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

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Date:   Thu, 16 Jan 2020 08:54:59 +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 17:32, Sergey Dyasli wrote:
> On 15/01/2020 11:09, Jürgen Groß wrote:
>> 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.
> 
> Your suggestion to use ifdef is really good, especially now when I
> figured out that CONFIG_XEN_PV implies X86. But I don't like the idea
> of kasan code calling a non-empty function from xen-ops.h when
> CONFIG_XEN_PV is not defined. I'd prefer to keep set_pmd_early_shadow()
> in mm/kasan/init.c with the suggested ifdef.

Fine with me.


Juergen