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

[Jessica Yu <jeyu@redhat.com>]

+++ Masami Hiramatsu [26/05/17 09:24 +0900]:
>On Thu, 25 May 2017 19:24:26 +0200
>"Luis R. Rodriguez" <mcgrof@kernel.org> 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@kernel.org>
>> > 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