linux-kernel - Re: [PATCH] x86-64/entry: add instruction suffix to SYSRET

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Date:   Mon, 16 Dec 2019 10:23:15 -0500
From:   Brian Gerst <brgerst@...il.com>
To:     Jan Beulich <jbeulich@...e.com>
Cc:     Andy Lutomirski <luto@...nel.org>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        lkml <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86-64/entry: add instruction suffix to SYSRET

On Mon, Dec 16, 2019 at 5:12 AM Jan Beulich <jbeulich@...e.com> wrote:
>
> On 13.12.2019 18:49, Andy Lutomirski wrote:
> > On Fri, Dec 13, 2019 at 1:55 AM Jan Beulich <jbeulich@...e.com> wrote:
> >>
> >> On 12.12.2019 22:43, Andy Lutomirski wrote:
> >>> On Tue, Dec 10, 2019 at 7:40 AM Jan Beulich <jbeulich@...e.com> wrote:
> >>>>
> >>>> On 10.12.2019 16:29, Andy Lutomirski wrote:
> >>>>>> On Dec 10, 2019, at 2:48 AM, Jan Beulich <JBeulich@...e.com> wrote:
> >>>>>>
> >>>>>> Omitting suffixes from instructions in AT&T mode is bad practice when
> >>>>>> operand size cannot be determined by the assembler from register
> >>>>>> operands, and is likely going to be warned about by upstream gas in the
> >>>>>> future. Add the missing suffix here.
> >>>>>>
> >>>>>> Signed-off-by: Jan Beulich <jbeulich@...e.com>
> >>>>>>
> >>>>>> --- a/arch/x86/entry/entry_64.S
> >>>>>> +++ b/arch/x86/entry/entry_64.S
> >>>>>> @@ -1728,7 +1728,7 @@ END(nmi)
> >>>>>> SYM_CODE_START(ignore_sysret)
> >>>>>>    UNWIND_HINT_EMPTY
> >>>>>>    mov    $-ENOSYS, %eax
> >>>>>> -    sysret
> >>>>>> +    sysretl
> >>>>>
> >>>>> Isn’t the default sysretq?  sysretl looks more correct, but that suggests
> >>>>> that your changelog is wrong.
> >>>>
> >>>> No, this is different from ret, and more like iret and lret.
> >>>>
> >>>>> Is this code even reachable?
> >>>>
> >>>> Yes afaict, supported by the comment ahead of the symbol. syscall_init()
> >>>> puts its address into MSR_CSTAR when !IA32_EMULATION.
> >>>>
> >>>
> >>> What I meant was: can a program actually get itself into 32-bit mode
> >>> to execute a 32-bit SYSCALL instruction?
> >>
> >> Why not? It can set up a 32-bit code segment descriptor, far-branch
> >> into it, and then execute SYSCALL. I can't see anything preventing
> >> this in the logic involved in descriptor adjustment system calls. In
> >> fact it looks to be at least partly the opposite - fill_ldt()
> >> disallows creation of 64-bit code segments (oddly enough
> >> fill_user_desc() then still copies the bit back, despite there
> >> apparently being no way for it to get set).
> >
> > Do we allow creation of 32-bit code segments on !IA32_EMULATION
> > kernels?
>
> As per above - I think so.
>
> >  I think we shouldn't, but I'm not really sure.
>
> It may be a little exotic, but I can't see any reason to disallow
> a 64-bit process to switch to compatibility mode temporarily. One
> contrived use case could be to be able to invoke INTO or BOUND.

I think it should be kept intact for future use by WINE.  WINE is
currently set up so that 32/16-bit Windows emulation needs a 32-bit
build against 32-bit Linux libraries, using the kernel compat layer.
With many distributions wanting to drop 32-bit support this has been a
big sticking point.  If WINE could be modified so that the core is
always built as 64-bit with 32-bit compatibility handled entirely in
userspace, that would remove its dependency on 32-bit Linux libraries
and thus wouldn't require IA32_EMULATION.

--
Brian Gerst