Messages in this thread |  | | Subject | Re: [PATCH] x86-64/entry: add instruction suffix to SYSRET | From | Jan Beulich <> | Date | Mon, 16 Dec 2019 11:11:51 +0100 |
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On 13.12.2019 18:49, Andy Lutomirski wrote: > On Fri, Dec 13, 2019 at 1:55 AM Jan Beulich <jbeulich@suse.com> wrote: >> >> On 12.12.2019 22:43, Andy Lutomirski wrote: >>> On Tue, Dec 10, 2019 at 7:40 AM Jan Beulich <jbeulich@suse.com> wrote: >>>> >>>> On 10.12.2019 16:29, Andy Lutomirski wrote: >>>>>> On Dec 10, 2019, at 2:48 AM, Jan Beulich <JBeulich@suse.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@suse.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.
Jan
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