[PATCH] mm: fix anon_vma->degree underflow in anon_vma endless growing prevention

[PATCH] mm: fix anon_vma->degree underflow in anon_vma endless growing prevention

[Leon Yu]

I have constantly stumbled upon "kernel BUG at mm/rmap.c:399!" after upgrading
to 3.19 and had no luck with 4.0-rc1 neither.

So, after looking into new logic introduced by commit 7a3ef208e662, ("mm:
prevent endless growth of anon_vma hierarchy"), I found chances are that
unlink_anon_vmas() is called without incrementing dst->anon_vma->degree in
anon_vma_clone() due to allocation failure. If dst->anon_vma is not NULL in
error path, its degree will be incorrectly decremented in unlink_anon_vmas()
and eventually underflow when exiting as a result of another call to
unlink_anon_vmas(). That's how "kernel BUG at mm/rmap.c:399!" is triggered
for me.

This patch fixes the underflow by dropping dst->anon_vma when allocation
fails. It's safe to do so regardless of original value of dst->anon_vma
because dst->anon_vma doesn't have valid meaning if anon_vma_clone() fails.
Besides, callers don't care dst->anon_vma in such case neither.

Signed-off-by: Leon Yu <chianglungyu@gmail.com>
Fixes: 7a3ef208e662 ("mm: prevent endless growth of anon_vma hierarchy")
Cc: stable@vger.kernel.org # v3.19
---
 mm/rmap.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/mm/rmap.c b/mm/rmap.c
index 5e3e090..bed3cf2 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -287,6 +287,13 @@ int anon_vma_clone(struct vm_area_struct *dst, struct vm_area_struct *src)
 	return 0;
 
  enomem_failure:
+	/*
+	 * dst->anon_vma is dropped here otherwise its degree can be incorrectly
+	 * decremented in unlink_anon_vmas().
+	 * We can safely do this because calllers of anon_vma_clone() wouldn't
+	 * care dst->anon_vma if anon_vma_clone() failed.
+	 */
+	dst->anon_vma = NULL;
 	unlink_anon_vmas(dst);
 	return -ENOMEM;
 }
-- 
2.3.1

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Re: [PATCH] mm: fix anon_vma->degree underflow in anon_vma endless growing prevention

[Konstantin Khlebnikov]

On Tue, Mar 3, 2015 at 3:02 PM, Leon Yu <chianglungyu@gmail.com> wrote:
> I have constantly stumbled upon "kernel BUG at mm/rmap.c:399!" after upgrading
> to 3.19 and had no luck with 4.0-rc1 neither.
>
> So, after looking into new logic introduced by commit 7a3ef208e662, ("mm:
> prevent endless growth of anon_vma hierarchy"), I found chances are that
> unlink_anon_vmas() is called without incrementing dst->anon_vma->degree in
> anon_vma_clone() due to allocation failure. If dst->anon_vma is not NULL in
> error path, its degree will be incorrectly decremented in unlink_anon_vmas()
> and eventually underflow when exiting as a result of another call to
> unlink_anon_vmas(). That's how "kernel BUG at mm/rmap.c:399!" is triggered
> for me.
>
> This patch fixes the underflow by dropping dst->anon_vma when allocation
> fails. It's safe to do so regardless of original value of dst->anon_vma
> because dst->anon_vma doesn't have valid meaning if anon_vma_clone() fails.
> Besides, callers don't care dst->anon_vma in such case neither.
>
> Signed-off-by: Leon Yu <chianglungyu@gmail.com>
> Fixes: 7a3ef208e662 ("mm: prevent endless growth of anon_vma hierarchy")
> Cc: stable@vger.kernel.org # v3.19

Good catch.

Signed-off-by: Konstantin Khlebnikov <koct9i@gmail.com>

That thing already backported into various stable branches so this's
not only for v3.19.

As I see other error paths are fine.

> ---
>  mm/rmap.c | 7 +++++++
>  1 file changed, 7 insertions(+)
>
> diff --git a/mm/rmap.c b/mm/rmap.c
> index 5e3e090..bed3cf2 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -287,6 +287,13 @@ int anon_vma_clone(struct vm_area_struct *dst, struct vm_area_struct *src)
>         return 0;
>
>   enomem_failure:
> +       /*
> +        * dst->anon_vma is dropped here otherwise its degree can be incorrectly
> +        * decremented in unlink_anon_vmas().
> +        * We can safely do this because calllers of anon_vma_clone() wouldn't
> +        * care dst->anon_vma if anon_vma_clone() failed.
> +        */
> +       dst->anon_vma = NULL;
>         unlink_anon_vmas(dst);
>         return -ENOMEM;
>  }
> --
> 2.3.1
>

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Re: [PATCH] mm: fix anon_vma->degree underflow in anon_vma endless growing prevention

[Michal Hocko]

On Tue 03-03-15 20:02:15, Leon Yu wrote:
> I have constantly stumbled upon "kernel BUG at mm/rmap.c:399!" after upgrading
> to 3.19 and had no luck with 4.0-rc1 neither.
> 
> So, after looking into new logic introduced by commit 7a3ef208e662, ("mm:
> prevent endless growth of anon_vma hierarchy"), I found chances are that
> unlink_anon_vmas() is called without incrementing dst->anon_vma->degree in
> anon_vma_clone() due to allocation failure. If dst->anon_vma is not NULL in
> error path, its degree will be incorrectly decremented in unlink_anon_vmas()
> and eventually underflow when exiting as a result of another call to
> unlink_anon_vmas(). That's how "kernel BUG at mm/rmap.c:399!" is triggered
> for me.
> 
> This patch fixes the underflow by dropping dst->anon_vma when allocation
> fails. It's safe to do so regardless of original value of dst->anon_vma
> because dst->anon_vma doesn't have valid meaning if anon_vma_clone() fails.
> Besides, callers don't care dst->anon_vma in such case neither.
> 
> Signed-off-by: Leon Yu <chianglungyu@gmail.com>
> Fixes: 7a3ef208e662 ("mm: prevent endless growth of anon_vma hierarchy")
> Cc: stable@vger.kernel.org # v3.19

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

I think we can safely remove the following code as well, because it is
anon_vma_clone which is responsible to do all the cleanups.

diff --git a/mm/mmap.c b/mm/mmap.c
index 943c6ad18b1d..06a6076c92e5 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -774,10 +774,8 @@ again:			remove_next = 1 + (end > next->vm_end);
 
 			importer->anon_vma = exporter->anon_vma;
 			error = anon_vma_clone(importer, exporter);
-			if (error) {
-				importer->anon_vma = NULL;
+			if (error)
 				return error;
-			}
 		}
 	}
 

> ---
>  mm/rmap.c | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/mm/rmap.c b/mm/rmap.c
> index 5e3e090..bed3cf2 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -287,6 +287,13 @@ int anon_vma_clone(struct vm_area_struct *dst, struct vm_area_struct *src)
>  	return 0;
>  
>   enomem_failure:
> +	/*
> +	 * dst->anon_vma is dropped here otherwise its degree can be incorrectly
> +	 * decremented in unlink_anon_vmas().
> +	 * We can safely do this because calllers of anon_vma_clone() wouldn't
> +	 * care dst->anon_vma if anon_vma_clone() failed.
> +	 */
> +	dst->anon_vma = NULL;
>  	unlink_anon_vmas(dst);
>  	return -ENOMEM;
>  }
> -- 
> 2.3.1
> 

-- 
Michal Hocko
SUSE Labs

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Re: [PATCH] mm: fix anon_vma->degree underflow in anon_vma endless growing prevention

[Leon Yu]

On Tue, Mar 3, 2015 at 9:36 PM, Michal Hocko <mhocko@suse.cz> wrote:
> On Tue 03-03-15 20:02:15, Leon Yu wrote:
>> I have constantly stumbled upon "kernel BUG at mm/rmap.c:399!" after upgrading
>> to 3.19 and had no luck with 4.0-rc1 neither.
>>
>> So, after looking into new logic introduced by commit 7a3ef208e662, ("mm:
>> prevent endless growth of anon_vma hierarchy"), I found chances are that
>> unlink_anon_vmas() is called without incrementing dst->anon_vma->degree in
>> anon_vma_clone() due to allocation failure. If dst->anon_vma is not NULL in
>> error path, its degree will be incorrectly decremented in unlink_anon_vmas()
>> and eventually underflow when exiting as a result of another call to
>> unlink_anon_vmas(). That's how "kernel BUG at mm/rmap.c:399!" is triggered
>> for me.
>>
>> This patch fixes the underflow by dropping dst->anon_vma when allocation
>> fails. It's safe to do so regardless of original value of dst->anon_vma
>> because dst->anon_vma doesn't have valid meaning if anon_vma_clone() fails.
>> Besides, callers don't care dst->anon_vma in such case neither.
>>
>> Signed-off-by: Leon Yu <chianglungyu@gmail.com>
>> Fixes: 7a3ef208e662 ("mm: prevent endless growth of anon_vma hierarchy")
>> Cc: stable@vger.kernel.org # v3.19
>
> Reviewed-by: Michal Hocko <mhocko@suse.cz>
>
> I think we can safely remove the following code as well, because it is
> anon_vma_clone which is responsible to do all the cleanups.

Thanks for the input, I'll send v2 with your cleanup.

- Leon

> diff --git a/mm/mmap.c b/mm/mmap.c
> index 943c6ad18b1d..06a6076c92e5 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -774,10 +774,8 @@ again:                     remove_next = 1 + (end > next->vm_end);
>
>                         importer->anon_vma = exporter->anon_vma;
>                         error = anon_vma_clone(importer, exporter);
> -                       if (error) {
> -                               importer->anon_vma = NULL;
> +                       if (error)
>                                 return error;
> -                       }
>                 }
>         }

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Re: [PATCH] mm: fix anon_vma->degree underflow in anon_vma endless growing prevention

[David Rientjes]

On Tue, 3 Mar 2015, Michal Hocko wrote:

> I think we can safely remove the following code as well, because it is
> anon_vma_clone which is responsible to do all the cleanups.
> 
> diff --git a/mm/mmap.c b/mm/mmap.c
> index 943c6ad18b1d..06a6076c92e5 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -774,10 +774,8 @@ again:			remove_next = 1 + (end > next->vm_end);
>  
>  			importer->anon_vma = exporter->anon_vma;
>  			error = anon_vma_clone(importer, exporter);
> -			if (error) {
> -				importer->anon_vma = NULL;
> +			if (error)
>  				return error;
> -			}
>  		}
>  	}
>  

When Michal proposes this on top of -mm, feel free to add my

Acked-by: David Rientjes <rientjes@google.com>

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Re: [PATCH] mm: fix anon_vma->degree underflow in anon_vma endless growing prevention

[David Rientjes]

On Tue, 3 Mar 2015, Leon Yu wrote:

> I have constantly stumbled upon "kernel BUG at mm/rmap.c:399!" after upgrading
> to 3.19 and had no luck with 4.0-rc1 neither.
> 
> So, after looking into new logic introduced by commit 7a3ef208e662, ("mm:
> prevent endless growth of anon_vma hierarchy"), I found chances are that
> unlink_anon_vmas() is called without incrementing dst->anon_vma->degree in
> anon_vma_clone() due to allocation failure. If dst->anon_vma is not NULL in
> error path, its degree will be incorrectly decremented in unlink_anon_vmas()
> and eventually underflow when exiting as a result of another call to
> unlink_anon_vmas(). That's how "kernel BUG at mm/rmap.c:399!" is triggered
> for me.
> 
> This patch fixes the underflow by dropping dst->anon_vma when allocation
> fails. It's safe to do so regardless of original value of dst->anon_vma
> because dst->anon_vma doesn't have valid meaning if anon_vma_clone() fails.
> Besides, callers don't care dst->anon_vma in such case neither.
> 
> Signed-off-by: Leon Yu <chianglungyu@gmail.com>
> Fixes: 7a3ef208e662 ("mm: prevent endless growth of anon_vma hierarchy")
> Cc: stable@vger.kernel.org # v3.19

Acked-by: David Rientjes <rientjes@google.com>

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