Re: [PATCH v5 2/5] powerpc/kprobes: Mark newly allocated probes as RO

Michael Ellerman
2019-11-02T10:45:43Z

Russell Currey <ruscur@russell.cc> writes:
> With CONFIG_STRICT_KERNEL_RWX=y and CONFIG_KPROBES=y, there will be one
> W+X page at boot by default. This can be tested with
> CONFIG_PPC_PTDUMP=y and CONFIG_PPC_DEBUG_WX=y set, and checking the
> kernel log during boot.
>
> powerpc doesn't implement its own alloc() for kprobes like other
> architectures do, but we couldn't immediately mark RO anyway since we do
> a memcpy to the page we allocate later. After that, nothing should be
> allowed to modify the page, and write permissions are removed well
> before the kprobe is armed.
>
> Thus mark newly allocated probes as read-only once it's safe to do so.
>
> Signed-off-by: Russell Currey <ruscur@russell.cc>
> ---
> arch/powerpc/kernel/kprobes.c | 3 +++
> 1 file changed, 3 insertions(+)
>
> diff --git a/arch/powerpc/kernel/kprobes.c b/arch/powerpc/kernel/kprobes.c
> index 2d27ec4feee4..2610496de7c7 100644
> --- a/arch/powerpc/kernel/kprobes.c
> +++ b/arch/powerpc/kernel/kprobes.c
> @@ -24,6 +24,7 @@
> #include <asm/sstep.h>
> #include <asm/sections.h>
> #include <linux/uaccess.h>
> +#include <linux/set_memory.h>
>
> DEFINE_PER_CPU(struct kprobe *, current_kprobe) = NULL;
> DEFINE_PER_CPU(struct kprobe_ctlblk, kprobe_ctlblk);
> @@ -131,6 +132,8 @@ int arch_prepare_kprobe(struct kprobe *p)
> (unsigned long)p->ainsn.insn + sizeof(kprobe_opcode_t));
> }
>
> + set_memory_ro((unsigned long)p->ainsn.insn, 1);
> +

That comes from:
p->ainsn.insn = get_insn_slot();

Which ends up in __get_insn_slot() I think. And that looks very much
like it's going to hand out multiple slots per page, which isn't going
to work because you've just marked the whole page RO.

So I would expect this to crash on the 2nd kprobe that's installed. Have
you tested it somehow?

I think this code should just use patch_instruction() rather than
memcpy().

cheers