linux-kernel - Re: [PATCH v2] x86: consider effective protection attributes in W+X check

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Message-Id: <5A8EA5CD02000078001AA4CA@prv-mh.provo.novell.com>
Date:   Thu, 22 Feb 2018 03:13:17 -0700
From:   "Jan Beulich" <JBeulich@...e.com>
To:     "Ingo Molnar" <mingo@...nel.org>
Cc:     <mingo@...e.hu>, <tglx@...utronix.de>,
        "Boris Ostrovsky" <boris.ostrovsky@...cle.com>,
        "Juergen Gross" <jgross@...e.com>, <linux-kernel@...r.kernel.org>,
        <hpa@...or.com>
Subject: Re: [PATCH v2] x86: consider effective protection attributes
 in W+X check

>>> On 21.02.18 at 17:53, <mingo@...nel.org> wrote:

> * Jan Beulich <JBeulich@...e.com> wrote:
> 
>> Using just the leaf page table entry flags would cause a false warning
>> in case _PAGE_RW is clear or _PAGE_NX is set in a higher level entry.
>> Hand through both the current entry's flags as well as the accumulated
>> effective value (the latter as pgprotval_t instead of pgprot_t, as it's
>> not an actual entry's value).
>> 
>> This in particular eliminates the false W+X warning when running under
>> Xen, as commit 2cc42bac1c ("x86-64/Xen: eliminate W+X mappings") has to
>> make the necessary adjustment in L2 rather than L1 (the reason is
>> explained there). I.e. _PAGE_RW is clear there in L1, but _PAGE_NX is
>> set in L2.
>> 
>> Signed-off-by: Jan Beulich <jbeulich@...e.com>
>> Reviewed-by: Juergen Gross <jgross@...e.com>
>> ---
>> v2: Re-base onto tip tree. Add Xen related paragraph to description.
>> ---
>>  arch/x86/mm/dump_pagetables.c |   92 
> ++++++++++++++++++++++++++----------------
>>  1 file changed, 57 insertions(+), 35 deletions(-)
> 
> There's a build failure with CONFIG_KASAN=y enabled:
> 
>  arch/x86/mm/dump_pagetables.c: In function ‘kasan_page_table’:
>  arch/x86/mm/dump_pagetables.c:365:3: error: too few arguments to function ‘note_page’
>  arch/x86/mm/dump_pagetables.c:238:13: note: declared here

Oh, I see. Question though is what to pass as the extra argument:
Do I need to pass in the caller's effective rights, or should I take
kasan_page_table()'s checking against kasan_zero_p?d as an
indication that the effective permission is zero here? I'm sorry for
this probably trivial question, but I know nothing about how KASAN
works.

Jan