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

[Thomas Gleixner]

Andy Lutomirski <luto@xxxxxxxxxx> writes:
> On Wed, Jul 1, 2020 at 8:42 AM Brian Gerst <brgerst@xxxxxxxxx> wrote:
> > On Fri, Jun 26, 2020 at 1:30 PM Andy Lutomirski <luto@xxxxxxxxxx> 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@xxxxxxxxxx>
>> > Cc: Juergen Gross <jgross@xxxxxxxx>
>> > Cc: Stefano Stabellini <sstabellini@xxxxxxxxxx>
>> > Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx
>> > Fixes: 1c3e5d3f60e2 ("x86/entry: Make entry_64_compat.S objtool clean")
>> > Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxx>
>> > ---
>> >  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