linux-kernel - Re: [PATCH 1/2] mm/debug_vm_pgtable/basic: Add validation for dirtiness after write protect

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Date:   Mon, 30 Nov 2020 09:38:42 +0000
From:   Catalin Marinas <catalin.marinas@....com>
To:     Anshuman Khandual <anshuman.khandual@....com>
Cc:     Christophe Leroy <christophe.leroy@...roup.eu>, linux-mm@...ck.org,
        akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
        steven.price@....com, gerald.schaefer@...ux.ibm.com,
        vgupta@...opsys.com, paul.walmsley@...ive.com
Subject: Re: [PATCH 1/2] mm/debug_vm_pgtable/basic: Add validation for
 dirtiness after write protect

On Mon, Nov 30, 2020 at 09:55:00AM +0530, Anshuman Khandual wrote:
> On 11/27/20 3:14 PM, Catalin Marinas wrote:
> > On Fri, Nov 27, 2020 at 09:22:24AM +0100, Christophe Leroy wrote:
> >> Le 27/11/2020 à 06:06, Anshuman Khandual a écrit :
> >>> This adds validation tests for dirtiness after write protect conversion for
> >>> each page table level. This is important for platforms such as arm64 that
> >>> removes the hardware dirty bit while making it an write protected one. This
> >>> also fixes pxx_wrprotect() related typos in the documentation file.
> >>
> >>> diff --git a/mm/debug_vm_pgtable.c b/mm/debug_vm_pgtable.c
> >>> index c05d9dcf7891..a5be11210597 100644
> >>> --- a/mm/debug_vm_pgtable.c
> >>> +++ b/mm/debug_vm_pgtable.c
> >>> @@ -70,6 +70,7 @@ static void __init pte_basic_tests(unsigned long pfn, pgprot_t prot)
> >>>   	WARN_ON(pte_young(pte_mkold(pte_mkyoung(pte))));
> >>>   	WARN_ON(pte_dirty(pte_mkclean(pte_mkdirty(pte))));
> >>>   	WARN_ON(pte_write(pte_wrprotect(pte_mkwrite(pte))));
> >>> +	WARN_ON(pte_dirty(pte_wrprotect(pte)));
> >>
> >> Wondering what you are testing here exactly.
> >>
> >> Do you expect that if PTE has the dirty bit, it gets cleared by
> >> pte_wrprotect() ?
> >>
> >> Powerpc doesn't do that, it only clears the RW bit but the dirty
> >> bit remains if it is set, until you call pte_mkclean() explicitely.
> > 
> > Arm64 has an unusual way of setting a hardware dirty "bit", it actually
> > clears the PTE_RDONLY bit. The pte_wrprotect() sets the PTE_RDONLY bit
> > back and we can lose the dirty information. Will found this and posted
> > patches to fix the arm64 pte_wprotect() to set a software PTE_DIRTY if
> > !PTE_RDONLY (we do this for ptep_set_wrprotect() already). My concern
> > was that we may inadvertently make a fresh/clean pte dirty with such
> > change, hence the suggestion for the test.
> > 
> > That said, I think we also need a test in the other direction,
> > pte_wrprotect() should preserve any dirty information:
> > 
> > 	WARN_ON(!pte_dirty(pte_wrprotect(pte_mkdirty(pte))));
> 
> This seems like a generic enough principle which all platforms should
> adhere to. But the proposed test WARN_ON(pte_dirty(pte_wrprotect(pte)))
> might fail on some platforms if the page table entry came in as a dirty
> one and pte_wrprotect() is not expected to alter the dirty state.

Ah, so do we have architectures where entries in protection_map[] are
already dirty? If those are valid, maybe the check should be:

	WARN_ON(!pte_dirty(pte) && pte_dirty(pte_wrprotect(pte)));

> Instead, should we just add the following two tests, which would ensure
> that pte_wrprotect() never alters the dirty state of a page table entry.
> 
> WARN_ON(!pte_dirty(pte_wrprotect(pte_mkdirty(pte))));
> WARN_ON(pte_dirty(pte_wrprotect(pte_mkclean(pte))));

These should be added as additional tests. However, my initial thought
was to check whether pte_wrprotect() on a new pte created from a
protection_map[] entry directly would inadvertently dirty it. On arm64,
that means a protection_map[] entry missing PTE_RDONLY. A pte_mkclean()
would set PTE_RDONLY, so we'd miss such check.

-- 
Catalin