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

Michal Hocko
2020-06-17T11:32:15Z

On Wed 17-06-20 04:08:20, Matthew Wilcox wrote:
> On Wed, Jun 17, 2020 at 09:12:12AM +0200, Michal Hocko wrote:
> > On Tue 16-06-20 17:37:11, Matthew Wilcox wrote:
> > > 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.
> >
> > FWIW __vfree already checks for atomic context and put the work into a
> > deferred context. So this should be safe. It should be used as a last
> > resort, though.
>
> Actually, it only checks for in_interrupt().

You are right. I have misremembered. You have made me look (thanks) ...

> If you call vfree() under
> a spinlock, you're in trouble. in_atomic() only knows if we hold a
> spinlock for CONFIG_PREEMPT, so it's not safe to check for in_atomic()
> in __vfree(). So we need the warning in order that preempt people can
> tell those without that there is a bug here.

... Unless I am missing something in_interrupt depends on preempt_count() as
well so neither of the two is reliable without PREEMPT_COUNT configured.

--
Michal Hocko
SUSE Labs