linux-kernel - Re: [PATCH v2] x86-64/Xen: eliminate W+X mappings

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Date:   Mon, 18 Dec 2017 07:52:37 -0800
From:   Joe Perches <joe@...ches.com>
To:     Ingo Molnar <mingo@...nel.org>, Jan Beulich <JBeulich@...e.com>
Cc:     Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Juergen Gross <jgross@...e.com>, mingo@...e.hu,
        tglx@...utronix.de, xen-devel <xen-devel@...ts.xenproject.org>,
        linux-kernel@...r.kernel.org, hpa@...or.com,
        Borislav Petkov <bp@...en8.de>
Subject: Re: [PATCH v2] x86-64/Xen: eliminate W+X mappings

On Mon, 2017-12-18 at 13:28 +0100, Ingo Molnar wrote:
> * Jan Beulich <JBeulich@...e.com> wrote:
> 
> > A few thousand such pages are usually left around due to the re-use of
> > L1 tables having been provided by the hypervisor (Dom0) or tool stack
> > (DomU). Set NX in the direct map variant, which needs to be done in L2
> > due to the dual use of the re-used L1s.
> > 
> > For x86_configure_nx() to actually do what it is supposed to do, call
> > get_cpu_cap() first. This was broken by commit 4763ed4d45 ("x86, mm:
> > Clean up and simplify NX enablement") when switching away from the
> > direct EFER read.
[]
> > --- 4.15-rc4/arch/x86/xen/mmu_pv.c
> > +++ 4.15-rc4-x86_64-Xen-avoid-W+X/arch/x86/xen/mmu_pv.c
> > @@ -1902,6 +1902,18 @@ void __init xen_setup_kernel_pagetable(p
> >  	/* Graft it onto L4[511][510] */
> >  	copy_page(level2_kernel_pgt, l2);
> >  
> > +	/*
> > +	 * Zap execute permission from the ident map. Due to the sharing of
> > +	 * L1 entries we need to do this in the L2.
> > +	 */
> > +	if (__supported_pte_mask & _PAGE_NX)
> > +		for (i = 0; i < PTRS_PER_PMD; ++i) {
> > +			if (pmd_none(level2_ident_pgt[i]))
> > +				continue;
> > +			level2_ident_pgt[i] = pmd_set_flags(level2_ident_pgt[i],
> > +							    _PAGE_NX);
> > +		}
> > +
> 
> This chunk has two stylistic problems:
> 
>  - Curly braces need to be added
>  - Line broken in an ugly fashion: just make it long and ignore the checkpatch col80 warning
> 
> looks good otherwise.

stylistic trivia:

Instead of repeating level2_ident_pgt[i] multiple times,
it might be nicer to use temporaries and not use i at all.

Something like:

	if (__supported_pte_mask & _PAGE_NX) {
		pmd_t *pmd = level2_ident_pgt;
		pmd_t *end = pmd + PTRS_PER_PMD;

		for (; pmd < end; pmd++) {
			if (!pmd_none(pmd))
				*pmd = pmd_set_flags(pmd, _PAGE_NX);
		}
	}