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

[Kees Cook]

On Thu, May 11, 2017 at 10:28 PM, Martin Schwidefsky
<schwidefsky@xxxxxxxxxx> wrote:
> On Thu, 11 May 2017 16:44:07 -0700
> Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
>
>> On Thu, May 11, 2017 at 4:17 PM, Thomas Garnier <thgarnie@xxxxxxxxxx> 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.
>
> This is exactly what Heiko did for the s390 backend as a result of this
> discussion. See the _CIF_ASCE_SECONDARY bit in arch/s390/kernel/entry.S,
> for the hot patch the check for the bit is included in the general
> _CIF_WORK test. Only the slow patch gets a bit slower.
>
> git commit b5a882fcf146c87cb6b67c6df353e1c042b8773d
> "s390: restore address space when returning to user space".

If I'm understanding this, it won't catch corruption of addr_limit
during fast-path syscalls, though (i.e. addr_limit changed without a
call to set_fs()). :( This addr_limit corruption is mostly only a risk
archs without THREAD_INFO_IN_TASK, but it would still be nice to catch
unbalanced set_fs() code, so I like the idea. I like getting rid of
addr_limit entirely even more, but that'll take some time. :)

-Kees


-- 
Kees Cook
Pixel Security