linux-kernel - Re: [PATCH 3/6] x86/entry/64/compat: Fix Xen PV SYSENTER frame setup

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Message-ID: <87k0zm9ivy.fsf@nanos.tec.linutronix.de>
Date:   Thu, 02 Jul 2020 14:54:57 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Andy Lutomirski <luto@...nel.org>, Brian Gerst <brgerst@...il.com>
Cc:     Andy Lutomirski <luto@...nel.org>,
        the arch/x86 maintainers <x86@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Andrew Cooper <andrew.cooper3@...rix.com>,
        Juergen Gross <jgross@...e.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Stefano Stabellini <sstabellini@...nel.org>,
        xen-devel <xen-devel@...ts.xenproject.org>
Subject: Re: [PATCH 3/6] x86/entry/64/compat: Fix Xen PV SYSENTER frame setup

Andy Lutomirski <luto@...nel.org> writes:
> On Wed, Jul 1, 2020 at 8:42 AM Brian Gerst <brgerst@...il.com> wrote:
> > On Fri, Jun 26, 2020 at 1:30 PM Andy Lutomirski <luto@...nel.org> wrote:
>> >
>> > The SYSENTER frame setup was nonsense.  It worked by accident
>> > because the normal code into which the Xen asm jumped
>> > (entry_SYSENTER_32/compat) threw away SP without touching the stack.
>> > entry_SYSENTER_compat was recently modified such that it relied on
>> > having a valid stack pointer, so now the Xen asm needs to invoke it
>> > with a valid stack.
>> >
>> > Fix it up like SYSCALL: use the Xen-provided frame and skip the bare
>> > metal prologue.
>> >
>> > Cc: Boris Ostrovsky <boris.ostrovsky@...cle.com>
>> > Cc: Juergen Gross <jgross@...e.com>
>> > Cc: Stefano Stabellini <sstabellini@...nel.org>
>> > Cc: xen-devel@...ts.xenproject.org
>> > Fixes: 1c3e5d3f60e2 ("x86/entry: Make entry_64_compat.S objtool clean")
>> > Signed-off-by: Andy Lutomirski <luto@...nel.org>
>> > ---
>> >  arch/x86/entry/entry_64_compat.S |  1 +
>> >  arch/x86/xen/xen-asm_64.S        | 20 ++++++++++++++++----
>> >  2 files changed, 17 insertions(+), 4 deletions(-)
>> >
>> > diff --git a/arch/x86/entry/entry_64_compat.S b/arch/x86/entry/entry_64_compat.S
>> > index 7b9d8150f652..381a6de7de9c 100644
>> > --- a/arch/x86/entry/entry_64_compat.S
>> > +++ b/arch/x86/entry/entry_64_compat.S
>> > @@ -79,6 +79,7 @@ SYM_CODE_START(entry_SYSENTER_compat)
>> >         pushfq                          /* pt_regs->flags (except IF = 0) */
>> >         pushq   $__USER32_CS            /* pt_regs->cs */
>> >         pushq   $0                      /* pt_regs->ip = 0 (placeholder) */
>> > +SYM_INNER_LABEL(entry_SYSENTER_compat_after_hwframe, SYM_L_GLOBAL)
>>
>> This skips over the section that truncates the syscall number to
>> 32-bits.  The comments present some doubt that it is actually
>> necessary, but the Xen path shouldn't differ from native.  That code
>> should be moved after this new label.
>
> Whoops.  I thought I caught that myself, but apparently not.  I'll fix it.

Darn. I already applied that lot. Can you please send a delta fix?

Thanks,

        tglx