Expose the state of the RFI flush (enabled/disabled) via debugfs, and
allow it to be enabled/disabled at runtime.

eg: $ cat /sys/kernel/debug/powerpc/rfi_flush
    1
    $ echo 0 > /sys/kernel/debug/powerpc/rfi_flush
    $ cat /sys/kernel/debug/powerpc/rfi_flush
    0

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Reviewed-by: Nicholas Piggin <npiggin@gmail.com>

The recent commit 87590ce6e373 ("sysfs/cpu: Add vulnerability folder")
added a generic folder and set of files for reporting information on
CPU vulnerabilities. One of those was for meltdown:

  /sys/devices/system/cpu/vulnerabilities/meltdown

This commit wires up that file for 64-bit Book3S powerpc.

For now we default to "Vulnerable" unless the RFI flush is enabled.
That may not actually be true on all hardware, further patches will
refine the reporting based on the CPU/platform etc. But for now we
default to being pessimists.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

This causes warnings from cpufreq mutex code. This is also rather
unnecessary and ineffective. If we really want to prevent concurrent
unplug, we could take the unplug read lock but I don't see this being
critical.

Fixes: cd77b5ce208c ("powerpc/powernv/cpufreq: Fix the frequency read by /proc/cpuinfo")
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

Because there may be some performance overhead of the RFI flush, add
kernel command line options to disable it.

We add a sensibly named 'no_rfi_flush' option, but we also hijack the
x86 option 'nopti'. The RFI flush is not the same as KPTI, but if we
see 'nopti' we can guess that the user is trying to avoid any overhead
of Meltdown mitigations, and it means we don't have to educate every
one about a different command line option.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

On some CPUs we can prevent the Meltdown vulnerability by flushing the
L1-D cache on exit from kernel to user mode, and from hypervisor to
guest.

This is known to be the case on at least Power7, Power8 and Power9. At
this time we do not know the status of the vulnerability on other CPUs
such as the 970 (Apple G5), pasemi CPUs (AmigaOne X1000) or Freescale
CPUs. As more information comes to light we can enable this, or other
mechanisms on those CPUs.

The vulnerability occurs when the load of an architecturally
inaccessible memory region (eg. userspace load of kernel memory) is
speculatively executed to the point where its result can influence the
address of a subsequent speculatively executed load.

In order for that to happen, the first load must hit in the L1,
because before the load is sent to the L2 the permission check is
performed. Therefore if no kernel addresses hit in the L1 the
vulnerability can not occur. We can ensure that is the case by
flushing the L1 whenever we return to userspace. Similarly for
hypervisor vs guest.

In order to flush the L1-D cache on exit, we add a section of nops at
each (h)rfi location that returns to a lower privileged context, and
patch that with some sequence. Newer firmwares are able to advertise
to us that there is a special nop instruction that flushes the L1-D.
If we do not see that advertised, we fall back to doing a displacement
flush in software.

For guest kernels we support migration between some CPU versions, and
different CPUs may use different flush instructions. So that we are
prepared to migrate to a machine with a different flush instruction
activated, we may have to patch more than one flush instruction at
boot if the hypervisor tells us to.

In the end this patch is mostly the work of Nicholas Piggin and
Michael Ellerman. However a cast of thousands contributed to analysis
of the issue, earlier versions of the patch, back ports testing etc.
Many thanks to all of them.

Tested-by: Jon Masters <jcm@redhat.com>
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

In the SLB miss handler we may be returning to user or kernel. We need
to add a check early on and save the result in the cr4 register, and
then we bifurcate the return path based on that.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

Similar to the syscall return path, in fast_exception_return we may be
returning to user or kernel context. We already have a test for that,
because we conditionally restore r13. So use that existing test and
branch, and bifurcate the return based on that.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

In the syscall exit path we may be returning to user or kernel
context. We already have a test for that, because we conditionally
restore r13. So use that existing test and branch, and bifurcate the
return based on that.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

This commit does simple conversions of rfi/rfid to the new macros that
include the expected destination context. By simple we mean cases
where there is a single well known destination context, and it's
simply a matter of substituting the instruction for the appropriate
macro.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

When we oops or otherwise call show_regs() we print the address of the
regs structure. Being able to see the address is fairly useful,
firstly to verify that the regs pointer is not completely bogus, and
secondly it allows you to dump the regs and surrounding memory with a
debugger if you have one.

In the normal case the regs will be located somewhere on the stack, so
printing their location discloses no further information than printing
the stack pointer does already.

So switch to %px and print the actual address, not the hashed value.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>