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

From: Yu-cheng Yu <yu-cheng.yu@intel.com>
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@oracle.com>,
	Nadav Amit <nadav.amit@gmail.com>,
	Oleg Nesterov <oleg@redhat.com>, Pavel Machek <pavel@ucw.cz>,
	Peter Zijlstra <peterz@infradead.org>,
	Randy Dunlap <rdunlap@infradead.org>,
	"Ravi V. Shankar" <ravi.v.shankar@intel.com>,
	Vedvyas Shanbhogue <vedvyas.shanbhogue@intel.com>,
	Dave Martin <Dave.Martin@arm.com>
Subject: Re: [PATCH v8 15/27] mm: Handle shadow stack page fault
Date: Wed, 14 Aug 2019 10:00:27 -0700	[thread overview]
Message-ID: <a47dd0338ff37ebaa7888f291d05a5fccf1cb44b.camel@intel.com> (raw)
In-Reply-To: <bf8a6390-97a6-1ab6-90ef-6399437ed38c@intel.com>

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