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Message-ID: <1531504984.11680.21.camel@intel.com>
Subject: Re: [RFC PATCH v2 17/27] x86/cet/shstk: User-mode shadow stack
 support
From:   Yu-cheng Yu <yu-cheng.yu@intel.com>
To:     Andy Lutomirski <luto@amacapital.net>, Jann Horn <jannh@google.com>
Cc:     the arch/x86 maintainers <x86@kernel.org>,
        "H . Peter Anvin" <hpa@zytor.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>,
        kernel list <linux-kernel@vger.kernel.org>,
        linux-doc@vger.kernel.org, Linux-MM <linux-mm@kvack.org>,
        linux-arch <linux-arch@vger.kernel.org>,
        Linux API <linux-api@vger.kernel.org>,
        Arnd Bergmann <arnd@arndb.de>, bsingharora@gmail.com,
        Cyrill Gorcunov <gorcunov@gmail.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Florian Weimer <fweimer@redhat.com>, hjl.tools@gmail.com,
        Jonathan Corbet <corbet@lwn.net>, keescook@chromiun.org,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Nadav Amit <nadav.amit@gmail.com>,
        Oleg Nesterov <oleg@redhat.com>, Pavel Machek <pavel@ucw.cz>,
        Peter Zijlstra <peterz@infradead.org>,
        ravi.v.shankar@intel.com, vedvyas.shanbhogue@intel.com
Date:   Fri, 13 Jul 2018 11:03:04 -0700
In-Reply-To: <A034EC34-13E7-4AEF-BB3C-FEF14143B601@amacapital.net>
References: <20180710222639.8241-1-yu-cheng.yu@intel.com>
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         <CAG48ez1ytOfQyNZMNPFp7XqKcpd7_aRai9G5s7rx0V=8ZG+r2A@mail.gmail.com>
         <6F5FEFFD-0A9A-4181-8D15-5FC323632BA6@amacapital.net>
         <CAG48ez1OwQMmhQfHaauo+vneywsQ_ERKr4uVcQebC=GbdqZWtA@mail.gmail.com>
         <A034EC34-13E7-4AEF-BB3C-FEF14143B601@amacapital.net>
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On Wed, 2018-07-11 at 15:21 -0700, Andy Lutomirski wrote:
> > 
> > On Jul 11, 2018, at 2:51 PM, Jann Horn <jannh@google.com> wrote:
> > 
> > On Wed, Jul 11, 2018 at 2:34 PM Andy Lutomirski <luto@amacapital.net> wrote:
> > > 
> > > > 
> > > > On Jul 11, 2018, at 2:10 PM, Jann Horn <jannh@google.com> wrote:
> > > > 
> > > > > 
> > > > > On Tue, Jul 10, 2018 at 3:31 PM Yu-cheng Yu <yu-cheng.yu@intel.com> wrote:
> > > > > 
> > > > > This patch adds basic shadow stack enabling/disabling routines.
> > > > > A task's shadow stack is allocated from memory with VM_SHSTK
> > > > > flag set and read-only protection.  The shadow stack is
> > > > > allocated to a fixed size.
> > > > > 
> > > > > Signed-off-by: Yu-cheng Yu <yu-cheng.yu@intel.com>
> > > > [...]
> > > > > 
> > > > > diff --git a/arch/x86/kernel/cet.c b/arch/x86/kernel/cet.c
> > > > > new file mode 100644
> > > > > index 000000000000..96bf69db7da7
> > > > > --- /dev/null
> > > > > +++ b/arch/x86/kernel/cet.c
> > > > [...]
> > > > > 
> > > > > +static unsigned long shstk_mmap(unsigned long addr, unsigned long len)
> > > > > +{
> > > > > +       struct mm_struct *mm = current->mm;
> > > > > +       unsigned long populate;
> > > > > +
> > > > > +       down_write(&mm->mmap_sem);
> > > > > +       addr = do_mmap(NULL, addr, len, PROT_READ,
> > > > > +                      MAP_ANONYMOUS | MAP_PRIVATE, VM_SHSTK,
> > > > > +                      0, &populate, NULL);
> > > > > +       up_write(&mm->mmap_sem);
> > > > > +
> > > > > +       if (populate)
> > > > > +               mm_populate(addr, populate);
> > > > > +
> > > > > +       return addr;
> > > > > +}
> > [...]
> > > 
> > > > 
> > > > Should the kernel enforce that two shadow stacks must have a guard
> > > > page between them so that they can not be directly adjacent, so that
> > > > if you have too much recursion, you can't end up corrupting an
> > > > adjacent shadow stack?
> > > I think the answer is a qualified “no”. I would like to instead enforce a general guard page on all mmaps that don’t use MAP_FORCE. We *might* need to exempt any mmap with an address hint for
> > > compatibility.
> > I like this idea a lot.
> > 
> > > 
> > > My commercial software has been manually adding guard pages on every single mmap done by tcmalloc for years, and it has caught a couple bugs and costs essentially nothing.
> > > 
> > > Hmm. Linux should maybe add something like Windows’ “reserved” virtual memory. It’s basically a way to ask for a VA range that explicitly contains nothing and can be subsequently be turned into
> > > something useful with the equivalent of MAP_FORCE.
> > What's the benefit over creating an anonymous PROT_NONE region? That
> > the kernel won't have to scan through the corresponding PTEs when
> > tearing down the mapping?
> Make it more obvious what’s happening and avoid accounting issues?  What I’ve actually used is MAP_NORESERVE | PROT_NONE, but I think this still counts against the VA rlimit. But maybe that’s
> actually the desired behavior.

We can put a NULL at both ends of a SHSTK to guard against corruption.

Yu-cheng