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

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Message-ID: <b4d4bec7-504e-2443-4cf3-0801b179000f@intel.com>
Date:   Thu, 21 Jan 2021 12:16:23 -0800
From:   "Yu, Yu-cheng" <yu-cheng.yu@...el.com>
To:     Borislav Petkov <bp@...en8.de>
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>,
        "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 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.

>> @@ -434,16 +469,40 @@ static inline pmd_t pmd_mkold(pmd_t pmd)
>>   
>>   static inline pmd_t pmd_mkclean(pmd_t pmd)
>>   {
>> -	return pmd_clear_flags(pmd, _PAGE_DIRTY);
>> +	return pmd_clear_flags(pmd, _PAGE_DIRTY_BITS);
>>   }
>>   
>>   static inline pmd_t pmd_wrprotect(pmd_t pmd)
>>   {
>> +	/*
>> +	 * Blindly clearing _PAGE_RW might accidentally create
>> +	 * a shadow stack PMD (RW=0, Dirty=1).  Move the hardware
>> +	 * dirty value to the software bit.
>> +	 */
>> +	if (cpu_feature_enabled(X86_FEATURE_SHSTK)) {
>> +		pmdval_t v = native_pmd_val(pmd);
>> +
>> +		v |= (v & _PAGE_DIRTY) >> _PAGE_BIT_DIRTY << _PAGE_BIT_COW;
> 
> As above.
> 
>> @@ -488,17 +554,35 @@ static inline pud_t pud_mkold(pud_t pud)
>>   
>>   static inline pud_t pud_mkclean(pud_t pud)
>>   {
>> -	return pud_clear_flags(pud, _PAGE_DIRTY);
>> +	return pud_clear_flags(pud, _PAGE_DIRTY_BITS);
>>   }
>>   
>>   static inline pud_t pud_wrprotect(pud_t pud)
>>   {
>> +	/*
>> +	 * Blindly clearing _PAGE_RW might accidentally create
>> +	 * a shadow stack PUD (RW=0, Dirty=1).  Move the hardware
>> +	 * dirty value to the software bit.
>> +	 */
>> +	if (cpu_feature_enabled(X86_FEATURE_SHSTK)) {
>> +		pudval_t v = native_pud_val(pud);
>> +
>> +		v |= (v & _PAGE_DIRTY) >> _PAGE_BIT_DIRTY << _PAGE_BIT_COW;
> 
> Ditto.
> 
>> @@ -1131,6 +1222,12 @@ extern int pmdp_clear_flush_young(struct vm_area_struct *vma,
>>   #define pmd_write pmd_write
>>   static inline int pmd_write(pmd_t pmd)
>>   {
>> +	/*
>> +	 * If _PAGE_DIRTY is set, then the PMD must either have _PAGE_RW or
>> +	 * be a shadow stack PMD, which is logically writable.
>> +	 */
>> +	if (cpu_feature_enabled(X86_FEATURE_SHSTK))
>> +		return pmd_flags(pmd) & (_PAGE_RW | _PAGE_DIRTY);
> 
> 	else
> 
> 
>>   	return pmd_flags(pmd) & _PAGE_RW;
>>   }
>>