linux-kernel - Re: [PATCH 25/43] x86/mm/kaiser: Unmap kernel from userspace page tables (core patch)

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Message-ID: <20171126151502.zrpwrdcx6yvrirb6@gmail.com>
Date:   Sun, 26 Nov 2017 16:15:02 +0100
From:   Ingo Molnar <mingo@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     linux-kernel@...r.kernel.org,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...capital.net>,
        Thomas Gleixner <tglx@...utronix.de>,
        "H . Peter Anvin" <hpa@...or.com>, Borislav Petkov <bp@...en8.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH 25/43] x86/mm/kaiser: Unmap kernel from userspace page
 tables (core patch)


* Peter Zijlstra <peterz@...radead.org> wrote:

> On Fri, Nov 24, 2017 at 10:14:30AM +0100, Ingo Molnar wrote:
> > +static pte_t *kaiser_shadow_pagetable_walk(unsigned long address,
> > +					   unsigned long flags)
> > +{
> > +	pte_t *pte;
> > +	pmd_t *pmd;
> > +	pud_t *pud;
> > +	p4d_t *p4d;
> > +	pgd_t *pgd = kernel_to_shadow_pgdp(pgd_offset_k(address));
> > +	gfp_t gfp = (GFP_KERNEL | __GFP_NOTRACK | __GFP_ZERO);
> > +
> > +	if (flags & KAISER_WALK_ATOMIC) {
> > +		gfp &= ~GFP_KERNEL;
> > +		gfp |= __GFP_HIGH | __GFP_ATOMIC;
> > +	}
> > +
> > +	if (address < PAGE_OFFSET) {
> > +		WARN_ONCE(1, "attempt to walk user address\n");
> > +		return NULL;
> > +	}
> > +
> > +	if (pgd_none(*pgd)) {
> > +		WARN_ONCE(1, "All shadow pgds should have been populated\n");
> > +		return NULL;
> > +	}
> > +	BUILD_BUG_ON(pgd_large(*pgd) != 0);
> > +
> > +	p4d = p4d_offset(pgd, address);
> > +	BUILD_BUG_ON(p4d_large(*p4d) != 0);
> > +	if (p4d_none(*p4d)) {
> > +		unsigned long new_pud_page = __get_free_page(gfp);
> > +		if (!new_pud_page)
> > +			return NULL;
> > +
> > +		spin_lock(&shadow_table_allocation_lock);
> > +		if (p4d_none(*p4d))
> > +			set_p4d(p4d, __p4d(_KERNPG_TABLE | __pa(new_pud_page)));
> > +		else
> > +			free_page(new_pud_page);
> > +		spin_unlock(&shadow_table_allocation_lock);
> 
> So mm/memory.c has two patterns here.. I prefer the other one:
> 
> 		spin_lock(&shadow_table_allocation_lock);
> 		if (p4d_none(*p4d)) {
> 			set_p4d(p4d, __p4d(_KERNEL_TABLE | __pa(new_pud_page)));
> 			new_pud_page = NULL;
> 		}
> 		spin_unlock(&shadow_table_allocation_lock);
> 		if (new_pud_page)
> 			free_page(new_pud_page);
> 
> > +	}

Ok, would be nice to get this cleanup as a delta patch, because the existing 
pattern has been tested to a fair degree already.

Thanks,

	Ingo