linux-kernel - Re: [PATCH v6 27/41] x86/mm: Warn if create Write=0,Dirty=1 with raw prot

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Message-ID: <63f28a4c.620a0220.cf190.b2fa@mx.google.com>
Date:   Sun, 19 Feb 2023 12:45:00 -0800
From:   Kees Cook <keescook@...omium.org>
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>,
        Borislav Petkov <bp@...en8.de>,
        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>,
        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 v6 27/41] x86/mm: Warn if create Write=0,Dirty=1 with raw
 prot

On Sat, Feb 18, 2023 at 01:14:19PM -0800, Rick Edgecombe wrote:
> When user shadow stack is use, Write=0,Dirty=1 is treated by the CPU as
> 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.
> 
> Tested-by: Pengfei Xu <pengfei.xu@...el.com>
> 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)
> ---
>  arch/x86/include/asm/pgtable.h | 10 +++++++++-
>  1 file changed, 9 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/include/asm/pgtable.h b/arch/x86/include/asm/pgtable.h
> index f3dc16fc4389..db8fe5511c74 100644
> --- a/arch/x86/include/asm/pgtable.h
> +++ b/arch/x86/include/asm/pgtable.h
> @@ -1032,7 +1032,15 @@ static inline unsigned long pmd_page_vaddr(pmd_t pmd)
>   * (Currently stuck as a macro because of indirect forward reference
>   * to linux/mm.h:page_to_nid())
>   */
> -#define mk_pte(page, pgprot)   pfn_pte(page_to_pfn(page), (pgprot))
> +#define mk_pte(page, pgprot)						 \
> +({									 \
> +	pgprot_t __pgprot = pgprot;					 \
> +									 \
> +	WARN_ON_ONCE(cpu_feature_enabled(X86_FEATURE_USER_SHSTK) &&	 \
> +		    (pgprot_val(__pgprot) & (_PAGE_DIRTY | _PAGE_RW)) == \
> +		    _PAGE_DIRTY);					 \
> +	pfn_pte(page_to_pfn(page), __pgprot);				 \
> +})

This only warns? Should it also enforce the state?

-- 
Kees Cook