linux-kernel - Re: [PATCH -next 3/4] arm64: mm: add support for page table check

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Date:   Fri, 18 Mar 2022 11:58:22 +0800
From:   Tong Tiangen <tongtiangen@...wei.com>
To:     Catalin Marinas <catalin.marinas@....com>
CC:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, <x86@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Pasha Tatashin <pasha.tatashin@...een.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Will Deacon" <will@...nel.org>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        "Palmer Dabbelt" <palmer@...belt.com>,
        Palmer Dabbelt <palmerdabbelt@...gle.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        <linux-kernel@...r.kernel.org>, <linux-mm@...ck.org>,
        <linux-arm-kernel@...ts.infradead.org>,
        <linux-riscv@...ts.infradead.org>
Subject: Re: [PATCH -next 3/4] arm64: mm: add support for page table check



在 2022/3/18 3:00, Catalin Marinas 写道:
> On Thu, Mar 17, 2022 at 02:12:02PM +0000, Tong Tiangen wrote:
>> @@ -628,6 +647,25 @@ static inline unsigned long pmd_page_vaddr(pmd_t pmd)
>>   #define pud_leaf(pud)		pud_sect(pud)
>>   #define pud_valid(pud)		pte_valid(pud_pte(pud))
>>   
>> +#ifdef CONFIG_PAGE_TABLE_CHECK
>> +static inline bool pte_user_accessible_page(pte_t pte)
>> +{
>> +	return (pte_val(pte) & PTE_VALID) && (pte_val(pte) & PTE_USER);
>> +}
> 
> There is another class of user mappings, execute-only, that have both
> PTE_USER and PTE_UXN cleared. So this logic should be:
> 
> 	pte_valid(pte) && (pte_user(pte) || pte_user_exec(pte))
> 
> with pte_user() as:
> 
> #define pte_user(pte)	(!!(pte_val(pte) & PTE_USER))

Good suggestion, the PTC(page table check) can cover UXN page and 
pte_user(pte) helper is required.

> 
> Do we care about PROT_NONE mappings here? They have the valid bit
> cleared but pte_present() is true.
>

PTC will not check this special type(PROT_NONE) of page.

>> +static inline bool pmd_user_accessible_page(pmd_t pmd)
>> +{
>> +	return pmd_leaf(pmd) && (pmd_val(pmd) & PTE_VALID) &&
>> +		(pmd_val(pmd) & PTE_USER);
>> +}
> 
> pmd_leaf() implies valid, so you can skip it if that's the aim.

PTC only checks whether the memory block corresponding to the pmd_leaf 
type can access, for !pmd_leaf, PTC checks at the pte level. So i think 
this is necessary.

> 
> Similar comment to the pte variant on execute-only and PROT_NONE
> mappings

Same considerations as above.

Thanks.
Tong
>