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

Andy Lutomirski
2018-02-15T23:30:05Z

On Thu, Feb 15, 2018 at 8:58 PM, Nadav Amit <nadav.amit@gmail.com> wrote:
> Andy Lutomirski <luto@kernel.org> wrote:
>
>> On Thu, Feb 15, 2018 at 4:36 PM, Nadav Amit <namit@vmware.com> wrote:
>>> Based on the understanding that there should be no way for userspace to
>>> address the kernel-space from compatibility mode, disable it while
>>> running in compatibility mode as long as the 64-bit code segment of the
>>> user is not used.
>>>
>>> Reenabling PTI is performed by restoring NX-bits to the userspace
>>> mappings, flushing the TLBs, and notifying all the CPUs that use the
>>> affected mm to disable PTI. Each core responds by removing the present
>>> bit for the 64-bit code-segment, and marking that PTI is disabled on
>>> that core.
>>
>> I dislike this patch because it's conflating two things. The patch
>> claims to merely disable PTI for compat tasks, whatever those are.
>> But it's also introducing a much stronger concept of what a compat
>> task is. The kernel currently mostly doesn't care whether a task is
>> "compat" or not, and I think that most remaining code paths that do
>> care are buggy and should be removed.
>>
>> I think the right way to approach this is to add a new arch_prctl()
>> that changes allowable bitness, like this:
>>
>> arch_prctl(ARCH_SET_ALLOWED_GDT_CS, X86_ALLOW_CS32 | X86_ALLOW_CS64);
>>
>> this would set the current task to work the normal way, where 32-bit
>> and 64-bit CS are available. You could set just X86_ALLOW_CS32 to
>> deny 64-bit mode and just X86_ALLOW_CS64 to deny 32-bit mode. This
>> would make nice attack surface reduction tools for the more paranoid
>> sandbox users to use. Doing arch_prctl(ARCH_SET_ALLOWED_GDT_CS, 0)
>> would return -EINVAL.
>>
>> A separate patch would turn PTI off if you set X86_ALLOW_CS32.
>>
>> This has the downside that old code doesn't get the benefit without
>> some code change, but that's not the end of the world.
>>
>>> +static void pti_cpu_update_func(void *info)
>>> +{
>>> + struct mm_struct *mm = (struct mm_struct *)info;
>>> +
>>> + if (mm != this_cpu_read(cpu_tlbstate.loaded_mm))
>>> + return;
>>> +
>>> + /*
>>> + * Keep CS64 and CPU settings in sync despite potential concurrent
>>> + * updates.
>>> + */
>>> + set_cpu_pti_disable(READ_ONCE(mm->context.pti_disable));
>>> +}
>>
>> I don't like this at all. IMO a sane implementation should never
>> change PTI status on a remote CPU. Just track it per task.
>
> From your comments in the past, I understood that this requirement
> (enabling/disabling PTI per mm) came from Linus. I can do it per task, but
> that means that the NX-bit will always be cleared on the kernel page-table
> for user mappings.

I disagree with Linus here, although I could be convinced. But
there's still no need for an IPI -- even if it were per-mm, PTI could
stay on until the next kernel entry.

>
>>> +void __pti_reenable(void)
>>> +{
>>> + struct mm_struct *mm = current->mm;
>>> + int cpu;
>>> +
>>> + if (!mm_pti_disable(mm))
>>> + return;
>>> +
>>> + /*
>>> + * Prevent spurious page-fault storm while we set the NX-bit and have
>>> + * yet not updated the per-CPU pti_disable flag.
>>> + */
>>> + down_write(&mm->mmap_sem);
>>> +
>>> + if (!mm_pti_disable(mm))
>>> + goto out;
>>> +
>>> + /*
>>> + * First, mark the PTI is enabled. Although we do anything yet, we are
>>> + * safe as long as we do not reenable CS64. Since we did not update the
>>> + * page tables yet, this may lead to spurious page-faults, but we need
>>> + * the pti_disable in mm to be set for __pti_set_user_pgd() to do the
>>> + * right thing. Holding mmap_sem would ensure matter we hold the
>>> + * mmap_sem to prevent them from swamping the system.
>>> + */
>>> + mm->context.pti_disable = PTI_DISABLE_OFF;
>>> +
>>> + /* Second, restore the NX bits. */
>>> + pti_update_user_pgds(mm, true);
>>
>> You're holding mmap_sem, but there are code paths that touch page
>> tables that don't hold mmap_sem, such as the stack extension code.
>
> Do they change user mappings? These are the only ones pti_update_user_pgds()
> changes.

I think they do -- it's the user stack. page_table_lock may be more
appropriate, although I'm far from an expert in this.

>
>>> +
>>> +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?