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

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Date:   Mon, 21 Mar 2022 14:15:36 +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 2022/3/19 1:18, Catalin Marinas 写道:
> On Fri, Mar 18, 2022 at 11:58:22AM +0800, Tong Tiangen wrote:
>> 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);
>>>> +}
> [...]
>>> 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.
> 
> PROT_NONE is just a permission but since we don't have independent read
> and write bits in the pte, we implement it as an invalid pte (bit 0
> cleared). The other content of the pte is fine, so pte_pfn() should
> still work. PTC could as well check this, I don't think it hurts.

You have a point and the logic should be:

	pte_present(pte) && (pte_user(pte) || pte_user_exec(pte))

> 
>>>> +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.
> 
> My point is that the (pmd_val(pmd) & PTE_VALID) check is superfluous
> since that's covered by pmd_leaf() already.

Oh，i got it，you're right and these will be fixed in v2.

Considering all your suggestions, The final logic should be:

+#define pte_user(pte)          (!!(pte_val(pte) & PTE_USER))

+#define pmd_user(pmd)		pte_user(pmd_pte(pmd))
+#define pmd_user_exec(pmd)	pte_user_exec(pmd_pte(pmd))

+#define pud_user(pud)          pte_user(pud_pte(pud))

+static inline bool pte_user_accessible_page(pte_t pte)
+{
+	return pte_present(pte) && (pte_user(pte)|| pte_user_exec(pte));
+}

+static inline bool pmd_user_accessible_page(pmd_t pmd)
+{
+	return pmd_present(pmd) && (pmd_user(pmd)|| pmd_user_exec(pmd));
+}

+static inline bool pud_user_accessible_page(pud_t pud)
+{
+	return pud_present(pud) && pud_user(pud);
+}

>