Re: [PATCH 1/2 v2] kprobe: Do not use uaccess functions to access kernel memory that can fault

[Steven Rostedt]

On Fri, 15 Feb 2019 15:49:35 -0800
Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:

> Iâm missing most of the context here, but even probe_kernel_...() is
> unwise for a totally untrustworthy address. It could be MMIO, for
> example.

True, but kprobes are used like modules, and only allowed by root. They
are used to poke literally anywhere one wants. That's the entire
purpose of kprobes.

> 
> If needed, we could come up with a safe-ish helper for tracing.  For
> direct-map addresses, probe_kernel_...() is probably okay.  Same for
> the current stack. Otherwise we could walk the page tables and check
> that the address is cacheable, I suppose, although this is slightly
> dubious if we donât also check MTRRs. We could also check that the PA
> is in main memory, I suppose, although this may have unfortunate
> interactions with the MCE code.

I added you just because I wanted help getting the change log correct,
as that's what Linus was complaining about. I kept using "kernel
address" when the sample bug used for the patch was really a
non-canonical address (as Linus said, it's just garbage. Neither kernel
or user space). But I pointed out that this can also bug if the
address is canonical and in the kernel address space. The old code
didn't complain about non-canonical or kernel address faulting before
commit 9da3f2b7405, which only talks about kernel address space
faulting (which is why I only mentioned that in my messages).

Would changing all the mention of "kernel address" to "non user space"
be accurate?

 For reference:

  http://lkml.kernel.org/r/20190215174945.557218316@xxxxxxxxxxx
  http://lkml.kernel.org/r/20190215142015.860423791@xxxxxxxxxxx

-- Steve