Re: [PATCH v28 09/32] x86/mm: Introduce _PAGE_COW

From: Borislav Petkov <bp@alien8.de>
To: Yu-cheng Yu <yu-cheng.yu@intel.com>
Cc: 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>,
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	Peter Zijlstra <peterz@infradead.org>,
	Randy Dunlap <rdunlap@infradead.org>,
	"Ravi V. Shankar" <ravi.v.shankar@intel.com>,
	Vedvyas Shanbhogue <vedvyas.shanbhogue@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>
Subject: Re: [PATCH v28 09/32] x86/mm: Introduce _PAGE_COW
Date: Mon, 16 Aug 2021 12:43:34 +0200	[thread overview]
Message-ID: <YRpBVu7dCBjks71I@zn.tnic> (raw)
In-Reply-To: <20210722205219.7934-10-yu-cheng.yu@intel.com>

On Thu, Jul 22, 2021 at 01:51:56PM -0700, Yu-cheng Yu wrote:
> @@ -153,13 +178,23 @@ static inline int pud_young(pud_t pud)
>  
>  static inline int pte_write(pte_t pte)
>  {
> -	return pte_flags(pte) & _PAGE_RW;
> +	/*
> +	 * Shadow stack pages are always writable - but not by normal
> +	 * instructions, and only by shadow stack operations.  Therefore,
> +	 * the W=0,D=1 test with pte_shstk().
> +	 */
> +	return (pte_flags(pte) & _PAGE_RW) || pte_shstk(pte);

Well, this is weird: if some kernel code queries a shstk page and this
here function says it is writable but then goes and tries to write into
it and that write fails, then it'll confuse the user.

IOW, from where I'm standing, that should be:

	return (pte_flags(pte) & _PAGE_RW) && !pte_shstk(pte);

as in, a writable page is one which has _PAGE_RW and it is *not* a
shadow stack page because latter is special and not really writable.

Hmmm?

-- 
Regards/Gruss,
    Boris.

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