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

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Message-ID: <YjS+2FVpq8D4Gx0S@arm.com>
Date:   Fri, 18 Mar 2022 17:18:16 +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 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.

> > > +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.

-- 
Catalin