linux-kernel - Re: [PATCH v3] mm: Add kvfree_sensitive() for freeing sensitive data objects

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Message-ID: <365c8e0c-5d92-f032-b9ff-f64a8d314dfe@redhat.com>
Date:   Sun, 17 May 2020 22:39:38 -0400
From:   Waiman Long <longman@...hat.com>
To:     Balbir Singh <bsingharora@...il.com>,
        Matthew Wilcox <willy@...radead.org>
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>,
        Joe Perches <joe@...ches.com>,
        David Rientjes <rientjes@...gle.com>
Subject: Re: [PATCH v3] mm: Add kvfree_sensitive() for freeing sensitive data
 objects

On 5/16/20 8:27 PM, Balbir Singh wrote:
>
> 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.

We can't disable preemption like that. The vfree() call may potentially 
sleep. It could be a mess to keep track of the preemption state to make 
that works.

The purpose of this API is to make sure that a newly allocated memory 
block won't contain secret left behind from another task. There is no 
guarantee on how long the freeing process will take.

Cheers,
Longman