Re: [PATCH v8 2/7] powerpc/kprobes: Mark newly allocated probes as RO

[Russell Currey <ruscur@russell.cc>]

On Fri, 2020-04-03 at 15:06 +0530, Naveen N. Rao wrote:
> Russell Currey wrote:
> > On Fri, 2020-04-03 at 00:18 +0530, Naveen N. Rao wrote:
> > > Naveen N. Rao wrote:
> > > > Russell Currey 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>
> > > > > ---
> > > > >  arch/powerpc/kernel/kprobes.c | 17 +++++++++++++----
> > > > >  1 file changed, 13 insertions(+), 4 deletions(-)
> > > > > 
> > > > > diff --git a/arch/powerpc/kernel/kprobes.c
> > > > > b/arch/powerpc/kernel/kprobes.c
> > > > > index 81efb605113e..fa4502b4de35 100644
> > > > > --- a/arch/powerpc/kernel/kprobes.c
> > > > > +++ b/arch/powerpc/kernel/kprobes.c
> > > > > @@ -24,6 +24,8 @@
> > > > >  #include <asm/sstep.h>
> > > > >  #include <asm/sections.h>
> > > > >  #include <linux/uaccess.h>
> > > > > +#include <linux/set_memory.h>
> > > > > +#include <linux/vmalloc.h>
> > > > >  
> > > > >  DEFINE_PER_CPU(struct kprobe *, current_kprobe) = NULL;
> > > > >  DEFINE_PER_CPU(struct kprobe_ctlblk, kprobe_ctlblk);
> > > > > @@ -102,6 +104,16 @@ kprobe_opcode_t
> > > > > *kprobe_lookup_name(const
> > > > > char *name, unsigned int offset)
> > > > >  	return addr;
> > > > >  }
> > > > >  
> > > > > +void *alloc_insn_page(void)
> > > > > +{
> > > > > +	void *page = vmalloc_exec(PAGE_SIZE);
> > > > > +
> > > > > +	if (page)
> > > > > +		set_memory_ro((unsigned long)page, 1);
> > > > > +
> > > > > +	return page;
> > > > > +}
> > > > > +
> > > > 
> > > > This crashes for me with KPROBES_SANITY_TEST during the
> > > > kretprobe
> > > > test.  
> > > 
> > > That isn't needed to reproduce this. After bootup, disabling
> > > optprobes 
> > > also shows the crash with kretprobes:
> > > 	sysctl debug.kprobes-optimization=0
> > > 
> > > The problem happens to be with patch_instruction() in 
> > > arch_prepare_kprobe(). During boot, on kprobe init, we register a
> > > probe 
> > > on kretprobe_trampoline for use with kretprobes (see 
> > > arch_init_kprobes()). This results in an instruction slot being 
> > > allocated, and arch_prepare_kprobe() to be called for copying
> > > the 
> > > instruction (nop) at kretprobe_trampoline. patch_instruction()
> > > is 
> > > failing resulting in corrupt instruction which we try to
> > > emulate/single 
> > > step causing the crash.
> > 
> > OK I think I've fixed it, KPROBES_SANITY_TEST passes too.  I'd
> > appreciate it if you could test v9, and thanks again for finding
> > this -
> > very embarrassing bug on my side.
> 
> Great! Thanks.
> 
> I think I should also add appropriate error checking to kprobes' use
> of 
> patch_instruction() which would have caught this much more easily.

Only kind of!  It turns out that if the initial setup fails for
KPROBES_SANITY_TEST, it silently doesn't run - so you miss the "Kprobe
smoke test" text, but you don't get any kind of error either.  I'll
send a patch so that it fails more loudly later.

> 
> On a related note, I notice that x86 seems to prefer not having any
> RWX 
> pages, and so they continue to do 'module_alloc()' followed by 
> 'set_memory_ro()' and then 'set_memory_x()'. Is that something worth 
> following for powerpc?

I just noticed that too.  arm64 doesn't set theirs executable, as far
as I can tell powerpc doesn't need to.

> 
> - Naveen
>