linux-kernel - Re: hv_hypercall

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Date:   Tue, 16 Jun 2020 09:23:18 +0200
From:   Christoph Hellwig <hch@....de>
To:     Dexuan Cui <decui@...rosoft.com>
Cc:     vkuznets <vkuznets@...hat.com>, Christoph Hellwig <hch@....de>,
        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>,
        Tom Lendacky <thomas.lendacky@....com>
Subject: Re: hv_hypercall_pg page permissios

On Mon, Jun 15, 2020 at 07:49:41PM +0000, Dexuan Cui wrote:
> 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?

It does.  But it also means every other user of PAGE_KERNEL_EXEC
should trigger this, of which there are a few (kexec, tboot, hibernate,
early xen pv mapping, early SEV identity mapping)

We really shouldn't create mappings like this by default.  Either we
need to flip PAGE_KERNEL_EXEC itself based on the needs of the above
users, or add another define to overload vmalloc_exec as there is no
other user of that for x86.