linux-kernel - Re: [PATCH v2 0/5] Optionally randomize kernel stack offset each syscall

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Date:   Thu, 26 Mar 2020 00:20:19 +0100
From:   Jann Horn <jannh@...gle.com>
To:     Kees Cook <keescook@...omium.org>
Cc:     "Reshetova, Elena" <elena.reshetova@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Potapenko <glider@...gle.com>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        Linux-MM <linux-mm@...ck.org>,
        kernel list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 0/5] Optionally randomize kernel stack offset each syscall

On Wed, Mar 25, 2020 at 9:27 PM Kees Cook <keescook@...omium.org> wrote:
> On Wed, Mar 25, 2020 at 12:15:12PM +0000, Reshetova, Elena wrote:
> > > > Also, are you sure that it isn't possible to make the syscall that
> > > > leaked its stack pointer never return to userspace (via ptrace or
> > > > SIGSTOP or something like that), and therefore never realign its
> > > > stack, while keeping some controlled data present on the syscall's
> > > > stack?
> >
> > How would you reliably detect that a stack pointer has been leaked
> > to userspace while it has been in a syscall? Does not seem to be a trivial
> > task to me.
>
> Well, my expectation is that folks using this defense are also using
> panic_on_warn sysctl, etc, so attackers don't get a chance to actually
> _use_ register values spilled to dmesg.

Uh... I thought that thing was exclusively for stuff like syzkaller,
because nuking the entire system because of a WARN is far too
excessive? WARNs should be safe to add almost anywhere in the kernel,
so that developers can put their assumptions about system behavior
into code without having to worry about bringing down the entire
system if that assumption turns out to have been false in some
harmless edgecase.

Also, there are other places that dump register state. In particular
the soft lockup detection, which you can IIRC easily trip even
accidentally if you play around with stuff like FUSE filesystems, or
if a disk becomes unresponsive. Sure, *theoretically* you can also set
the "panic on soft lockup" flag, but that seems like a really terrible
idea to me.

As far as I can tell, the only clean way to fix this is to tell
distros that give non-root users access to dmesg (Ubuntu in
particular) that they have to stop doing that. E.g. Debian seems to
get by just fine with root-restricted dmesg.