From: Masami Hiramatsu <mhiramat@kernel•org>
To: Christophe Leroy <christophe.leroy@c-s•fr>
Cc: linux-kernel@vger•kernel.org,
Anil S Keshavamurthy <anil.s.keshavamurthy@intel•com>,
Paul Mackerras <paulus@samba•org>,
stable@kernel•vger.org,
"Naveen N. Rao" <naveen.n.rao@linux•vnet.ibm.com>,
linuxppc-dev@lists•ozlabs.org,
"David S. Miller" <davem@davemloft•net>,
Larry Finger <Larry.Finger@lwfinger•net>
Subject: Re: [PATCH] powerpc/kprobes: Fix trap address when trap happened in real mode
Date: Mon, 17 Feb 2020 19:27:35 +0900 [thread overview]
Message-ID: <20200217192735.5070f0925c4159ccffa4e465@kernel.org> (raw)
In-Reply-To: <baee8186-549a-f6cf-3619-884b6d708185@c-s.fr>
On Mon, 17 Feb 2020 10:03:22 +0100
Christophe Leroy <christophe.leroy@c-s•fr> wrote:
>
>
> Le 16/02/2020 à 13:34, Masami Hiramatsu a écrit :
> > On Sat, 15 Feb 2020 11:28:49 +0100
> > Christophe Leroy <christophe.leroy@c-s•fr> wrote:
> >
> >> Hi,
> >>
> >> Le 14/02/2020 à 14:54, Masami Hiramatsu a écrit :
> >>> Hi,
> >>>
> >>> On Fri, 14 Feb 2020 12:47:49 +0000 (UTC)
> >>> Christophe Leroy <christophe.leroy@c-s•fr> wrote:
> >>>
> >>>> When a program check exception happens while MMU translation is
> >>>> disabled, following Oops happens in kprobe_handler() in the following
> >>>> test:
> >>>>
> >>>> } else if (*addr != BREAKPOINT_INSTRUCTION) {
> >>>
> >>> Thanks for the report and patch. I'm not so sure about powerpc implementation
> >>> but at where the MMU translation is disabled, can the handler work correctly?
> >>> (And where did you put the probe on?)
> >>>
> >>> Your fix may fix this Oops, but if the handler needs special care, it is an
> >>> option to blacklist such place (if possible).
> >>
> >> I guess that's another story. Here we are not talking about a place
> >> where kprobe has been illegitimately activated, but a place where there
> >> is a valid trap, which generated a valid 'program check exception'. And
> >> kprobe was off at that time.
> >
> > Ah, I got it. It is not a kprobe breakpoint, but to check that correctly,
> > it has to know the address where the breakpoint happens. OK.
> >
> >>
> >> As any 'program check exception' due to a trap (ie a BUG_ON, a WARN_ON,
> >> a debugger breakpoint, a perf breakpoint, etc...) calls
> >> kprobe_handler(), kprobe_handler() must be prepared to handle the case
> >> where the MMU translation is disabled, even if probes are not supposed
> >> to be set for functions running with MMU translation disabled.
> >
> > Can't we check the MMU is disabled there (as same as checking the exception
> > happened in user space or not)?
> >
>
> 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.
> 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;
/*
Thank you,
--
Masami Hiramatsu <mhiramat@kernel•org>
next prev parent reply other threads:[~2020-02-17 10:30 UTC|newest]
Thread overview: 16+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-02-14 12:47 [PATCH] powerpc/kprobes: Fix trap address when trap happened in real mode Christophe Leroy
2020-02-14 13:54 ` Masami Hiramatsu
2020-02-15 10:28 ` Christophe Leroy
2020-02-16 12:34 ` Masami Hiramatsu
2020-02-17 9:03 ` Christophe Leroy
2020-02-17 10:27 ` Masami Hiramatsu [this message]
2020-02-17 15:38 ` Christophe Leroy
2020-02-17 17:41 ` Christophe Leroy
2020-02-18 0:44 ` Masami Hiramatsu
2020-02-18 5:58 ` Christophe Leroy
2020-02-18 10:29 ` Masami Hiramatsu
2020-02-18 11:04 ` Christophe Leroy
2020-02-18 12:33 ` Masami Hiramatsu
2020-02-18 13:58 ` Christophe Leroy
2020-02-18 14:06 ` Naveen N. Rao
2020-02-15 10:19 ` Christophe Leroy
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