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From:   Andy Lutomirski <luto@amacapital.net>
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Subject: Re: [PATCH v28 09/32] x86/mm: Introduce _PAGE_COW
Date:   Tue, 17 Aug 2021 13:13:09 -0700
Message-Id: <1A27F5DF-477B-45B7-AD33-CC68D9B7CB89@amacapital.net>
References: <YRwT7XX36fQ2GWXn@zn.tnic>
Cc:     "Yu, Yu-cheng" <yu-cheng.yu@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>,
        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>,
        Pengfei Xu <pengfei.xu@intel.com>,
        Haitao Huang <haitao.huang@intel.com>,
        Rick P Edgecombe <rick.p.edgecombe@intel.com>,
        "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>
In-Reply-To: <YRwT7XX36fQ2GWXn@zn.tnic>
To:     Borislav Petkov <bp@alien8.de>
Precedence: bulk


> On Aug 17, 2021, at 12:53 PM, Borislav Petkov <bp@alien8.de> wrote:
>=20
> =EF=BB=BFOn Tue, Aug 17, 2021 at 11:24:29AM -0700, Yu, Yu-cheng wrote:
>> Indeed, this can be looked at in a few ways.  We can visualize pte_write(=
)
>> as 'CPU can write to it with MOV' or 'CPU can write to it with any opcode=
s'.
>> Depending on whatever pte_write() is, copy-on-write code can be adjusted
>> accordingly.
>=20
> Can be?
>=20
> I think you should exclude shadow stack pages from being writable
> and treat them as read-only. How the CPU writes them is immaterial -
> pte/pmd_write() is used by normal kernel code to query whether the page
> is writable or not by any instruction - not by the CPU.
>=20
> And since normal kernel code cannot write shadow stack pages, then for
> that code those pages are read-only.
>=20
> If special kernel code using shadow stack management insns needs
> to modify a shadow stack, then it can check whether a page is
> pte/pmd_shstk() but that code is special anyway.
>=20
> Hell, a shadow stack page is (Write=3D0, Dirty=3D1) so calling it writable=

>                  ^^^^^^^
> is simply wrong.

But it *is* writable using WRUSS, and it=E2=80=99s also writable by CALL, WR=
SS, etc.

Now if the mm code tries to write protect it and expects sensible semantics,=
 the results could be interesting. At the very least, someone would need to v=
alidate that RET reading a read only shadow stack page does the right thing.=


>=20
> Thx.
>=20
> --=20
> Regards/Gruss,
>    Boris.
>=20
> https://people.kernel.org/tglx/notes-about-netiquette