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

[Christophe Leroy]

Le 03/02/2020 Ã 05:50, Russell Currey a ÃcritÂ:
> 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@xxxxxxxxxx>
> > Signed-off-by: Russell Currey <ruscur@xxxxxxxxxx>
> > Signed-off-by: Christophe Leroy <christophe.leroy@xxxxxx>
> > ---
> > 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;
> >   }

Ok. I'll send out v3 as patch 1 fails on PPC64, so I'll take that in.

Christophe