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

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Message-ID: <CALCETrXoK+6gNn=3_yZdkHScd=N-a2f_VPC-svkFfHVsiVusVw@mail.gmail.com>
Date:   Fri, 13 Dec 2019 09:49:28 -0800
From:   Andy Lutomirski <luto@...nel.org>
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 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?  I think we shouldn't, but I'm not really sure.

Anyway, this is irrelevant to the patch at hand.

--Andy