linux-kernel - RE: hv_hypercall

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Date:   Mon, 15 Jun 2020 19:49:41 +0000
From:   Dexuan Cui <decui@...rosoft.com>
To:     vkuznets <vkuznets@...hat.com>, Christoph Hellwig <hch@....de>
CC:     Stephen Hemminger <stephen@...workplumber.org>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...capital.net>,
        Michael Kelley <mikelley@...rosoft.com>,
        Ju-Hyoung Lee <juhlee@...rosoft.com>,
        "x86@...nel.org" <x86@...nel.org>,
        "linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        KY Srinivasan <kys@...rosoft.com>
Subject: RE: hv_hypercall_pg page permissios

> From: linux-hyperv-owner@...r.kernel.org
> <linux-hyperv-owner@...r.kernel.org> On Behalf Of Dexuan Cui
> Sent: Monday, June 15, 2020 10:42 AM
> > >
> > > Hi hch,
> > > The patch is merged into the mainine recently, but unluckily we noticed
> > > a warning with CONFIG_DEBUG_WX=y
> > >
> > > Should we revert this patch, or figure out a way to ask the DEBUG_WX
> > > code to ignore this page?
> >
> > Are you sure it is hv_hypercall_pg?
> Yes, 100% sure. I printed the value of hv_hypercall_pg and and it matched the
> address in the warning line " x86/mm: Found insecure W+X mapping at
> address".

I did this experiment:
  1. export vmalloc_exec and ptdump_walk_pgd_level_checkwx.
  2. write a test module that calls them.
  3. It turns out that every call of vmalloc_exec() triggers such a warning.

vmalloc_exec() uses PAGE_KERNEL_EXEC, which is defined as
   (__PP|__RW|   0|___A|   0|___D|   0|___G)

It looks the logic in note_page() is: for_each_RW_page, if the NX bit is unset,
then report the page as an insecure W+X mapping. IMO this explains the
warning?

Thanks,
-- Dexuan