Re: [PATCH] mm/page_alloc: silence a KASAN false positive

Re: [PATCH] mm/page_alloc: silence a KASAN false positive

[Vasily Gorbik]

On Wed, Jun 10, 2020 at 08:26:00AM -0400, Qian Cai wrote:
> On Wed, Jun 10, 2020 at 07:54:50AM +0200, Dmitry Vyukov wrote:
> > On Wed, Jun 10, 2020 at 7:22 AM Qian Cai <cai@lca.pw> wrote:
> > >
> > > kernel_init_free_pages() will use memset() on s390 to clear all pages
> > > from kmalloc_order() which will override KASAN redzones because a
> > > redzone was setup from the end of the allocation size to the end of the
> > > last page. Silence it by not reporting it there. An example of the
> > > report is,
> > 
> > Interesting. The reason why we did not hit it on x86_64 is because
> > clear_page is implemented in asm (arch/x86/lib/clear_page_64.S) and
> > thus is not instrumented. Arm64 probably does the same. However, on
> > s390 clear_page is defined to memset.
> > clear_[high]page are pretty extensively used in the kernel.
> > We can either do this, or make clear_page non instrumented on s390 as
> > well to match the existing implicit assumption. The benefit of the
> > current approach is that we can find some real use-after-free's and
> > maybe out-of-bounds on clear_page. The downside is that we may need
> > more of these annotations. Thoughts?
> 
> Since we had already done the same thing in poison_page(), I suppose we
> could do the same here. Also, clear_page() has been used in many places
> on s390, and it is not clear to me if those are all safe like this.
> 
> There might be more annotations required, so it probably up to s390
> maintainers (CC'ed) if they prefer not instrumenting clear_page() like
> other arches.
> 

Sorry for delay. I assume you tested it without CONFIG_JUMP_LABEL.
I had to fix couple of things before I was able to use init_on_alloc=1
and init_on_free=1 boot options on s390 to reproduce KASAN problem:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?h=v5.8-rc3&id=998f5bbe3dbdab81c1cfb1aef7c3892f5d24f6c7
https://git.kernel.org/pub/scm/linux/kernel/git/s390/linux.git/commit/?h=fixes&id=95e61b1b5d6394b53d147c0fcbe2ae70fbe09446
https://git.kernel.org/pub/scm/linux/kernel/git/s390/linux.git/commit/?h=fixes&id=d6df52e9996dcc2062c3d9c9123288468bb95b52

Back to clear_page - we could certainly make it non-instrumented. But
it didn't cause any problems so far. And as Dmitry pointed out we
could potentially find additional bugs with it. So, I'm leaning
towards original solution proposed. For that you have my

Acked-by: Vasily Gorbik <gor@linux.ibm.com>
Tested-by: Vasily Gorbik <gor@linux.ibm.com>

Thank you for looking into this!

Andrew, would you pick this change up?
Thank you

Vasily