Re: [PATCH -tip v2 0/2] kprobes/x86: RO text code bugfix and cleanup

[Masami Hiramatsu <mhiramat@kernel.org>]

On Thu, 17 Aug 2017 11:55:30 +0200
Ingo Molnar <mingo@kernel.org> wrote:

> 
> * Masami Hiramatsu <mhiramat@kernel.org> wrote:
> 
> > Hi,
> > 
> > This series fixes a kprobe-x86 bug related to RO text and
> > cleans up addressof operators.
> > 
> > The first one is an obvious bug that misses to set memory
> > RO when the function fails. And the second one is just a
> > cleanup patch to remove addressof operators ("&") since
> > it is meaningless anymore.
> > 
> > V2 has just a following update:
> >  - [1/2] Use a helper variable instead of using p->ainsn.insn
> >    directly.
> > 
> > Thanks,
> > 
> > ---
> > 
> > Masami Hiramatsu (2):
> >       kprobes/x86: Don't forget to set memory back to RO on failure
> >       kprobes/x86: Remove addressof operators
> > 
> > 
> >  arch/x86/include/asm/kprobes.h |    4 ++--
> >  arch/x86/kernel/kprobes/core.c |   15 +++++++++------
> >  arch/x86/kernel/kprobes/opt.c  |    9 +++++----
> >  3 files changed, 16 insertions(+), 12 deletions(-)
> 
> So I'm totally opposed to the whole approach of modifying the permissions of the 
> kernel text virtual memory area.
> 
> Firstly, it's racy against other kernel subsystems: what happens if some other 
> code patching mechanism does a similar 'mark RWX and then back to RX'? Who 
> provides the synchronization? set_memory_*() certainly does not.

Hmm, this sounds common problem for set_memory_*() users.

> Secondly, it's racy against attackers: if an attacker can time the attack to the 
> time when a kprobe is installed, then the kernel is still vulnerable.

Right, since this is not against for attackers, but for some unexpected memory
corruption bugs. Yes, this is vulnerable against attackers.

> So how about avoiding the problem altogether by patching the kernel not in its 
> virtual text address, but in the direct mappings? Then page permissions won't have 
> to be modified, and the whole solution will be more robust and secure.

So would you mean using text_poke()?
OK, that's a good idea. I'll try to rewrite it again with text_poke().

Thank you!

> 
> Is there anything I'm missing?
> 
> Thanks,
> 
> 	Ingo


-- 
Masami Hiramatsu <mhiramat@kernel.org>