linux-kernel - Re: [PATCH v7 27/41] x86/mm: Warn if create Write=0,Dirty=1 with raw prot

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Date:   Wed, 8 Mar 2023 10:23:24 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Rick Edgecombe <rick.p.edgecombe@...el.com>
Cc:     x86@...nel.org, "H . Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
        linux-doc@...r.kernel.org, linux-mm@...ck.org,
        linux-arch@...r.kernel.org, linux-api@...r.kernel.org,
        Arnd Bergmann <arnd@...db.de>,
        Andy Lutomirski <luto@...nel.org>,
        Balbir Singh <bsingharora@...il.com>,
        Cyrill Gorcunov <gorcunov@...il.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Eugene Syromiatnikov <esyr@...hat.com>,
        Florian Weimer <fweimer@...hat.com>,
        "H . J . Lu" <hjl.tools@...il.com>, Jann Horn <jannh@...gle.com>,
        Jonathan Corbet <corbet@....net>,
        Kees Cook <keescook@...omium.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Nadav Amit <nadav.amit@...il.com>,
        Oleg Nesterov <oleg@...hat.com>, Pavel Machek <pavel@....cz>,
        Peter Zijlstra <peterz@...radead.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        Weijiang Yang <weijiang.yang@...el.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        John Allen <john.allen@....com>, kcc@...gle.com,
        eranian@...gle.com, rppt@...nel.org, jamorris@...ux.microsoft.com,
        dethoma@...rosoft.com, akpm@...ux-foundation.org,
        Andrew.Cooper3@...rix.com, christina.schimpe@...el.com,
        david@...hat.com, debug@...osinc.com
Subject: Re: [PATCH v7 27/41] x86/mm: Warn if create Write=0,Dirty=1 with raw
 prot

On Mon, Feb 27, 2023 at 02:29:43PM -0800, Rick Edgecombe wrote:
> When user shadow stack is use, Write=0,Dirty=1 is treated by the CPU as
			   ^
			   in

> shadow stack memory. So for shadow stack memory this bit combination is
> valid, but when Dirty=1,Write=1 (conventionally writable) memory is being
> write protected, the kernel has been taught to transition the Dirty=1
> bit to SavedDirty=1, to avoid inadvertently creating shadow stack
> memory. 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.


> 
> However, when a PTE is created by a raw prot using mk_pte(), mk_pte()
> can't know whether to adjust Dirty=1 to SavedDirty=1. It can't
> distinguish between the caller intending to create a shadow stack PTE or
> needing the SavedDirty shift.
> 
> The kernel has been updated to not do this, and so Write=0,Dirty=1
> memory should only be created by the pte_mkfoo() helpers. Add a warning
> to make sure no new mk_pte() start doing this.

Might wanna add the note from below here:

"... start doing this, like, for example, set_memory_rox() did."

> Tested-by: Pengfei Xu <pengfei.xu@...el.com>
> Tested-by: John Allen <john.allen@....com>
> Tested-by: Kees Cook <keescook@...omium.org>
> Acked-by: Mike Rapoport (IBM) <rppt@...nel.org>
> Signed-off-by: Rick Edgecombe <rick.p.edgecombe@...el.com>
> 
> ---
> v6:
>  - New patch (Note, this has already been a useful warning, it caught the
>    newly added set_memory_rox() doing this)

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette