Re: [PATCH v2 2/7] powerpc/kprobes: Mark newly allocated probes as RO

[Russell Currey <ruscur@russell.cc>]

On Fri, 2020-01-31 at 13:34 +0000, Christophe Leroy wrote:
> 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.
> 
> The memcpy() would fail if >1 probes were allocated, so use
> patch_instruction() instead which is safe for RO.
> 
> Reviewed-by: Daniel Axtens <dja@axtens.net>
> Signed-off-by: Russell Currey <ruscur@russell.cc>
> Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
> ---
> v2: removed the redundant flush
> ---
>  arch/powerpc/kernel/kprobes.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/powerpc/kernel/kprobes.c
> b/arch/powerpc/kernel/kprobes.c
> index 2d27ec4feee4..d3e594e6094c 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);
> @@ -124,13 +125,12 @@ int arch_prepare_kprobe(struct kprobe *p)
>  	}
>  
>  	if (!ret) {
> -		memcpy(p->ainsn.insn, p->addr,
> -				MAX_INSN_SIZE *
> sizeof(kprobe_opcode_t));
> +		patch_instruction(p->ainsn.insn, *p->addr);
>  		p->opcode = *p->addr;
> -		flush_icache_range((unsigned long)p->ainsn.insn,
> -			(unsigned long)p->ainsn.insn +
> sizeof(kprobe_opcode_t));
>  	}
>  
> +	set_memory_ro((unsigned long)p->ainsn.insn, 1);
> +


Since this can be called multiple times on the same page, can avoid by
implementing:

void *alloc_insn_page(void)
{
	void *page;

	page = vmalloc_exec(PAGE_SIZE);
	if (page)
		set_memory_ro((unsigned long)page, 1);

	return page;
}

Which is pretty much the same as what's in arm64.  Works for me and
passes ftracetest, I was originally doing this but cut it because it
broke with the memcpy, but works with patch_instruction().

>  	p->ainsn.boostable = 0;
>  	return ret;
>  }