[PATCH] powerpc: Avoid code patching freed init sections

[Michal Suchánek]

On Mon, 10 Sep 2018 15:44:05 +1000
Michael Neuling <mikey at neuling.org> wrote:

> This stops us from doing code patching in init sections after they've
> been freed.
> 
> In this chain:
>   kvm_guest_init() ->
>     kvm_use_magic_page() ->
>       fault_in_pages_readable() ->
> 	 __get_user() ->
> 	   __get_user_nocheck() ->
> 	     barrier_nospec();
> 
> We have a code patching location at barrier_nospec() and
> kvm_guest_init() is an init function. This whole chain gets inlined,
> so when we free the init section (hence kvm_guest_init()), this code
> goes away and hence should no longer be patched.
> 
> We seen this as userspace memory corruption when using a memory
> checker while doing partition migration testing on powervm (this
> starts the code patching post migration via
> /sys/kernel/mobility/migration). In theory, it could also happen when
> using /sys/kernel/debug/powerpc/barrier_nospec.
> 
> With this patch there is a small change of a race if we code patch
> between the init section being freed and setting SYSTEM_RUNNING (in
> kernel_init()) but that seems like an impractical time and small
> window for any code patching to occur.
> 
> cc: stable at vger.kernel.org # 4.13+
> Signed-off-by: Michael Neuling <mikey at neuling.org>
> 
> ---
> For stable I've marked this as v4.13+ since that's when we refactored
> code-patching.c but it could go back even further than that. In
> reality though, I think we can only hit this since the first
> spectre/meltdown changes.

Which means it affects all maintained stable trees because the
spectre/meltdown changes are backported.

Thanks

Michal