linux-kernel - Re: [PATCH v11 25/25] x86/cet/shstk: Add arch

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Date:   Wed, 2 Sep 2020 14:58:35 +0100
From:   Dave Martin <Dave.Martin@....com>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     Andy Lutomirski <luto@...nel.org>,
        "Yu, Yu-cheng" <yu-cheng.yu@...el.com>,
        "H.J. Lu" <hjl.tools@...il.com>,
        Florian Weimer <fweimer@...hat.com>, X86 ML <x86@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>,
        Arnd Bergmann <arnd@...db.de>,
        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>,
        Jann Horn <jannh@...gle.com>, Jonathan Corbet <corbet@....net>,
        Kees Cook <keescook@...omium.org>,
        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>,
        Vedvyas Shanbhogue <vedvyas.shanbhogue@...el.com>,
        Weijiang Yang <weijiang.yang@...el.com>
Subject: Re: [PATCH v11 25/25] x86/cet/shstk: Add arch_prctl functions for
 shadow stack

On Tue, Sep 01, 2020 at 11:11:37AM -0700, Dave Hansen wrote:
> On 9/1/20 10:45 AM, Andy Lutomirski wrote:
> >>> For arm64 (and sparc etc.) we continue to use the regular mmap/mprotect
> >>> family of calls.  One or two additional arch-specific mmap flags are
> >>> sufficient for now.
> >>>
> >>> Is x86 definitely not going to fit within those calls?
> >> That can work for x86.  Andy, what if we create PROT_SHSTK, which can
> >> been seen only from the user.  Once in kernel, it is translated to
> >> VM_SHSTK.  One question for mremap/mprotect is, do we allow a normal
> >> data area to become shadow stack?
> > I'm unconvinced that we want to use a somewhat precious PROT_ or VM_
> > bit for this.  Using a flag bit makes sense if we expect anyone to
> > ever map an fd or similar as a shadow stack, but that seems a bit odd
> > in the first place.  To me, it seems more logical for a shadow stack
> > to be a special sort of mapping with a special vm_ops, not a normal
> > mapping with a special flag set.  Although I realize that we want
> > shadow stacks to work like anonymous memory with respect to fork().
> > Dave?
> 
> I actually don't like the idea of *creating* mappings much.
> 
> I think the pkey model has worked out pretty well where we separate
> creating the mapping from doing something *to* it, like changing
> protections.  For instance, it would be nice if we could preserve things
> like using hugetlbfs or heck even doing KSM for shadow stacks.
> 
> If we're *creating* mappings, we've pretty much ruled out things like
> hugetlbfs.
> 
> Something like mprotect_shstk() would allow an implementation today that
> only works on anonymous memory *and* sets up a special vm_ops.  But, the
> same exact ABI could do wonky stuff in the future if we decided we
> wanted to do shadow stacks on DAX or hugetlbfs or whatever.
> 
> I don't really like the idea of PROT_SHSTK those are plumbed into a
> bunch of interfaces.  But, I also can't deny that it seems to be working
> fine for the arm64 folks.

Note, there are some rough edges, such as what happens when someone
calls mprotect() on memory marked with PROT_BTI.  Unless the caller
knows whether PROT_BTI should be set for that page, the flag may get
unintentionally cleared.  Since the flag only applies to text pages
though, it's not _that_ much of a concern.  Software that deals with
writable text pages is also usually involved in generating the code and
so will know about PROT_BTI.  That's was the theory anyway.

In the longer term, it might be preferable to have a mprotect2() that
can leave some flags unmodified, and that doesn't silently ignore
unknown flags (at least one of mmap or mprotect does; I don't recall
which).  We attempt didn't go this far, for now.

For arm64 it seemed fairly natural for the BTI flag to be a PROT_ flag,
but I don't know enough detail about x86 shstk to know whether it's a
natural fit there.

Cheers
---Dave