On Fri, Jun 27, 2014 at 12:55 PM, Oleg Nesterov [off-list ref] wrote:

On 06/27, Andy Lutomirski wrote:

quoted

On Fri, Jun 27, 2014 at 12:27 PM, Oleg Nesterov [off-list ref] wrote:

quoted

On 06/27, Kees Cook wrote:

quoted

It looks like SMP ARM issues dsb for rmb, which seems a bit expensive.
http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dui0204g/CIHJFGFE.htm

...

I really want to avoid adding anything to the secure_computing()
execution path. :(

I must have missed something but I do not understand your concerns.

__secure_computing() is not trivial, and we are going to execute the
filters. Do you really think rmb() can add the noticeable difference?

Not to mention that we can only get here if we take the slow syscall
enter path due to TIF_SECCOMP...

On my box, with my fancy multi-phase seccomp patches, the total
seccomp overhead for a very short filter is about 13ns.  Adding a full
barrier would add several ns, I think.

I am just curious, does this 13ns overhead include the penalty from the
slow syscall enter path triggered by TIF_SECCOMP ?

quoted

Admittedly, this is x86, not ARM, so comparisons here are completely
bogus.  And that read memory barrier doesn't even need an instruction
on x86.  But still, let's try to keep this fast.

Well, I am not going to insist...

But imo it would be better to make it correct in a most simple way, then
we can optimize this code and see if there is a noticeable difference.

Not only we can change the ordering, we can remove the BUG_ON's and just
accept the fact the __secure_computing() can race with sys_seccomp() from
another thread.

If nothing else, it would be much simpler to discuss this patch if it comes
as a separate change.

Yeah, the way I want to go is to add the rmb() for now (it gets us
"correctness"), and then later deal with any potential performance
problems on ARM at a later time. (At present, I'm unable to perform
any timings on ARM -- maybe next week.)

I will send the v9 series in a moment here...

-Kees

-- 
Kees Cook
Chrome OS Security