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

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Date:   Fri, 13 Dec 2019 10:55:31 +0100
From:   Jan Beulich <jbeulich@...e.com>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     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 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).

> Anyway, the change itself is Acked-by: Andy Lutomirski <luto@...nel.org>
> 
> But let's please clarify the changelog:
> 
> ignore_sysret contains an unsuffixed 'sysret' instruction.  gas
> correctly interprets this as sysretl, but leaving it up to gas to
> guess when there is no register operand that implies a size is bad
> practice, and upstream gas is likely to warn about this in the future.
> Use 'sysretl' explicitly.  This does not change the assembled output.

Fine with me, changed.

Jan