linux-kernel - Re: [PATCH] mm: Add kvfree_sensitive() for freeing sensitive data objects

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Message-ID: <3f6658e5-0082-7759-1d08-eda9cb3eba2f@redhat.com>
Date:   Mon, 6 Apr 2020 10:36:07 -0400
From:   Waiman Long <longman@...hat.com>
To:     David Rientjes <rientjes@...gle.com>
Cc:     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>, linux-mm@...ck.org,
        keyrings@...r.kernel.org, linux-kernel@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH] mm: Add kvfree_sensitive() for freeing sensitive data
 objects

On 4/6/20 12:20 AM, David Rientjes wrote:
> On Sun, 5 Apr 2020, Waiman Long wrote:
>
>> For kvmalloc'ed data object that contains sensitive information like
>> cryptographic key, we need to make sure that the buffer is always
>> cleared before freeing it. Using memset() alone for buffer clearing may
>> not provide certainty as the compiler may compile it away. To be sure,
>> the special memzero_explicit() has to be used.
>>
>> This patch introduces a new kvfree_sensitive() for freeing those
>> sensitive data objects allocated by kvmalloc(). The relevnat places
>> where kvfree_sensitive() can be used are modified to use it.
>>
>> Fixes: 4f0882491a14 ("KEYS: Avoid false positive ENOMEM error on key read")
>> Suggested-by: Linus Torvalds <torvalds@...ux-foundation.org>
>> Signed-off-by: Waiman Long <longman@...hat.com>
>> ---
>>  include/linux/mm.h       | 17 +++++++++++++++++
>>  security/keys/internal.h | 11 -----------
>>  security/keys/keyctl.c   | 16 +++++-----------
>>  3 files changed, 22 insertions(+), 22 deletions(-)
>>
>> diff --git a/include/linux/mm.h b/include/linux/mm.h
>> index 7dd5c4ccbf85..c26f279f1956 100644
>> --- a/include/linux/mm.h
>> +++ b/include/linux/mm.h
>> @@ -758,6 +758,23 @@ static inline void *kvcalloc(size_t n, size_t size, gfp_t flags)
>>  
>>  extern void kvfree(const void *addr);
>>  
>> +/**
>> + * kvfree_sensitive - free a data object containing sensitive information
>> + * @addr - address of the data object to be freed
>> + * @len  - length of the data object
>> + *
>> + * Use the special memzero_explicit() function to clear the content of a
>> + * kvmalloc'ed object containing sensitive data to make sure that the
>> + * compiler won't optimize out the data clearing.
>> + */
>> +static inline void kvfree_sensitive(const void *addr, size_t len)
>> +{
>> +	if (addr) {
> Shouldn't this be if (unlikely(ZERO_OR_NULL_PTR(addr))?
>
ZERO_OR_NULL_PTR is defined in slab.h. Using it may cause some header
file dependency problem. To guard against the possibility of 0-length
allocation request, how about just

    if (likely(addr && len)) {

Cheers,
Longman