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From:   Dmitry Vyukov <dvyukov@google.com>
Date:   Tue, 13 Apr 2021 20:27:57 +0200
Message-ID: <CACT4Y+b6g7DNQTRo0VSFgPzAZF2vMJOcnijuLWeLxUtOWL1nrA@mail.gmail.com>
Subject: Re: [syzbot] WARNING in unsafe_follow_pfn
To:     Jason Gunthorpe <jgg@ziepe.ca>
Cc:     Dan Carpenter <dan.carpenter@oracle.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        syzbot <syzbot+015dd7cdbbbc2c180c65@syzkaller.appspotmail.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Borislav Petkov <bp@alien8.de>,
        Daniel Vetter <daniel.vetter@ffwll.ch>,
        daniel.vetter@intel.com, "H. Peter Anvin" <hpa@zytor.com>,
        Jim Mattson <jmattson@google.com>,
        James Morris <jmorris@namei.org>,
        Joerg Roedel <joro@8bytes.org>, KVM list <kvm@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Linux Media Mailing List <linux-media@vger.kernel.org>,
        Linux-MM <linux-mm@kvack.org>,
        linux-security-module <linux-security-module@vger.kernel.org>,
        m.szyprowski@samsung.com,
        Mauro Carvalho Chehab <mchehab@kernel.org>,
        Ingo Molnar <mingo@redhat.com>,
        Sean Christopherson <seanjc@google.com>,
        "Serge E. Hallyn" <serge@hallyn.com>,
        syzkaller-bugs <syzkaller-bugs@googlegroups.com>,
        Tomasz Figa <tfiga@chromium.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        "the arch/x86 maintainers" <x86@kernel.org>
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On Tue, Apr 13, 2021 at 8:11 PM Jason Gunthorpe <jgg@ziepe.ca> wrote:
>
> On Tue, Apr 13, 2021 at 07:20:12PM +0200, Dmitry Vyukov wrote:
> > > > Plus users are going to be seeing this as well.  According to the commit
> > > > message for 69bacee7f9ad ("mm: Add unsafe_follow_pfn") "Unfortunately
> > > > there's some users where this is not fixable (like v4l userptr of iomem
> > > > mappings)".  It sort of seems crazy to dump this giant splat and then
> > > > tell users to ignore it forever because it can't be fixed...  0_0
> > >
> > > I think the discussion conclusion was that this interface should not
> > > be used by userspace anymore, it is obsolete by some new interface?
> > >
> > > It should be protected by some kconfig and the kconfig should be
> > > turned off for syzkaller runs.
> >
> > If this is not a kernel bug, then it must not use WARN_ON[_ONCE]. It
> > makes the kernel untestable for both automated systems and humans:
>
> It is a kernel security bug triggerable by userspace.
>
> > And if it's a kernel bug reachable from user-space, then I think this
> > code should be removed entirely, not just on all testing systems. Or
> > otherwise if we are not removing it for some reason, then it needs to
> > be fixed.
>
> Legacy embedded systems apparently require it.
>
> It should be blocked by a kconfig. Distributions and syzkaller runs
> should not enable that kconfig. What else can we do for insane uapi?

I see. Adding a config gives at least some path forward, so if there
are no better options, that's do that. If we default it to 'n' and add
a bold warning in the description, it may work.