netdev - Re: [PATCH v2 2/2] crypto: Remove unnecessary memzero

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Date:   Tue, 14 Apr 2020 12:24:36 -0400
From:   Waiman Long <longman@...hat.com>
To:     Christophe Leroy <christophe.leroy@....fr>,
        Andrew Morton <akpm@...ux-foundation.org>,
        David Howells <dhowells@...hat.com>,
        Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>,
        James Morris <jmorris@...ei.org>,
        "Serge E. Hallyn" <serge@...lyn.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Joe Perches <joe@...ches.com>,
        Matthew Wilcox <willy@...radead.org>,
        David Rientjes <rientjes@...gle.com>
Cc:     linux-mm@...ck.org, keyrings@...r.kernel.org,
        linux-kernel@...r.kernel.org, x86@...nel.org,
        linux-crypto@...r.kernel.org, linux-s390@...r.kernel.org,
        linux-pm@...r.kernel.org, linux-stm32@...md-mailman.stormreply.com,
        linux-arm-kernel@...ts.infradead.org,
        linux-amlogic@...ts.infradead.org,
        linux-mediatek@...ts.infradead.org, linuxppc-dev@...ts.ozlabs.org,
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        linux-integrity@...r.kernel.org
Subject: Re: [PATCH v2 2/2] crypto: Remove unnecessary memzero_explicit()

On 4/14/20 2:08 AM, Christophe Leroy wrote:
>
>
> Le 14/04/2020 à 00:28, Waiman Long a écrit :
>> Since kfree_sensitive() will do an implicit memzero_explicit(), there
>> is no need to call memzero_explicit() before it. Eliminate those
>> memzero_explicit() and simplify the call sites. For better correctness,
>> the setting of keylen is also moved down after the key pointer check.
>>
>> Signed-off-by: Waiman Long <longman@...hat.com>
>> ---
>>   .../allwinner/sun8i-ce/sun8i-ce-cipher.c      | 19 +++++-------------
>>   .../allwinner/sun8i-ss/sun8i-ss-cipher.c      | 20 +++++--------------
>>   drivers/crypto/amlogic/amlogic-gxl-cipher.c   | 12 +++--------
>>   drivers/crypto/inside-secure/safexcel_hash.c  |  3 +--
>>   4 files changed, 14 insertions(+), 40 deletions(-)
>>
>> diff --git a/drivers/crypto/allwinner/sun8i-ce/sun8i-ce-cipher.c
>> b/drivers/crypto/allwinner/sun8i-ce/sun8i-ce-cipher.c
>> index aa4e8fdc2b32..8358fac98719 100644
>> --- a/drivers/crypto/allwinner/sun8i-ce/sun8i-ce-cipher.c
>> +++ b/drivers/crypto/allwinner/sun8i-ce/sun8i-ce-cipher.c
>> @@ -366,10 +366,7 @@ void sun8i_ce_cipher_exit(struct crypto_tfm *tfm)
>>   {
>>       struct sun8i_cipher_tfm_ctx *op = crypto_tfm_ctx(tfm);
>>   -    if (op->key) {
>> -        memzero_explicit(op->key, op->keylen);
>> -        kfree(op->key);
>> -    }
>> +    kfree_sensitive(op->key);
>>       crypto_free_sync_skcipher(op->fallback_tfm);
>>       pm_runtime_put_sync_suspend(op->ce->dev);
>>   }
>> @@ -391,14 +388,11 @@ int sun8i_ce_aes_setkey(struct crypto_skcipher
>> *tfm, const u8 *key,
>>           dev_dbg(ce->dev, "ERROR: Invalid keylen %u\n", keylen);
>>           return -EINVAL;
>>       }
>> -    if (op->key) {
>> -        memzero_explicit(op->key, op->keylen);
>> -        kfree(op->key);
>> -    }
>> -    op->keylen = keylen;
>> +    kfree_sensitive(op->key);
>>       op->key = kmemdup(key, keylen, GFP_KERNEL | GFP_DMA);
>>       if (!op->key)
>>           return -ENOMEM;
>> +    op->keylen = keylen;
>
> Does it matter at all to ensure op->keylen is not set when of->key is
> NULL ? I'm not sure.
>
> But if it does, then op->keylen should be set to 0 when freeing op->key. 

My thinking is that if memory allocation fails, we just don't touch
anything and return an error code. I will not explicitly set keylen to 0
in this case unless it is specified in the API documentation.

Cheers,
Longman