linux-kernel - Re: [PATCH v1 1/4] kasan: introduce set_pmd_early

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Date:   Wed, 15 Jan 2020 12:09:50 +0100
From:   Jürgen Groß <jgross@...e.com>
To:     Sergey Dyasli <sergey.dyasli@...rix.com>
Cc:     xen-devel@...ts.xen.org, kasan-dev@...glegroups.com,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Alexander Potapenko <glider@...gle.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Stefano Stabellini <sstabellini@...nel.org>,
        George Dunlap <george.dunlap@...rix.com>,
        Ross Lagerwall <ross.lagerwall@...rix.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH v1 1/4] kasan: introduce set_pmd_early_shadow()

On 15.01.20 11:54, Sergey Dyasli wrote:
> Hi Juergen,
> 
> On 08/01/2020 15:20, Sergey Dyasli wrote:
>> It is incorrect to call pmd_populate_kernel() multiple times for the
>> same page table. Xen notices it during kasan_populate_early_shadow():
>>
>>      (XEN) mm.c:3222:d155v0 mfn 3704b already pinned
>>
>> This happens for kasan_early_shadow_pte when USE_SPLIT_PTE_PTLOCKS is
>> enabled. Fix this by introducing set_pmd_early_shadow() which calls
>> pmd_populate_kernel() only once and uses set_pmd() afterwards.
>>
>> Signed-off-by: Sergey Dyasli <sergey.dyasli@...rix.com>
> 
> Looks like the plan to use set_pmd() directly has failed: it's an
> arch-specific function and can't be used in arch-independent code
> (as kbuild test robot has proven).
> 
> Do you see any way out of this other than disabling SPLIT_PTE_PTLOCKS
> for PV KASAN?

Change set_pmd_early_shadow() like the following:

#ifdef CONFIG_XEN_PV
static inline void set_pmd_early_shadow(pmd_t *pmd, pte_t *early_shadow)
{
	static bool pmd_populated = false;

	if (likely(pmd_populated)) {
		set_pmd(pmd, __pmd(__pa(early_shadow) | _PAGE_TABLE));
	} else {
		pmd_populate_kernel(&init_mm, pmd, early_shadow);
		pmd_populated = true;
	}
}
#else
static inline void set_pmd_early_shadow(pmd_t *pmd, pte_t *early_shadow)
{
	pmd_populate_kernel(&init_mm, pmd, early_shadow);
}
#endif

... and move it to include/xen/xen-ops.h and call it with
lm_alias(kasan_early_shadow_pte) as the second parameter.


Juergen