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Message-ID: <bc184460-70be-0fe2-0a01-a97ee96652c0@c-s.fr>
Date: Tue, 31 Mar 2020 08:28:04 +0200
From: Christophe Leroy <christophe.leroy@....fr>
To: "Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com>,
Benjamin Herrenschmidt <benh@...nel.crashing.org>,
Michael Ellerman <mpe@...erman.id.au>,
Paul Mackerras <paulus@...ba.org>
Cc: linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH 10/12] powerpc/entry32: Blacklist exception entry points
for kprobe.
Le 31/03/2020 à 08:17, Naveen N. Rao a écrit :
> Christophe Leroy wrote:
>>
>>
>> Le 30/03/2020 à 20:33, Christophe Leroy a écrit :
>>>
>>>
>>> Le 30/03/2020 à 19:08, Naveen N. Rao a écrit :
>>>> Christophe Leroy wrote:
>>>>> kprobe does not handle events happening in real mode.
>>>>>
>>>>> As exception entry points are running with MMU disabled,
>>>>> blacklist them.
>>>>>
>>>>> Signed-off-by: Christophe Leroy <christophe.leroy@....fr>
>>>>> ---
>>>>> arch/powerpc/kernel/entry_32.S | 7 +++++++
>>>>> 1 file changed, 7 insertions(+)
>>>>>
>>>>> diff --git a/arch/powerpc/kernel/entry_32.S
>>>>> b/arch/powerpc/kernel/entry_32.S
>>>>> index 94f78c03cb79..9a1a45d6038a 100644
>>>>> --- a/arch/powerpc/kernel/entry_32.S
>>>>> +++ b/arch/powerpc/kernel/entry_32.S
>>>>> @@ -51,6 +51,7 @@ mcheck_transfer_to_handler:
>>>>> mfspr r0,SPRN_DSRR1
>>>>> stw r0,_DSRR1(r11)
>>>>> /* fall through */
>>>>> +_ASM_NOKPROBE_SYMBOL(mcheck_transfer_to_handler)
>>>>>
>>>>> .globl debug_transfer_to_handler
>>>>> debug_transfer_to_handler:
>>>>> @@ -59,6 +60,7 @@ debug_transfer_to_handler:
>>>>> mfspr r0,SPRN_CSRR1
>>>>> stw r0,_CSRR1(r11)
>>>>> /* fall through */
>>>>> +_ASM_NOKPROBE_SYMBOL(debug_transfer_to_handler)
>>>>>
>>>>> .globl crit_transfer_to_handler
>>>>> crit_transfer_to_handler:
>>>>> @@ -94,6 +96,7 @@ crit_transfer_to_handler:
>>>>> rlwinm r0,r1,0,0,(31 - THREAD_SHIFT)
>>>>> stw r0,KSP_LIMIT(r8)
>>>>> /* fall through */
>>>>> +_ASM_NOKPROBE_SYMBOL(crit_transfer_to_handler)
>>>>> #endif
>>>>>
>>>>> #ifdef CONFIG_40x
>>>>> @@ -115,6 +118,7 @@ crit_transfer_to_handler:
>>>>> rlwinm r0,r1,0,0,(31 - THREAD_SHIFT)
>>>>> stw r0,KSP_LIMIT(r8)
>>>>> /* fall through */
>>>>> +_ASM_NOKPROBE_SYMBOL(crit_transfer_to_handler)
>>>>> #endif
>>>>>
>>>>> /*
>>>>> @@ -127,6 +131,7 @@ crit_transfer_to_handler:
>>>>> .globl transfer_to_handler_full
>>>>> transfer_to_handler_full:
>>>>> SAVE_NVGPRS(r11)
>>>>> +_ASM_NOKPROBE_SYMBOL(transfer_to_handler_full)
>>>>> /* fall through */
>>>>>
>>>>> .globl transfer_to_handler
>>>>> @@ -286,6 +291,8 @@ reenable_mmu:
>>>>> lwz r2, GPR2(r11)
>>>>> b fast_exception_return
>>>>> #endif
>>>>> +_ASM_NOKPROBE_SYMBOL(transfer_to_handler)
>>>>> +_ASM_NOKPROBE_SYMBOL(transfer_to_handler_cont)
>>>>
>>>> These are added after 'reenable_mmu', which is itself not
>>>> blacklisted. Is that intentional?
>>>
>>> Yes I put it as the complete end of the entry part, ie just before
>>> stack_ovf which is a function by itself.
>>>
>>> Note that reenable_mmu is inside an #ifdef CONFIG_TRACE_IRQFLAGS.
>>>
>>> I'm not completely sure where to put the _ASM_NOKPROBE_SYMBOL()s,
>>> that's the reason why I put it close to the symbol itself in my first
>>> series.
>>>
>>> Could you have a look at the code and tell me what looks the most
>>> appropriate as a location to you ?
>>>
>>> https://elixir.bootlin.com/linux/v5.6/source/arch/powerpc/kernel/entry_32.S#L230
>>
>>
>> Ok, thinking about it once more, I guess we have a problem as
>> everything after that reenable_mmu will be visible.
>
> I see that we reach reenable_mmu through a 'rfi' with MSR_KERNEL, which
> seems safe to me. So, I figured it can be probed without issues?
Yes it can. And that's the reason why I didn't blacklist it. However the
4: and 7: which are after reenable_mmu are called from earlier, at a
time we are still in real mode. So I need to do something about that I
guess.
Christophe
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