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

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Date:   Tue, 20 Feb 2018 01:22:14 -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 RESEND] x86: consider effective protection
 attributes in W+X check

>>> On 20.02.18 at 09:10, <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.
> 
> Under what circumstances did you see false positive warnings?

As explained in the 2-patch series this was originally part of, there
continues to be that 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.

>> 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).
>> 
>> Signed-off-by: Jan Beulich <jbeulich@...e.com>
>> Reviewed-by: Juergen Gross <jgross@...e.com>
>> ---
>>  arch/x86/mm/dump_pagetables.c |   92 ++++++++++++++++++++++++++----------------
>>  1 file changed, 57 insertions(+), 35 deletions(-)
> 
> Could you please rebase this on top of latest tip:master, which changed 
> arch/x86/mm/dump_pagetables.c non-trivially due the dynamic 5 level paging 
> changes?

I'll see what I can do; it's a pity that the change here, which had
been sent weeks ago and is a bug fix, hadn't gone in before that
other change (being more an improvement than a bug fix). At
least it doesn't look like the re-basing would be very difficult.

Jan