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

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Date:   Thu, 17 Mar 2022 19:00:04 +0000
From:   Catalin Marinas <catalin.marinas@....com>
To:     Tong Tiangen <tongtiangen@...wei.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

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))

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

> +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.

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

-- 
Catalin