Re: [PATCH v2 1/4] KVM: arm64: Forbid kprobing of the VHE world-switch code

[Christoffer Dall <christoffer.dall@arm.com>]

On Thu, Jan 31, 2019 at 06:53:06PM +0000, James Morse wrote:
> Hey Christoffer,
> 
> On 31/01/2019 08:08, Christoffer Dall wrote:
> > On Thu, Jan 24, 2019 at 04:32:54PM +0000, James Morse wrote:
> >> On systems with VHE the kernel and KVM's world-switch code run at the
> >> same exception level. Code that is only used on a VHE system does not
> >> need to be annotated as __hyp_text as it can reside anywhere in the
> >> kernel text.
> >>
> >> __hyp_text was also used to prevent kprobes from patching breakpoint
> >> instructions into this region, as this code runs at a different
> >> exception level. While this is no longer true with VHE, KVM still
> >> switches VBAR_EL1, meaning a kprobe's breakpoint executed in the
> >> world-switch code will cause a hyp-panic.
> > 
> > Forgive potentially very stupid questions here, but:
> > 
> >  (1) Would it make sense to move the save/restore VBAR_EL1 to the last
> >      possible moment, and would that actually allow kprobes to work for
> >      the world-switch code, or does that just result in other weird
> >      problems?
> 
> This would work for taking the debug exception. But next kprobes wants to
> single-step the probed instruction in an out-of-line slot. I don't think we can
> do this if we've already configured the debug hardware for the guest.
> (If could at least turn single-step off when we return to guest-EL0, which
> guest-EL1 was single-stepping)
> 
> 

I suspected something like that, let's not go there.

> >  (2) Are we sure that this catches every call path of every non-inlined
> >      function called after switchign VBAR_EL1?  Can kprobes only be
> >      called on exported symbols, or can you (if you know the address
> >      somehow) put a kprobe on a static function as well.  If there are
> >      any concerns in this area, we might want to consider (1) more
> >      closely.
> 
> Hmmm, good question. The blacklisting applies to whole symbols as seen by
> kallsyms, the compiler has no idea what is going on.
> 
> If it chose not to inline something, it would be kprobe'able yes.
> 
> __kprobes uses a section function-attribute instead. The gcc manual[0] doesn't
> say what happens when inline and the section attributes are used together. (or
> at least I couldn't find it)
> 
> A quick experiment with gcc 8.2.0 shows adding __kprobes on the inlines gets
> discarded when they are inlined. I'm not sure how to trick the compiler into
> not-inlining it to see what happens, but adding 'noinline' to the header file
> causes it to duplicate the function everywhere, but puts it in the __kprobes
> section.
> 
> (For KVM we could use the 'flatten' attribute, but that does say 'if possible'.
> Alternatively we can decorate all the inline helpers we know we use with
> __kprobes as a safety net.)
> 
> I think this is a wider problem with kprobes.
> 

Sounds like it.  Probably in the "you did something crazy, and your
kernel is going to suffer from it" category.

Let's stick to your approach.

Thanks for the explanation.

    Christoffer

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