CVE-2021-46990

In the Linux kernel, the following vulnerability has been resolved:
powerpc/64s: Fix crashes when toggling entry flush barrier The entry flush
mitigation can be enabled/disabled at runtime via a debugfs file
(entry_flush), which causes the kernel to patch itself to enable/disable
the relevant mitigations. However depending on which mitigation we’re
using, it may not be safe to do that patching while other CPUs are active.
For example the following crash: sleeper[15639]: segfault (11) at
c000000000004c20 nip c000000000004c20 lr c000000000004c20 Shows that we
returned to userspace with a corrupted LR that points into the kernel, due
to executing the partially patched call to the fallback entry flush (ie. we
missed the LR restore). Fix it by doing the patching under stop machine.
The CPUs that aren’t doing the patching will be spinning in the core of the
stop machine logic. That is currently sufficient for our purposes, because
none of the patching we do is to that code or anywhere in the vicinity.