From mboxrd@z Thu Jan  1 00:00:00 1970
From: Yu-cheng Yu <yu-cheng.yu@intel.com>
Subject: Re: [PATCH v8 15/27] mm: Handle shadow stack page fault
Date: Wed, 14 Aug 2019 09:27:21 -0700
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To: Andy Lutomirski <luto@kernel.org>
Cc: X86 ML <x86@kernel.org>, "H. Peter Anvin" <hpa@zytor.com>, Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>, LKML <linux-kernel@vger.kernel.org>, "open list:DOCUMENTATION" <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>, Balbir Singh <bsingharora@gmail.com>, Borislav Petkov <bp@alien8.de>, Cyrill Gorcunov <gorcunov@gmail.com>, Dave Hansen <dave.hansen@linux.intel.com>, Eugene Syromiatnikov <esyr@redhat.com>, Florian Weimer <fweimer@redhat.com>, "H.J. Lu" <hjl.tools@gmail.com>, Jann Horn <jannh@google.com>, Jonathan Corbet <corbet@lwn.net>, Kees Cook <keescook@chromium.org>, Mike Kravetz <mike.kravetz@or>
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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@intel.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