On Fri, 14 Sep 2018 11:14:11 +1000
Michael Neuling <mi...@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.
>
> cc: sta...@vger.kernel.org # 4.13+
> Signed-off-by: Michael Neuling <mi...@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.
>
> v4:
> Feedback from Christophe Leroy:
> - init_mem_free -> init_mem_is_free
> - prlog %lx -> %px
>
> v3:
> Add init_mem_free flag to avoid potential race.
> Feedback from Christophe Leroy:
> - use init_section_contains()
> - change order of init test for performance
> - use pr_debug()
> - remove blank line
>
> v2:
> Print when we skip an address
> ---
> arch/powerpc/include/asm/setup.h | 1 +
> arch/powerpc/lib/code-patching.c | 6 ++++++
> arch/powerpc/mm/mem.c | 2 ++
> 3 files changed, 9 insertions(+)
>
> diff --git a/arch/powerpc/include/asm/setup.h
> b/arch/powerpc/include/asm/setup.h
> index 1a951b0046..1fffbba8d6 100644
> --- a/arch/powerpc/include/asm/setup.h
> +++ b/arch/powerpc/include/asm/setup.h
> @@ -9,6 +9,7 @@ extern void ppc_printk_progress(char *s, unsigned short hex);
>
> extern unsigned int rtas_data;
> extern unsigned long long memory_limit;
> +extern bool init_mem_is_free;
> extern unsigned long klimit;
> extern void *zalloc_maybe_bootmem(size_t size, gfp_t mask);
>
> diff --git a/arch/powerpc/lib/code-patching.c
> b/arch/powerpc/lib/code-patching.c
> index 850f3b8f4d..6ae2777c22 100644
> --- a/arch/powerpc/lib/code-patching.c
> +++ b/arch/powerpc/lib/code-patching.c
> @@ -28,6 +28,12 @@ static int __patch_instruction(unsigned int *exec_addr,
> unsigned int instr,
> {
> int err;
>
> + /* Make sure we aren't patching a freed init section */
> + if (init_mem_is_free && init_section_contains(exec_addr, 4)) {
> + pr_debug("Skipping init section patching addr: 0x%px\n",
> exec_addr);
> + return 0;
> + }
What we should do is a whitelist, make sure it's only patching the
sections we want it to.
That's a bigger job when you consider modules and things too though,
so this looks good for now. Thanks,
Reviewed-by: Nicholas Piggin <npig...@gmail.com>
