diff mbox series

[v4,1/3] mm/slab: Use memzero_explicit() in kzfree()

Message ID 20200616015718.7812-2-longman@redhat.com
State New
Headers show
Series mm, treewide: Rename kzfree() to kfree_sensitive() | expand

Commit Message

Waiman Long June 16, 2020, 1:57 a.m. UTC
The kzfree() function is normally used to clear some sensitive
information, like encryption keys, in the buffer before freeing it back
to the pool. Memset() is currently used for the buffer clearing. However,
it is entirely possible that the compiler may choose to optimize away the
memory clearing especially if LTO is being used. To make sure that this
optimization will not happen, memzero_explicit(), which is introduced
in v3.18, is now used in kzfree() to do the clearing.

Fixes: 3ef0e5ba4673 ("slab: introduce kzfree()")
Cc: stable@vger.kernel.org
Signed-off-by: Waiman Long <longman@redhat.com>
---
 mm/slab_common.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

Comments

Eric Biggers June 16, 2020, 3:30 a.m. UTC | #1
On Mon, Jun 15, 2020 at 09:57:16PM -0400, Waiman Long wrote:
> The kzfree() function is normally used to clear some sensitive
> information, like encryption keys, in the buffer before freeing it back
> to the pool. Memset() is currently used for the buffer clearing. However,
> it is entirely possible that the compiler may choose to optimize away the
> memory clearing especially if LTO is being used. To make sure that this
> optimization will not happen, memzero_explicit(), which is introduced
> in v3.18, is now used in kzfree() to do the clearing.
> 
> Fixes: 3ef0e5ba4673 ("slab: introduce kzfree()")
> Cc: stable@vger.kernel.org
> Signed-off-by: Waiman Long <longman@redhat.com>
> ---
>  mm/slab_common.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/slab_common.c b/mm/slab_common.c
> index 9e72ba224175..37d48a56431d 100644
> --- a/mm/slab_common.c
> +++ b/mm/slab_common.c
> @@ -1726,7 +1726,7 @@ void kzfree(const void *p)
>  	if (unlikely(ZERO_OR_NULL_PTR(mem)))
>  		return;
>  	ks = ksize(mem);
> -	memset(mem, 0, ks);
> +	memzero_explicit(mem, ks);
>  	kfree(mem);
>  }
>  EXPORT_SYMBOL(kzfree);

This is a good change, but the commit message isn't really accurate.  AFAIK, no
one has found any case where this memset() gets optimized out.  And even with
LTO, it would be virtually impossible due to all the synchronization and global
data structures that kfree() uses.  (Remember that this isn't the C standard
function "free()", so the compiler can't assign it any special meaning.)
Not to mention that LTO support isn't actually upstream yet.

I still agree with the change, but it might be helpful if the commit message
were honest that this is really a hardening measure and about properly conveying
the intent.  As-is this sounds like a critical fix, which might confuse people.

- Eric
Michal Hocko June 16, 2020, 6:42 a.m. UTC | #2
On Mon 15-06-20 21:57:16, Waiman Long wrote:
> The kzfree() function is normally used to clear some sensitive
> information, like encryption keys, in the buffer before freeing it back
> to the pool. Memset() is currently used for the buffer clearing. However,
> it is entirely possible that the compiler may choose to optimize away the
> memory clearing especially if LTO is being used. To make sure that this
> optimization will not happen, memzero_explicit(), which is introduced
> in v3.18, is now used in kzfree() to do the clearing.
> 
> Fixes: 3ef0e5ba4673 ("slab: introduce kzfree()")
> Cc: stable@vger.kernel.org
> Signed-off-by: Waiman Long <longman@redhat.com>

Acked-by: Michal Hocko <mhocko@suse.com>

Although I am not really sure this is a stable material. Is there any
known instance where the memset was optimized out from kzfree?

> ---
>  mm/slab_common.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/slab_common.c b/mm/slab_common.c
> index 9e72ba224175..37d48a56431d 100644
> --- a/mm/slab_common.c
> +++ b/mm/slab_common.c
> @@ -1726,7 +1726,7 @@ void kzfree(const void *p)
>  	if (unlikely(ZERO_OR_NULL_PTR(mem)))
>  		return;
>  	ks = ksize(mem);
> -	memset(mem, 0, ks);
> +	memzero_explicit(mem, ks);
>  	kfree(mem);
>  }
>  EXPORT_SYMBOL(kzfree);
> -- 
> 2.18.1
>
Dan Carpenter June 16, 2020, 9:08 a.m. UTC | #3
On Tue, Jun 16, 2020 at 08:42:08AM +0200, Michal Hocko wrote:
> On Mon 15-06-20 21:57:16, Waiman Long wrote:
> > The kzfree() function is normally used to clear some sensitive
> > information, like encryption keys, in the buffer before freeing it back
> > to the pool. Memset() is currently used for the buffer clearing. However,
> > it is entirely possible that the compiler may choose to optimize away the
> > memory clearing especially if LTO is being used. To make sure that this
> > optimization will not happen, memzero_explicit(), which is introduced
> > in v3.18, is now used in kzfree() to do the clearing.
> > 
> > Fixes: 3ef0e5ba4673 ("slab: introduce kzfree()")
> > Cc: stable@vger.kernel.org
> > Signed-off-by: Waiman Long <longman@redhat.com>
> 
> Acked-by: Michal Hocko <mhocko@suse.com>
> 
> Although I am not really sure this is a stable material. Is there any
> known instance where the memset was optimized out from kzfree?

I told him to add the stable.  Otherwise it will just get reported to
me again.  It's a just safer to backport it before we forget.

regards,
dan carpenter
Waiman Long June 16, 2020, 1:05 p.m. UTC | #4
On 6/15/20 11:30 PM, Eric Biggers wrote:
> On Mon, Jun 15, 2020 at 09:57:16PM -0400, Waiman Long wrote:
>> The kzfree() function is normally used to clear some sensitive
>> information, like encryption keys, in the buffer before freeing it back
>> to the pool. Memset() is currently used for the buffer clearing. However,
>> it is entirely possible that the compiler may choose to optimize away the
>> memory clearing especially if LTO is being used. To make sure that this
>> optimization will not happen, memzero_explicit(), which is introduced
>> in v3.18, is now used in kzfree() to do the clearing.
>>
>> Fixes: 3ef0e5ba4673 ("slab: introduce kzfree()")
>> Cc: stable@vger.kernel.org
>> Signed-off-by: Waiman Long <longman@redhat.com>
>> ---
>>   mm/slab_common.c | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/mm/slab_common.c b/mm/slab_common.c
>> index 9e72ba224175..37d48a56431d 100644
>> --- a/mm/slab_common.c
>> +++ b/mm/slab_common.c
>> @@ -1726,7 +1726,7 @@ void kzfree(const void *p)
>>   	if (unlikely(ZERO_OR_NULL_PTR(mem)))
>>   		return;
>>   	ks = ksize(mem);
>> -	memset(mem, 0, ks);
>> +	memzero_explicit(mem, ks);
>>   	kfree(mem);
>>   }
>>   EXPORT_SYMBOL(kzfree);
> This is a good change, but the commit message isn't really accurate.  AFAIK, no
> one has found any case where this memset() gets optimized out.  And even with
> LTO, it would be virtually impossible due to all the synchronization and global
> data structures that kfree() uses.  (Remember that this isn't the C standard
> function "free()", so the compiler can't assign it any special meaning.)
> Not to mention that LTO support isn't actually upstream yet.
>
> I still agree with the change, but it might be helpful if the commit message
> were honest that this is really a hardening measure and about properly conveying
> the intent.  As-is this sounds like a critical fix, which might confuse people.

Yes, I agree that the commit log may look a bit scary. How about the 
following:

The kzfree() function is normally used to clear some sensitive
information, like encryption keys, in the buffer before freeing it back
to the pool. Memset() is currently used for buffer clearing. However
unlikely, there is still a non-zero probability that the compiler may
choose to optimize away the memory clearing especially if LTO is being
used in the future. To make sure that this optimization will never
happen, memzero_explicit(), which is introduced in v3.18, is now used
in kzfree() to future-proof it.

Cheers,
Longman
David Howells June 16, 2020, 3:46 p.m. UTC | #5
Waiman Long <longman@redhat.com> wrote:

> The kzfree() function is normally used to clear some sensitive
> information, like encryption keys, in the buffer before freeing it back
> to the pool. Memset()

"memset()" is all lowercase.

> is currently used for buffer clearing. However unlikely, there is still a
> non-zero probability

I'd say "a possibility".

> that

and I'd move "in [the] future" here.

> the compiler may choose to optimize away the
> memory clearing especially if LTO is being used in the future. To make sure
> that this optimization will never happen

"in these cases"

> , memzero_explicit(), which is introduced in v3.18, is now used in

"instead of"?

> kzfree() to future-proof it.

Davod
diff mbox series

Patch

diff --git a/mm/slab_common.c b/mm/slab_common.c
index 9e72ba224175..37d48a56431d 100644
--- a/mm/slab_common.c
+++ b/mm/slab_common.c
@@ -1726,7 +1726,7 @@  void kzfree(const void *p)
 	if (unlikely(ZERO_OR_NULL_PTR(mem)))
 		return;
 	ks = ksize(mem);
-	memset(mem, 0, ks);
+	memzero_explicit(mem, ks);
 	kfree(mem);
 }
 EXPORT_SYMBOL(kzfree);