Re: [PATCH v5 7/7] mm: kasan: Initial memory quarantine implementation

From: Andrew Morton
Date: Thu Mar 10 2016 - 15:14:34 EST

On Thu, 10 Mar 2016 14:50:56 +0100 Alexander Potapenko <glider@xxxxxxxxxx> wrote:

> On Wed, Mar 9, 2016 at 9:21 PM, Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
> > On Wed, 9 Mar 2016 12:05:48 +0100 Alexander Potapenko <glider@xxxxxxxxxx> wrote:
> >
> >> Quarantine isolates freed objects in a separate queue. The objects are
> >> returned to the allocator later, which helps to detect use-after-free
> >> errors.
> >
> > I'd like to see some more details on precisely *how* the parking of
> > objects in the qlists helps "detect use-after-free"?
> When the object is freed, its state changes from KASAN_STATE_ALLOC to
> KASAN_STATE_QUARANTINE. The object is poisoned and put into quarantine
> instead of being returned to the allocator, therefore every subsequent
> access to that object triggers a KASAN error, and the error handler is
> able to say where the object has been allocated and deallocated.
> When it's time for the object to leave quarantine, its state becomes
> KASAN_STATE_FREE and it's returned to the allocator. From now on the
> allocator may reuse it for another allocation.
> Before that happens, it's still possible to detect a use-after free on
> that object (it retains the allocation/deallocation stacks).
> When the allocator reuses this object, the shadow is unpoisoned and
> old allocation/deallocation stacks are wiped. Therefore a use of this
> object, even an incorrect one, won't trigger ASan warning.
> Without the quarantine, it's not guaranteed that the objects aren't
> reused immediately, that's why the probability of catching a
> use-after-free is lower than with quarantine in place.

I see, thanks. I'll slurp that into the changelog for posterity.

> >> +}
> >
> > We could avoid th4ese ifdefs in the usual way: an empty version of
> > quarantine_remove_cache() if CONFIG_SLAB=n.
> Yes, agreed.
> I am sorry, I don't fully understand the review process now, when
> you've pulled the patches into mm-tree.
> Shall I send the new patch series version, as before, or is anything
> else needs to be done?
> Do I need to rebase against mm- or linux-next? Thanks in advance.

I like to queue a delta patch so I and others can see what changed and
also to keep track of who fixed what and why. It's a bit harsh on the
reviewers to send them a slightly altered version of a 500 line patch
which they've already read through.

Before sending the patch up to Linus I'll clump everything into a
single patch and a lot of that history is somewhat lost.

Sending a replacement patch is often more convenient for the originator
so that's fine - I'll turn the replacement into a delta locally and
will review then queue that delta. Also a new revision of a patch has
an altered changelog so I'll manually move that into the older original
patch's changelog immediately.

IOW: either a new patch or a delta is fine.

Your patch is in linux-next now so a diff against -next will work OK.

Probably the easiest thing for you to do is to just alter the patch you
have in-place and send out the new one. A "[v2" in the Subject: helps
people keep track of things.