linux-kernel - Re: [Xen-devel] Linux 5.0 regression: BUG: unable to handle kernel paging request at ffff888023e26778

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Date:   Sat, 9 Feb 2019 19:44:08 +0000
From:   Andrew Cooper <andrew.cooper3@...rix.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Sander Eikelenboom <linux@...elenboom.it>
CC:     Juergen Gross <jgross@...e.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        "Kirill A. Shutemov" <kirill@...temov.name>,
        "Joel Fernandes (Google)" <joel@...lfernandes.org>,
        "xen-devel@...ts.xenproject.org" <xen-devel@...ts.xenproject.org>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>
Subject: Re: [Xen-devel] Linux 5.0 regression: BUG: unable to handle kernel
 paging request at ffff888023e26778

On 09/02/2019 18:54, Linus Torvalds wrote:
> On Sat, Feb 9, 2019 at 12:24 AM Sander Eikelenboom <linux@...elenboom.it> wrote:
>> I haven't got a reproducer so i might be hard to hit it again,
>> system is AMD and this is from the host kernel running under
>> the Xen hypervisor might it matter.
> I think this is a Xen bug.
>
> In particular, there's a few poison values in there that look like
> zen. Like this:
>
>    R10: deadbeefdeadf00d
>
> looks like a special poison value that is from Xen itself.

Xen's hypercall ABI states that parameters in registers may be changed
as part of the hypercall.  This is used restart hypercalls midway
through their processing if we had to deliver an interrupt to the vcpu.

As a result, debug builds of Xen deliberately poison all hypercall
parameters, to help catch guest code which doesn't follow the rules.

~Andrew