[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.
> ---
>  arch/powerpc/lib/code-patching.c | 19 +++++++++++++++++++
>  1 file changed, 19 insertions(+)
> 
> diff --git a/arch/powerpc/lib/code-patching.c
> b/arch/powerpc/lib/code-patching.c index 850f3b8f4d..a2bc08bfd8 100644
> --- a/arch/powerpc/lib/code-patching.c
> +++ b/arch/powerpc/lib/code-patching.c
> @@ -23,11 +23,30 @@
>  #include <asm/code-patching.h>
>  #include <asm/setup.h>
>  
> +
> +static inline bool in_init_section(unsigned int *patch_addr)
> +{
> +	if (patch_addr < (unsigned int *)__init_begin)
> +		return false;
> +	if (patch_addr >= (unsigned int *)__init_end)
> +		return false;
> +	return true;
> +}
> +
> +static inline bool init_freed(void)
> +{
> +	return (system_state >= SYSTEM_RUNNING);
> +}
> +
>  static int __patch_instruction(unsigned int *exec_addr, unsigned int
> instr, unsigned int *patch_addr)
>  {
>  	int err;
>  
> +	/* Make sure we aren't patching a freed init section */
> +	if (in_init_section(patch_addr) && init_freed())
> +		return 0;
> +

Do we even need the init_freed() check?

What user input can we process in init-only code?

Also it would be nice to write the function+offset of the skipped patch
location into the kernel log.

Thanks

Michal