Re: [PATCH RFC v2 4/6] x86: Disable PTI on compatibility mode

Dmitry Safonov
2018-02-16T22:07:37Z

2018-02-16 7:11 GMT+00:00 Cyrill Gorcunov <gorcunov@gmail.com>:
> On Thu, Feb 15, 2018 at 11:29:42PM +0000, Andy Lutomirski wrote:
> ...
>> >>> +bool pti_handle_segment_not_present(long error_code)
>> >>> +{
>> >>> + if (!static_cpu_has(X86_FEATURE_PTI))
>> >>> + return false;
>> >>> +
>> >>> + if ((unsigned short)error_code != GDT_ENTRY_DEFAULT_USER_CS << 3)
>> >>> + return false;
>> >>> +
>> >>> + pti_reenable();
>> >>> + return true;
>> >>> +}
>> >>
>> >> Please don't. You're trying to emulate the old behavior here, but
>> >> you're emulating it wrong. In particular, you won't trap on LAR.
>> >
>> > Yes, I thought I’ll manage to address LAR, but failed. I thought you said
>> > this is not a “show-stopper”. I’ll adapt your approach of using prctl, although
>> > it really limits the benefit of this mechanism.
>> >
>>
>> It's possible we could get away with adding the prctl but making the
>> default be that only the bitness that matches the program being run is
>> allowed. After all, it's possible that CRIU is literally the only
>> program that switches bitness using the GDT. (DOSEMU2 definitely does
>> cross-bitness stuff, but it uses the LDT as far as I know.) And I've
>> never been entirely sure that CRIU fully counts toward the Linux
>> "don't break ABI" guarantee.
>>
>> Linus, how would you feel about, by default, preventing 64-bit
>> programs from long-jumping to __USER32_CS and vice versa? I think it
>> has some value as a hardening measure. I've certainly engaged in some
>> exploit shenanigans myself that took advantage of the ability to long
>> jump/ret to change bitness at will. This wouldn't affect users of
>> modify_ldt() -- 64-bit programs could still create and use their own
>> private 32-bit segments with modify_ldt(), and seccomp can (and
>> should!) prevent that in sandboxed programs.
>>
>> In general, I prefer an approach where everything is explicit to an
>> approach where we almost, but not quite, emulate the weird historical
>> behavior.
>>
>> Pavel and Cyrill, how annoying would it be if CRIU had to do an extra
>> arch_prctl() to enable its cross-bitness shenanigans when
>> checkpointing and restoring a 32-bit program?
>
> I think this should not be a problem for criu (CC'ing Dima, who has
> been working on compat mode support in criu). As far as I remember
> we initiate restoring of 32 bit tasks in native 64 bit mode (well,
> ia32e to be precise :) mode and then, once everything is ready,
> we changing the mode by doing a return to __USER32_CS descriptor.
> So this won't be painful to add additional prctl call here.

Yeah, restoring will still be easy..
But checkpointing will be harder if we can't switch to 64-bit mode.
ATM we have one 64-bit parasite binary, which does all seizing job
for both 64 and 32 bit binaries.
So, if you can't switch back to 64-bit from 32-bit mode, we'll need
to keep two parasites.

--
Dmitry