[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: [PATCH] xen/x86: irq: Avoid a TOCTOU race in pirq_spin_lock_irq_desc()





On 17/08/2020 16:03, Roger Pau Monné wrote:
On Mon, Aug 17, 2020 at 03:39:52PM +0100, Julien Grall wrote:


On 17/08/2020 15:01, Roger Pau Monné wrote:
On Mon, Aug 17, 2020 at 02:14:01PM +0100, Julien Grall wrote:
Hi,

On 17/08/2020 13:46, Roger Pau Monné wrote:
On Fri, Aug 14, 2020 at 08:25:28PM +0100, Julien Grall wrote:
Hi Andrew,

Sorry for the late answer.

On 23/07/2020 14:59, Andrew Cooper wrote:
On 23/07/2020 14:22, Julien Grall wrote:
Hi Jan,

On 23/07/2020 12:23, Jan Beulich wrote:
On 22.07.2020 18:53, Julien Grall wrote:
--- a/xen/arch/x86/irq.c
+++ b/xen/arch/x86/irq.c
@@ -1187,7 +1187,7 @@ struct irq_desc *pirq_spin_lock_irq_desc(
            for ( ; ; )
          {
-        int irq = pirq->arch.irq;
+        int irq = read_atomic(&pirq->arch.irq);

There we go - I'd be fine this way, but I'm pretty sure Andrew
would want this to be ACCESS_ONCE(). So I guess now is the time
to settle which one to prefer in new code (or which criteria
there are to prefer one over the other).

I would prefer if we have a single way to force the compiler to do a
single access (read/write).

Unlikely to happen, I'd expect.

But I would really like to get rid of (or at least rename)
read_atomic()/write_atomic() specifically because they've got nothing to
do with atomic_t's and the set of functionality who's namespace they share.

Would you be happy if I rename both to READ_ONCE() and WRITE_ONCE()? I would
also suggest to move them implementation in a new header asm/lib.h.

Maybe {READ/WRITE}_SINGLE (to note those should be implemented using a
single instruction)?

The asm volatile statement contains only one instruction, but this doesn't
mean the helper will generate a single instruction.

Well, the access should be done using a single instruction, which is
what we care about when using this helpers.

You may have other instructions to get the registers ready for the access.


ACCESS_ONCE (which also has the _ONCE suffix) IIRC could be
implemented using several instructions, and hence doesn't seem right
that they all have the _ONCE suffix.

The goal here is the same, we want to access the variable *only* once.

Right, but this is not guaranteed by the current implementation of
ACCESS_ONCE AFAICT, as the compiler *might* split the access into two
(or more) instructions, and hence won't be an atomic access anymore?
 From my understanding, at least on GCC/Clang, ACCESS_ONCE() should be atomic
if you are using aligned address and the size smaller than a register size.

Yes, any sane compiler shouldn't split such access, but this is not
guaranteed by the current code in ACCESS_ONCE.
To be sure, your concern here is not about GCC/Clang but other compilers. Am I correct?

We already have a collection of compiler specific macros in compiler.h. So how about we classify this macro as a compiler specific one? (See more below).



May I ask why we would want to expose the difference to the user?

I'm not saying we should, but naming them using the _ONCE suffix seems
misleading IMO, as they have different guarantees than what
ACCESS_ONCE currently provides.

Lets leave aside how ACCESS_ONCE() is implemented for a moment.

If ACCESS_ONCE() doesn't guarantee atomicy, then it means you may read a mix
of the old and new value. This would most likely break quite a few of the
users because the result wouldn't be coherent.

Do you have place in mind where the non-atomicity would be useful?

Not that I'm aware, I think they could all be safely switched to use
the atomic variants
There is concern that read_atomic(), write_atomic() prevent the compiler to do certain optimization. Andrew gave the example of:

ACCESS_ONCE(...) |= ...


In fact I wouldn't be surprised if users of ACCESS_ONCE break if the
access was split into multiple instructions.

My comment was to notice that just renaming the atomic read/write
helpers to use the _ONCE prefix is IMO weird as they offer different
properties than ACCESS_ONCE, and hence might confuse users.Just
looking at READ_ONCE users could assume all _ONCE helpers would
guarantee atomicity, which is not the case.

Our implementation of ACCESS_ONCE() is very similar to what Linux used to have. There READ_ONCE()/WRITE_ONCE() are also using the same principles.

From my understanding, you can safely assume the access will be atomic if the following conditions are met:
        - The address is correctly size
        - The size is smaller than the word machine size

I would agree this may not be correct on all the existing compilers. But this macro could easily be re-implemented if we add support for a compiler with different guarantee.

Therefore, I fail to see why we can't use the same guarantee in Xen.

Cheers,

--
Julien Grall



 


Rackspace

Lists.xenproject.org is hosted with RackSpace, monitoring our
servers 24x7x365 and backed by RackSpace's Fanatical Support®.