Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Message-ID: <fc2274d4-4f1d-d86b-38ad-d80141c3115c@intel.com>
Date:   Thu, 10 Feb 2022 11:27:34 -0800
MIME-Version: 1.0
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101
 Thunderbird/91.5.0
Content-Language: en-US
To:     Rick Edgecombe <rick.p.edgecombe@intel.com>, x86@kernel.org,
        "H . Peter Anvin" <hpa@zytor.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, linux-kernel@vger.kernel.org,
        linux-doc@vger.kernel.org, linux-mm@kvack.org,
        linux-arch@vger.kernel.org, linux-api@vger.kernel.org,
        Arnd Bergmann <arnd@arndb.de>,
        Andy Lutomirski <luto@kernel.org>,
        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>,
        Dave Martin <Dave.Martin@arm.com>,
        Weijiang Yang <weijiang.yang@intel.com>,
        "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
        joao.moreira@intel.com, John Allen <john.allen@amd.com>,
        kcc@google.com, eranian@google.com
Cc:     Yu-cheng Yu <yu-cheng.yu@intel.com>
References: <20220130211838.8382-1-rick.p.edgecombe@intel.com>
 <20220130211838.8382-22-rick.p.edgecombe@intel.com>
From:   Dave Hansen <dave.hansen@intel.com>
Subject: Re: [PATCH 21/35] mm/mprotect: Exclude shadow stack from
 preserve_write
In-Reply-To: <20220130211838.8382-22-rick.p.edgecombe@intel.com>
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 7bit
Precedence: bulk

On 1/30/22 13:18, Rick Edgecombe wrote:
> In change_pte_range(), when a PTE is changed for prot_numa, _PAGE_RW is
> preserved to avoid the additional write fault after the NUMA hinting fault.
> However, pte_write() now includes both normal writable and shadow stack
> (RW=0, Dirty=1) PTEs, but the latter does not have _PAGE_RW and has no need
> to preserve it.

This series creates an interesting situation: it causes a logical
disconnection between things that were tightly coupled before.  For
instance, before this series, _PAGE_RW=1 and "writable" really were
synonyms.  They meant the same thing.

One of the complexities in this series is differentiating the two.  For
instance, a shadow stack page can be written to, even though it has
_PAGE_RW=0.

This particular patch seems to be hacking around the problem that a
p*_mkwrite() doesn't work on shadow stack PTE/PMDs.  First, that makes
me wonder what *actually* happens if we do a plain pte_mkwrite() on a
shadow stack PTE.  I *think* it will take the [Write=0,Dirty=1] PTE and

       pte = pte_set_flags(pte, _PAGE_RW);

so we'll end up with [Write=1,Dirty=1], which is bad.

Let's say pte_mkwrite() can't be fixed.  We should probably make it
VM_BUG_ON() if it's ever asked to muck with a shadow stack PTE.

It's also weird because we have this pte_write()==1 PTE in a !VM_WRITE
VMA.  Then, we're trying to pte_mkwrite() under this !VM_WRITE VMA.

	pte_write() <-- returns true for on shadow stack PTE!
	pte_mkwrite() <-- illegal on shadow stack PTE

I need to think about this a little more.  I don't have a solution.
But, as-is, it seems untenable.  The rules are just too counter
intuitive to live.