Messages in this thread | | | Subject | Re: [PATCH RFC v2 4/6] x86: Disable PTI on compatibility mode | From | Nadav Amit <> | Date | Fri, 16 Feb 2018 14:11:32 -0800 |
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Dmitry Safonov <0x7f454c46@gmail.com> wrote:
> 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.
I can allow to switch back and forth by dynamically enabling/disabling PTI. Andy, Dave, do you think it makes it a viable option? Should I respin another version of the patch-set?
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