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

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Date:   Tue, 23 Jun 2020 10:48:24 +0200
From:   Heiko Carstens <heiko.carstens@...ibm.com>
To:     Qian Cai <cai@....pw>
Cc:     Dmitry Vyukov <dvyukov@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Alexander Potapenko <glider@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        kasan-dev <kasan-dev@...glegroups.com>,
        Linux-MM <linux-mm@...ck.org>,
        linux-s390 <linux-s390@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Vasily Gorbik <gor@...ux.ibm.com>
Subject: Re: [PATCH] mm/page_alloc: silence a KASAN false positive

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@....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.

Vasily will look into this and come up with a proper solution for s390.