linux-kernel - Re: [PATCH v8 15/27] mm: Handle shadow stack page fault

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Message-ID: <eabd0c16bd2028ad9fef8d10ddf570b3a10d5680.camel@intel.com>
Date:   Wed, 14 Aug 2019 09:27:21 -0700
From:   Yu-cheng Yu <yu-cheng.yu@...el.com>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     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>,
        Florian Weimer <fweimer@...hat.com>,
        "H.J. Lu" <hjl.tools@...il.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>,
        Dave Martin <Dave.Martin@....com>
Subject: Re: [PATCH v8 15/27] mm: Handle shadow stack page fault

On Tue, 2019-08-13 at 15:55 -0700, Andy Lutomirski wrote:
> On Tue, Aug 13, 2019 at 2:02 PM Yu-cheng Yu <yu-cheng.yu@...el.com> wrote:
> > 
> > When a task does fork(), its shadow stack (SHSTK) must be duplicated
> > for the child.  This patch implements a flow similar to copy-on-write
> > of an anonymous page, but for SHSTK.
> > 
> > A SHSTK PTE must be RO and dirty.  This dirty bit requirement is used
> > to effect the copying.  In copy_one_pte(), clear the dirty bit from a
> > SHSTK PTE to cause a page fault upon the next SHSTK access.  At that
> > time, fix the PTE and copy/re-use the page.
> 
> Is using VM_SHSTK and special-casing all of this really better than
> using a special mapping or other pseudo-file-backed VMA and putting
> all the magic in the vm_operations?

A special mapping is cleaner.  However, we also need to exclude normal [RO +
dirty] pages from shadow stack.

Yu-cheng