Date:   Mon, 27 Feb 2023 14:54:51 -0800
From:   Kees Cook <kees@kernel.org>
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>,
        Weijiang Yang <weijiang.yang@intel.com>,
        "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
        John Allen <john.allen@amd.com>, kcc@google.com,
        eranian@google.com, rppt@kernel.org, jamorris@linux.microsoft.com,
        dethoma@microsoft.com, akpm@linux-foundation.org,
        Andrew.Cooper3@citrix.com, christina.schimpe@intel.com,
        david@redhat.com, debug@rivosinc.com
CC:     rick.p.edgecombe@intel.com
Subject: =?US-ASCII?Q?Re=3A_=5BPATCH_v7_27/41=5D_x86/mm=3A_Warn_if?= =?US-ASCII?Q?_create_Write=3D0=2CDirty=3D1_with_raw_prot?=
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On February 27, 2023 2:29:43 PM PST, Rick Edgecombe <rick=2Ep=2Eedgecombe@i=
ntel=2Ecom> wrote:
>When user shadow stack is use, Write=3D0,Dirty=3D1 is treated by the CPU =
as
>shadow stack memory=2E So for shadow stack memory this bit combination is
>valid, but when Dirty=3D1,Write=3D1 (conventionally writable) memory is b=
eing
>write protected, the kernel has been taught to transition the Dirty=3D1
>bit to SavedDirty=3D1, to avoid inadvertently creating shadow stack
>memory=2E It does this inside pte_wrprotect() because it knows the PTE is
>not intended to be a writable shadow stack entry, it is supposed to be
>write protected=2E
>
>However, when a PTE is created by a raw prot using mk_pte(), mk_pte()
>can't know whether to adjust Dirty=3D1 to SavedDirty=3D1=2E It can't
>distinguish between the caller intending to create a shadow stack PTE or
>needing the SavedDirty shift=2E
>
>The kernel has been updated to not do this, and so Write=3D0,Dirty=3D1
>memory should only be created by the pte_mkfoo() helpers=2E Add a warning
>to make sure no new mk_pte() start doing this=2E
>
>Tested-by: Pengfei Xu <pengfei=2Exu@intel=2Ecom>
>Tested-by: John Allen <john=2Eallen@amd=2Ecom>
>Tested-by: Kees Cook <keescook@chromium=2Eorg>
>Acked-by: Mike Rapoport (IBM) <rppt@kernel=2Eorg>
>Signed-off-by: Rick Edgecombe <rick=2Ep=2Eedgecombe@intel=2Ecom>

Reviewed-by: Kees Cook <keescook@chromium=2Eorg>


--=20
Kees Cook