linux-kernel - Re: [PATCH v23 14/28] x86/mm: Shadow Stack page fault error checking

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Date:   Mon, 22 Mar 2021 10:27:53 -0700
From:   "Yu, Yu-cheng" <yu-cheng.yu@...el.com>
To:     "Kirill A. Shutemov" <kirill@...temov.name>
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>,
        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>,
        Haitao Huang <haitao.huang@...el.com>
Subject: Re: [PATCH v23 14/28] x86/mm: Shadow Stack page fault error checking

On 3/22/2021 3:38 AM, Kirill A. Shutemov wrote:
> On Tue, Mar 16, 2021 at 08:10:40AM -0700, Yu-cheng Yu wrote:
[...]
>>   
>> diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
>> index a73347e2cdfc..4316732a18c6 100644
>> --- a/arch/x86/mm/fault.c
>> +++ b/arch/x86/mm/fault.c
>> @@ -1100,6 +1100,17 @@ access_error(unsigned long error_code, struct vm_area_struct *vma)
>>   				       (error_code & X86_PF_INSTR), foreign))
>>   		return 1;
>>   
>> +	/*
>> +	 * Verify a shadow stack access is within a shadow stack VMA.
>> +	 * It is always an error otherwise.  Normal data access to a
>> +	 * shadow stack area is checked in the case followed.
>> +	 */
>> +	if (error_code & X86_PF_SHSTK) {
>> +		if (!(vma->vm_flags & VM_SHSTK))
>> +			return 1;
>> +		return 0;
> 
> Any reason to return 0 here? I would rather keep the single return 0 in
> the function, after all checks are done.
> 

For shadow stack fault, X86_PF_SHSTK and X86_PF_WRITE both can be set. 
So for shadow stack fault, return from here and don't go into the normal 
write fault case.

Thanks,
Yu-cheng

>> +	}
>> +
>>   	if (error_code & X86_PF_WRITE) {
>>   		/* write, present and write, not present: */
>>   		if (unlikely(!(vma->vm_flags & VM_WRITE)))
>> @@ -1293,6 +1304,14 @@ void do_user_addr_fault(struct pt_regs *regs,
>>   
>>   	perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS, 1, regs, address);
>>   
>> +	/*
>> +	 * Clearing _PAGE_DIRTY is used to detect shadow stack access.
>> +	 * This method cannot distinguish shadow stack read vs. write.
>> +	 * For valid shadow stack accesses, set FAULT_FLAG_WRITE to effect
>> +	 * copy-on-write.
>> +	 */
>> +	if (error_code & X86_PF_SHSTK)
>> +		flags |= FAULT_FLAG_WRITE;
>>   	if (error_code & X86_PF_WRITE)
>>   		flags |= FAULT_FLAG_WRITE;
>>   	if (error_code & X86_PF_INSTR)
>> -- 
>> 2.21.0
>>
>