Re: [PATCH] powerpc/kprobes: Fix trap address when trap happened in real mode

On Tue, 18 Feb 2020 06:58:06 +0100
Christophe Leroy [off-list ref] wrote:

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What do you mean by 'there' ? At the entry of kprobe_handler() ?

That's what my patch does, it checks whether MMU is disabled or not. If
it is, it converts the address to a virtual address.

Do you mean kprobe_handler() should bail out early as it does when the
trap happens in user mode ?

Yes, that is what I meant.

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Of course we can do that, I don't know
enough about kprobe to know if kprobe_handler() should manage events
that happened in real-mode or just ignore them. But I tested adding an
event on a function that runs in real-mode, and it (now) works.

So, what should we do really ?

I'm not sure how the powerpc kernel runs in real mode.
But clearly, at least kprobe event can not handle that case because
it tries to access memory by probe_kernel_read(). Unless that function
correctly handles the address translation, I want to prohibit kprobes
on such address.

So what I would like to see is, something like below.

diff --git a/arch/powerpc/kernel/kprobes.c b/arch/powerpc/kernel/kprobes.c
index 2d27ec4feee4..4771be152416 100644
--- a/arch/powerpc/kernel/kprobes.c
+++ b/arch/powerpc/kernel/kprobes.c

@@ -261,7 +261,7 @@ int kprobe_handler(struct pt_regs *regs)
          unsigned int *addr = (unsigned int *)regs->nip;
          struct kprobe_ctlblk *kcb;
   
-       if (user_mode(regs))
+       if (user_mode(regs) || !(regs->msr & MSR_IR))
                  return 0;
   
          /*

With this instead change of my patch, I get an Oops everytime a kprobe
event occurs in real-mode.

This is because kprobe_handler() is now saying 'this trap doesn't belong
to me' for a trap that has been installed by it.

Hmm, on powerpc, kprobes is allowed to probe on the code which runs
in the real mode? I think we should also prohibit it by blacklisting.
(It is easy to add blacklist by NOKPROBE_SYMBOL(func))

Yes, I see a lot of them tagged with _ASM_NOKPROBE_SYMBOL() on PPC64, 
but none on PPC32. I suppose that's missing and have to be added. 

Ah, you are using PPC32. 

Nevertheless, if one symbol has been forgotten in the blacklist, I think 
it is a problem if it generate Oopses.

There is a long history also on x86 to make a blacklist. Anyway, how did
you get this error on PPC32? Somewhere would you like to probe and
it is a real mode function? Or, it happened unexpectedly?

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Or, some parts are possble to run under both real mode and kernel mode?

I don't think so, at least on PPC32

OK, that's a good news. Also, are there any independent section where such
real mode functions are stored? (I can see start_real_trampolines in
sections.h) If that kind of sections are defined, it is easy to make
a blacklist in arch_populate_kprobe_blacklist(). See arch/arm64/kernel/probes/kprobes.c.

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So the 'program check' exception handler doesn't find the owner of the
trap hence generate an Oops.

Even if we don't want kprobe() to proceed with the event entirely
(allthough it works at least for simple events), I'd expect it to fail
gracefully.

Agreed. I thought it was easy to identify real mode code. But if it is
hard, we should apply your first patch and also skip user handlers
if we are in the real mode (and increment missed count).

user handlers are already skipped.

Yes, if you don't put a kprobes on real mode code. However, if user
(accidentally) puts a probe on real mode code, it might call a
user handler?

What do you think about my latest proposal below ? If a trap is 
encoutered in real mode, if checks if the matching virtual address 
corresponds to a valid kprobe. If it is, it skips it. If not, it returns 
0 to tell "it's no me". You are also talking about incrementing the 
missed count. Who do we do that ?

I rather like your first patch. If there is a kprobes, we can not skip
the instruction, because there is an instruction which must be executed.
(or single-skipped, but I'm not sure the emulator works correctly on
real mode)

Thank you,

-- 
Masami Hiramatsu [off-list ref]