linux-kernel - Re: [PATCH v2 00/39] Shadowstacks for userspace

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Date:   Mon, 3 Oct 2022 10:04:34 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Rick Edgecombe <rick.p.edgecombe@...el.com>
Cc:     x86@...nel.org, "H . Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
        linux-doc@...r.kernel.org, linux-mm@...ck.org,
        linux-arch@...r.kernel.org, linux-api@...r.kernel.org,
        Arnd Bergmann <arnd@...db.de>,
        Andy Lutomirski <luto@...nel.org>,
        Balbir Singh <bsingharora@...il.com>,
        Borislav Petkov <bp@...en8.de>,
        Cyrill Gorcunov <gorcunov@...il.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Eugene Syromiatnikov <esyr@...hat.com>,
        Florian Weimer <fweimer@...hat.com>,
        "H . J . Lu" <hjl.tools@...il.com>, Jann Horn <jannh@...gle.com>,
        Jonathan Corbet <corbet@....net>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Nadav Amit <nadav.amit@...il.com>,
        Oleg Nesterov <oleg@...hat.com>, Pavel Machek <pavel@....cz>,
        Peter Zijlstra <peterz@...radead.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        "Ravi V . Shankar" <ravi.v.shankar@...el.com>,
        Weijiang Yang <weijiang.yang@...el.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        joao.moreira@...el.com, John Allen <john.allen@....com>,
        kcc@...gle.com, eranian@...gle.com, rppt@...nel.org,
        jamorris@...ux.microsoft.com, dethoma@...rosoft.com
Subject: Re: [PATCH v2 00/39] Shadowstacks for userspace

On Thu, Sep 29, 2022 at 03:28:57PM -0700, Rick Edgecombe wrote:
> This is an overdue followup to the “Shadow stacks for userspace” CET series. 
> Thanks for all the comments on the first version [0]. They drove a decent 
> amount of changes for v2. Since it has been awhile, I’ll try to summarize the 
> areas that got major changes since last time. Smaller changes are listed in 
> each patch.

Thanks for the write-up!

> [...]
>         GUP
>         ---
>         Shadow stack memory is generally treated as writable by the kernel, but
>         it behaves differently then other writable memory with respect to GUP.
>         FOLL_WRITE will not GUP shadow stack memory unless FOLL_FORCE is also
>         set. Shadow stack memory is writable from the perspective of being
>         changeable by userspace, but it is also protected memory from
>         userspace’s perspective. So preventing it from being writable via
>         FOLL_WRITE help’s make it harder for userspace to arbitrarily write to
>         it. However, like read-only memory, FOLL_FORCE can still write through
>         it. This means shadow stacks can be written to via things like
>         “/proc/self/mem”. Apps that want extra security will have to prevent
>         access to kernel features that can write with FOLL_FORCE.

This seems like a problem to me -- the point of SS is that there cannot be
a way to write to them without specific instruction sequences. The fact
that /proc/self/mem bypasses memory protections was an old design mistake
that keeps leading to surprising behaviors. It would be much nicer to
draw the line somewhere and just say that FOLL_FORCE doesn't work on
VM_SHADOW_STACK. Why must FOLL_FORCE be allowed to write to SS?

> [...]
> Shadow stack signal format
> --------------------------
> So to handle alt shadow stacks we need to push some data onto a stack. To 
> prevent SROP we need to push something to the shadow stack that the kernel can 
> [...]
> shadow stack return address or a shadow stack tokens. To make sure it can’t be 
> used, data is pushed with the high bit (bit 63) set. This bit is a linear 
> address bit in both the token format and a normal return address, so it should 
> not conflict with anything. It puts any return address in the kernel half of 
> the address space, so would never be created naturally by a userspace program. 
> It will not be a valid restore token either, as the kernel address will never 
> be pointing to the previous frame in the shadow stack.
> 
> When a signal hits, the format pushed to the stack that is handling the signal 
> is four 8 byte values (since we are 64 bit only):
> |1...old SSP|1...alt stack size|1...alt stack base|0|

Do these end up being non-canonical addresses? (To avoid confusion with
"real" kernel addresses?)

-Kees

-- 
Kees Cook