Linux Virtualization - Jun 2020 - [PATCH v4 0/3] mm, treewide: Rename kzfree() to kfree_sensitive()

On Tue 16-06-20 17:37:11, Matthew Wilcox wrote:
> 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 at
perches.com/
> > > 
> > > 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.
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.

-- 
Michal Hocko
SUSE Labs

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().  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.

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