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From: Waiman Long <longman@redhat.com>
To: Eric Biggers <ebiggers@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	David Howells <dhowells@redhat.com>,
	Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>,
	James Morris <jmorris@namei.org>,
	"Serge E. Hallyn" <serge@hallyn.com>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Joe Perches <joe@perches.com>,
	Matthew Wilcox <willy@infradead.org>,
	David Rientjes <rientjes@google.com>,
	Michal Hocko <mhocko@suse.com>,
	Johannes Weiner <hannes@cmpxchg.org>,
	Dan Carpenter <dan.carpenter@oracle.com>,
	David Sterba <dsterba@suse.cz>,
	"Jason A . Donenfeld" <Jason@zx2c4.com>,
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	linux-integrity@vger.kernel.org, stable@vger.kernel.org
Subject: Re: [PATCH v4 1/3] mm/slab: Use memzero_explicit() in kzfree()
Date: Tue, 16 Jun 2020 09:05:00 -0400
Message-ID: <56c2304c-73cc-8f48-d8d0-5dd6c39f33f3@redhat.com> (raw)
In-Reply-To: <20200616033035.GB902@sol.localdomain>

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


  reply index

Thread overview: 28+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-06-16  1:57 [PATCH v4 0/3] mm, treewide: Rename kzfree() to kfree_sensitive() Waiman Long
2020-06-16  1:57 ` [PATCH v4 1/3] mm/slab: Use memzero_explicit() in kzfree() Waiman Long
2020-06-16  3:30   ` Eric Biggers
2020-06-16 13:05     ` Waiman Long [this message]
2020-06-16 15:46     ` David Howells
2020-06-16  6:42   ` Michal Hocko
2020-06-16  9:08     ` Dan Carpenter
2020-06-16  1:57 ` [PATCH v4 2/3] mm, treewide: Rename kzfree() to kfree_sensitive() Waiman Long
2020-06-16 14:26   ` Dan Carpenter
2020-06-16 15:05     ` Waiman Long
2020-06-16  1:57 ` [PATCH v4 3/3] btrfs: Use kfree() in btrfs_ioctl_get_subvol_info() Waiman Long
2020-06-16 14:48   ` David Sterba
2020-06-16 15:05     ` Waiman Long
2020-06-16 18:53 ` [PATCH v4 0/3] mm, treewide: Rename kzfree() to kfree_sensitive() Joe Perches
2020-06-16 19:43   ` Waiman Long
2020-06-16 19:46   ` Jason A. Donenfeld
2020-06-16 20:01   ` Waiman Long
2020-06-16 21:14   ` Matthew Wilcox
2020-06-16 23:01   ` David Sterba
2020-06-17  0:37     ` Matthew Wilcox
2020-06-17  7:12       ` Michal Hocko
2020-06-17 11:08         ` Matthew Wilcox
2020-06-17 11:31           ` Michal Hocko
2020-06-17 12:23             ` Matthew Wilcox
2020-06-17 12:55               ` Michal Hocko
2020-06-17  8:03       ` Jo -l
2020-06-17 21:31   ` Denis Efremov
2020-06-17 23:12     ` Joe Perches

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