Re: [PATCH v4 0/3] mm, treewide: Rename kzfree() to kfree_sensitive()

[Matthew Wilcox <willy-wEGCiKHe2LqWVfeAwA7xHQ@public.gmane.org>]

On Wed, Jun 17, 2020 at 01:01:30AM +0200, David Sterba wrote:
> On Tue, Jun 16, 2020 at 11:53:50AM -0700, Joe Perches wrote:
> > On Mon, 2020-06-15 at 21:57 -0400, Waiman Long wrote:
> > >  v4:
> > >   - Break out the memzero_explicit() change as suggested by Dan Carpenter
> > >     so that it can be backported to stable.
> > >   - Drop the "crypto: Remove unnecessary memzero_explicit()" patch for
> > >     now as there can be a bit more discussion on what is best. It will be
> > >     introduced as a separate patch later on after this one is merged.
> > 
> > To this larger audience and last week without reply:
> > https://lore.kernel.org/lkml/573b3fbd5927c643920e1364230c296b23e7584d.camel-6d6DIl74uiNBDgjK7y7TUQ@public.gmane.org/
> > 
> > Are there _any_ fastpath uses of kfree or vfree?
> 
> I'd consider kfree performance critical for cases where it is called
> under locks. If possible the kfree is moved outside of the critical
> section, but we have rbtrees or lists that get deleted under locks and
> restructuring the code to do eg. splice and free it outside of the lock
> is not always possible.

Not just performance critical, but correctness critical.  Since kvfree()
may allocate from the vmalloc allocator, I really think that kvfree()
should assert that it's !in_atomic().  Otherwise we can get into trouble
if we end up calling vfree() and have to take the mutex.