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

[Matthew Wilcox <willy@infradead.org>]

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?