;
>>> 	hstart = (vma->vm_start + ~HPAGE_PMD_MASK) & HPAGE_PMD_MASK;
>>> 	hend = vma->vm_end & HPAGE_PMD_MASK;
>>> 	if (address < hstart || address + HPAGE_PMD_SIZE > hend)
>>> -- 
>>> 1.8.2.1
>>>
>>>
>>> --
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>>
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> 
> 
> .
> 


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Re: [PATCH] mm: Fix potential NULL pointer dereference

[Libin]

On 2013/7/25 22:01, Michal Hocko wrote:
> On Wed 24-07-13 11:48:19, Libin wrote:
>> find_vma may return NULL, thus check the return
>> value to avoid NULL pointer dereference.
> 
> Please add a note that the check matters only because
> khugepaged_alloc_page drops mmap_sem.
> 

Thanks for your suggestion. I will add the information
and post the patch soon.

Regards,
Libin.

>> Signed-off-by: Libin <huawei.libin@huawei.com>
> 
> Other than that
> Reviewed-by: Michal Hocko <mhocko@suse.cz>
> 
> + I guess this is worth backporting to the stable trees. This goes back
> to when khugepaged was introduced AFAICS.
> 
>> ---
>>  mm/huge_memory.c | 2 ++
>>  1 file changed, 2 insertions(+)
>>
>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>> index 243e710..d4423f4 100644
>> --- a/mm/huge_memory.c
>> +++ b/mm/huge_memory.c
>> @@ -2294,6 +2294,8 @@ static void collapse_huge_page(struct mm_struct *mm,
>>  		goto out;
>>  
>>  	vma = find_vma(mm, address);
>> +	if (!vma)
>> +		goto out;
>>  	hstart = (vma->vm_start + ~HPAGE_PMD_MASK) & HPAGE_PMD_MASK;
>>  	hend = vma->vm_end & HPAGE_PMD_MASK;
>>  	if (address < hstart || address + HPAGE_PMD_SIZE > hend)
>> -- 
>> 1.8.2.1
>>
>>
>> --
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> 


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[PATCH] mm: Fix potential NULL pointer dereference

[Libin]

v1->v2: Add description about the bug potential trigger condition.
	Thanks for the review/suggestion of Michal Hocko &
	Wanpeng Li.

In collapse_huge_page, there is a race window between release
the mmap_sem read lock and hold the mmap_sem write lock, so
find_vma() may return NULL, thus check the return value to
avoid NULL pointer dereference.

collapse_huge_page
	khugepaged_alloc_page
		up_read(&mm->mmap_sem)
	down_write(&mm->mmap_sem)
	vma = find_vma(mm, address)

Signed-off-by: Libin <huawei.libin@huawei.com>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Reviewed-by: Wanpeng Li <liwanp@linux.vnet.ibm.com>
Reviewed-by: Michal Hocko <mhocko@suse.cz>
Cc: <stable@vger.kernel.org> # v3.0+
---
 mm/huge_memory.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 243e710..d4423f4 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -2294,6 +2294,8 @@ static void collapse_huge_page(struct mm_struct *mm,
 		goto out;
 
 	vma = find_vma(mm, address);
+	if (!vma)
+		goto out;
 	hstart = (vma->vm_start + ~HPAGE_PMD_MASK) & HPAGE_PMD_MASK;
 	hend = vma->vm_end & HPAGE_PMD_MASK;
 	if (address < hstart || address + HPAGE_PMD_SIZE > hend)
-- 
1.8.2.1


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