Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Subject: Re: [PATCH v3] mm: Add kvfree_sensitive() for freeing sensitive data
 objects
To:     Matthew Wilcox <willy@infradead.org>
Cc:     Waiman Long <longman@redhat.com>,
        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>, linux-mm@kvack.org,
        keyrings@vger.kernel.org, linux-kernel@vger.kernel.org,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Joe Perches <joe@perches.com>,
        David Rientjes <rientjes@google.com>
References: <20200407200318.11711-1-longman@redhat.com>
 <1158ff38-c65d-379f-8ae7-6f507d9fc8dd@gmail.com>
 <20200514120018.GA16070@bombadil.infradead.org>
From:   Balbir Singh <bsingharora@gmail.com>
Message-ID: <f779dea1-3b50-e354-3914-7394b4473f5b@gmail.com>
Date:   Sun, 17 May 2020 10:27:39 +1000
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101
 Thunderbird/68.7.0
MIME-Version: 1.0
In-Reply-To: <20200514120018.GA16070@bombadil.infradead.org>
Content-Type: text/plain; charset=utf-8
Content-Language: en-US
Content-Transfer-Encoding: 7bit
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk


On 14/5/20 10:00 pm, Matthew Wilcox wrote:
> On Thu, May 14, 2020 at 09:00:40PM +1000, Balbir Singh wrote:
>> I wonder if the right thing to do is also to disable pre-emption, just so that the thread does not linger on with sensitive data.
>>
>> void kvfree_sensitive(const void *addr, size_t len)
>> {
>> 	preempt_disable();
>> 	if (likely(!ZERO_OR_NULL_PTR(addr))) {
>> 		memzero_explicit((void *)addr, len);
>> 		kvfree(addr);
>> 	}
>> 	preempt_enable();
>> }
>> EXPORT_SYMBOL(kvfree_sensitive);
> 
> If it's _that_ sensitive then the caller should have disabled preemption.
> Because preemption could otherwise have occurred immediately before
> kvfree_sensitive() was called.
> 

May be, but the callers of the API have to be explictly aware of the contract.
I don't disagree with you on what you've said, but I was referring to the
intent of freeing sensitive data vs the turn around time for doing so.

Balbir Singh.