TLDRVuln - Cybersecurity Vulnerabilities Made Simple

In the Linux kernel, the following vulnerability has been resolved:

LoongArch: Define the __io_aw() hook as mmiowb()

Commit fb24ea52f78e0d595852e ("drivers: Remove explicit invocations of

mmiowb()") remove all mmiowb() in drivers, but it says:

"NOTE: mmiowb() has only ever guaranteed ordering in conjunction with

spin_unlock(). However, pairing each mmiowb() removal in this patch with

the corresponding call to spin_unlock() is not at all trivial, so there

is a small chance that this change may regress any drivers incorrectly

relying on mmiowb() to order MMIO writes between CPUs using lock-free

synchronisation."

The mmio in radeon_ring_commit() is protected by a mutex rather than a

spinlock, but in the mutex fastpath it behaves similar to spinlock. We

can add mmiowb() calls in the radeon driver but the maintainer says he

doesn't like such a workaround, and radeon is not the only example of

mutex protected mmio.

So we should extend the mmiowb tracking system from spinlock to mutex,

and maybe other locking primitives. This is not easy and error prone, so

we solve it in the architectural code, by simply defining the __io_aw()

hook as mmiowb(). And we no longer need to override queued_spin_unlock()

so use the generic definition.

Without this, we get such an error when run 'glxgears' on weak ordering

architectures such as LoongArch:

radeon 0000:04:00.0: ring 0 stalled for more than 10324msec

radeon 0000:04:00.0: ring 3 stalled for more than 10240msec

radeon 0000:04:00.0: GPU lockup (current fence id 0x000000000001f412 last fence id 0x000000000001f414 on ring 3)

radeon 0000:04:00.0: GPU lockup (current fence id 0x000000000000f940 last fence id 0x000000000000f941 on ring 0)