Re: [BUGFIX PATCH] kprobes/x86: Fix to set RWX bits correctly before releasing trampoline

[Jessica Yu]

+++ Masami Hiramatsu [26/05/17 09:24 +0900]:
> On Thu, 25 May 2017 19:24:26 +0200
> "Luis R. Rodriguez" <mcgrof@xxxxxxxxxx> wrote:
>
> > On Thu, May 25, 2017 at 07:38:17PM +0900, Masami Hiramatsu wrote:
> > > Fix kprobes to set(recover) RWX bits correctly on trampoline
> > > buffer before releasing it. Releasing readonly page to
> > > module_memfree() crash the kernel.
> > >
> > > Without this fix, if kprobes user register a bunch of kprobes
> > > in function body (since kprobes on function entry usually
> > > use ftrace) and unregister it, kernel hits a BUG and crash.
> > >
> > > Signed-off-by: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
> > > Fixes: d0381c81c2f7 ("kprobes/x86: Set kprobes pages read-only")
> > > ---
> > >  arch/x86/kernel/kprobes/core.c |    9 +++++++++
> > >  kernel/kprobes.c               |    2 +-
> > >  2 files changed, 10 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/arch/x86/kernel/kprobes/core.c b/arch/x86/kernel/kprobes/core.c
> > > index 5b2bbfb..6b87780 100644
> > > --- a/arch/x86/kernel/kprobes/core.c
> > > +++ b/arch/x86/kernel/kprobes/core.c
> > > @@ -52,6 +52,7 @@
> > >  #include <linux/ftrace.h>
> > >  #include <linux/frame.h>
> > >  #include <linux/kasan.h>
> > > +#include <linux/moduleloader.h>
> > >
> > >  #include <asm/text-patching.h>
> > >  #include <asm/cacheflush.h>
> > > @@ -417,6 +418,14 @@ static void prepare_boost(struct kprobe *p, struct insn *insn)
> > >  	}
> > >  }
> > >
> > > +/* Recover page to RW mode before releasing it */
> > > +void free_insn_page(void *page)
> > > +{
> > > +	set_memory_nx((unsigned long)page & PAGE_MASK, 1);
> > > +	set_memory_rw((unsigned long)page & PAGE_MASK, 1);
> > > +	module_memfree(page);
> > > +}
> >
> > Is this needed for all module_memfree() ? If so should / could it just do it
> > for alloc users ?
>
> Hmm, would you mean setting those bits in module_memfree()?
> I think it should be discussed with other users, kmodule, bpf and ftrace.
> It could be, but I'm not so sure about that because setting nx
> timing would be critical for some users. As far as I can see,
> for ftrace and kprobes, that is OK.

Memory does need to be rw before calling module_memfree(), although I
think it might be better leave that responsibility/flexibility to the
callers, instead of blanket calls to set_memory_rw/x. At least in the
case of the module loader, we have finer-grained control of page
protections; not all pages within the module_alloc'd region need
set_memory_rw/x to be called before freeing (see disable_ro_nx() in
module.c).

Jessica