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

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Message-ID: <20200517004437.GN16070@bombadil.infradead.org>
Date:   Sat, 16 May 2020 17:44:37 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     Balbir Singh <bsingharora@...il.com>
Cc:     Waiman Long <longman@...hat.com>,
        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 Sun, May 17, 2020 at 10:27:39AM +1000, 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.

It's the caller's information.  They should be aware of their own
requirements.  If they do something like:

p = kmalloc();
preempt_disable();
construct(p);
use(p);
preempt_enable();
kvfree_sensitive(p);

there's really nothing we can do to help them inside kvfree_sensitive().
Actually, can you come up with a scenario where disabling preemption
inside kvfree_sensitive() will help with anything?