Re: [kernel-hardening] Re: [PATCH v9 1/4] syscalls: Verify address limit before returning to user-mode

Thomas Garnier
2017-05-12T17:05:58Z

On Thu, May 11, 2017 at 11:58 PM, Ingo Molnar <mingo@kernel.org> wrote:
>
> * Linus Torvalds <torvalds@linux-foundation.org> wrote:
>
>> On Thu, May 11, 2017 at 4:17 PM, Thomas Garnier <thgarnie@google.com> wrote:
>> >
>> > Ingo: Do you want the change as-is? Would you like it to be optional?
>> > What do you think?
>>
>> I'm not ingo, but I don't like that patch. It's in the wrong place -
>> that system call return code is too timing-critical to add address
>> limit checks.
>>
>> Now what I think you *could* do is:
>>
>> - make "set_fs()" actually set a work flag in the current thread flags
>>
>> - do the test in the slow-path (syscall_return_slowpath).
>>
>> Yes, yes, that ends up being architecture-specific, but it's fairly simple.
>>
>> And it only slows down the system calls that actually use "set_fs()".
>> Sure, it will slow those down a fair amount, but they are hopefully a
>> small subset of all cases.
>>
>> How does that sound to people? Thats' where we currently do that
>>
>> if (IS_ENABLED(CONFIG_PROVE_LOCKING) &&
>> WARN(irqs_disabled(), "syscall %ld left IRQs disabled",
>> regs->orig_ax))
>> local_irq_enable();
>>
>> check too, which is a fairly similar issue.
>
> I really like that idea and I'd be perfectly fine with that solution, because it
> puts the overhead where the problem comes from, and adds an extra incentive for
> code to move away from set_fs() facilities. Win-win.

Great, I will adapt the patch for that.

>
> Thanks,
>
> Ingo

--
Thomas