linux-kernel - Re: [PATCH v17 08/26] x86/mm: Introduce _PAGE

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite for Android: free password hash cracker in your pocket

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <1b9cd39a-fe66-d237-b847-2b62ff1477e7@intel.com>
Date:   Fri, 22 Jan 2021 13:54:53 -0800
From:   "Yu, Yu-cheng" <yu-cheng.yu@...el.com>
To:     David Laight <David.Laight@...LAB.COM>,
        'Randy Dunlap' <rdunlap@...radead.org>,
        Borislav Petkov <bp@...en8.de>
Cc:     "x86@...nel.org" <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-kernel@...r.kernel.org>,
        "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
        "linux-api@...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>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
        Vedvyas Shanbhogue <vedvyas.shanbhogue@...el.com>,
        Dave Martin <Dave.Martin@....com>,
        Weijiang Yang <weijiang.yang@...el.com>,
        Pengfei Xu <pengfei.xu@...el.com>
Subject: Re: [PATCH v17 08/26] x86/mm: Introduce _PAGE_COW

On 1/21/2021 2:32 PM, David Laight wrote:
> From: Randy Dunlap
>> Sent: 21 January 2021 22:19
>>
>> On 1/21/21 2:16 PM, David Laight wrote:
>>> From: Yu, Yu-cheng
>>>>
>>>> On 1/21/2021 10:44 AM, Borislav Petkov wrote:
>>>>> On Tue, Dec 29, 2020 at 01:30:35PM -0800, Yu-cheng Yu wrote:
>>>> [...]
>>>>>> @@ -343,6 +349,16 @@ static inline pte_t pte_mkold(pte_t pte)
>>>>>>
>>>>>>    static inline pte_t pte_wrprotect(pte_t pte)
>>>>>>    {
>>>>>> +	/*
>>>>>> +	 * Blindly clearing _PAGE_RW might accidentally create
>>>>>> +	 * a shadow stack PTE (RW=0, Dirty=1).  Move the hardware
>>>>>> +	 * dirty value to the software bit.
>>>>>> +	 */
>>>>>> +	if (cpu_feature_enabled(X86_FEATURE_SHSTK)) {
>>>>>> +		pte.pte |= (pte.pte & _PAGE_DIRTY) >> _PAGE_BIT_DIRTY << _PAGE_BIT_COW;
>>>>>
>>>>> Why the unreadable shifting when you can simply do:
>>>>>
>>>>>                   if (pte.pte & _PAGE_DIRTY)
>>>>>                           pte.pte |= _PAGE_COW;
>>>>>
>>>
>>>>> ?
>>>>
>>>> It clears _PAGE_DIRTY and sets _PAGE_COW.  That is,
>>>>
>>>> if (pte.pte & _PAGE_DIRTY) {
>>>> 	pte.pte &= ~_PAGE_DIRTY;
>>>> 	pte.pte |= _PAGE_COW;
>>>> }
>>>>
>>>> So, shifting makes resulting code more efficient.
>>>
>>> Does the compiler manage to do one shift?
>>>
>>> How can it clear anything?
>>
>> It could shift it off either end since there are both << and >>.
> 
> It is still:
> 	pte.pte |= xxxxxxx;
> 
>>> There is only an |= against the target.
>>>
>>> Something horrid with ^= might set and clear.
> 
> It could be 4 instructions:
> 	is_dirty = pte.pte & PAGE_DIRTY;
> 	pte.pte &= ~PAGE_DIRTY; // or pte.pte ^= is_dirty
> 	is_cow = is_dirty << (BIT_COW - BIT_DIRTY); // or equivalent >>
> 	pte.pte |= is_cow;
> provided you've a three operand form for one of the first two instructions.
> Something like ARM might manage to merge the last two as well.
> But the register dependency chain length may matter more than
> the number of instructions.
> The above is likely to be three long.

I see what you are saying.  The patch is like...

	if (cpu_feature_enabled(X86_FEATURE_SHSTK)) {
		pte.pte |= (pte.pte & _PAGE_DIRTY) >> _PAGE_BIT_DIRTY << _PAGE_BIT_COW;
		pte = pte_clear_flags(pte, _PAGE_DIRTY);
	}

It is not necessary to do the shifting.  I will make it, simply,

if (pte.pte & _PAGE_DIRTY) {
	pte.pte &= ~PAGE_DIRTY;
	pte.pte |= _PAGE_COW;
}

Thanks for your comments.

--
Yu-cheng