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 10:00:27 -0700
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To: Dave Hansen <dave.hansen@intel.com>, 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 Wed, 2019-08-14 at 09:48 -0700, Dave Hansen wrote:
> On 8/14/19 9:27 AM, Yu-cheng Yu wrote:
> > 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.
> 
> I don't understand what you are saying.
> 
> Are you saying that we need this VM_SHSTK flag in order to exclude
> RO+HW-Dirty pages from being created in non-shadow-stack VMAs?

We use VM_SHSTK for page fault handling (the special-casing).  If we have a
special mapping, all these become cleaner (but more code).  However, we still
need most of the PTE macros (e.g. ptep_set_wrprotect, PAGE_DIRTY_SW, etc.).

Yu-cheng