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Subject: Re: [PATCH] powerpc/kprobes: Fix trap address when trap happened in
 real mode
To:     Masami Hiramatsu <mhiramat@kernel.org>
Cc:     Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Paul Mackerras <paulus@samba.org>,
        Michael Ellerman <mpe@ellerman.id.au>,
        Larry Finger <Larry.Finger@lwfinger.net>,
        "Naveen N. Rao" <naveen.n.rao@linux.vnet.ibm.com>,
        linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org,
        stable@kernel.vger.org,
        Anil S Keshavamurthy <anil.s.keshavamurthy@intel.com>,
        "David S. Miller" <davem@davemloft.net>
References: <b1451438f7148ad0e03306a1f1409f4ad1d6ec7c.1581684263.git.christophe.leroy@c-s.fr>
 <20200214225434.464ec467ad9094961abb8ddc@kernel.org>
From:   Christophe Leroy <christophe.leroy@c-s.fr>
Message-ID: <e09d3c42-542e-48c1-2f1e-cfe605b05bec@c-s.fr>
Date:   Sat, 15 Feb 2020 11:28:49 +0100
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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.

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.

Christophe

> 
> Anyway, Naveen, can you review it?
> 
> Thank you,
> 
>>
>> [   33.098554] BUG: Unable to handle kernel data access on read at 0x0000e268
>> [   33.105091] Faulting instruction address: 0xc000ec34
>> [   33.110010] Oops: Kernel access of bad area, sig: 11 [#1]
>> [   33.115348] BE PAGE_SIZE=16K PREEMPT CMPC885
>> [   33.119540] Modules linked in:
>> [   33.122591] CPU: 0 PID: 429 Comm: cat Not tainted 5.6.0-rc1-s3k-dev-00824-g84195dc6c58a #3267
>> [   33.131005] NIP:  c000ec34 LR: c000ecd8 CTR: c019cab8
>> [   33.136002] REGS: ca4d3b58 TRAP: 0300   Not tainted  (5.6.0-rc1-s3k-dev-00824-g84195dc6c58a)
>> [   33.144324] MSR:  00001032 <ME,IR,DR,RI>  CR: 2a4d3c52  XER: 00000000
>> [   33.150699] DAR: 0000e268 DSISR: c0000000
>> [   33.150699] GPR00: c000b09c ca4d3c10 c66d0620 00000000 ca4d3c60 00000000 00009032 00000000
>> [   33.150699] GPR08: 00020000 00000000 c087de44 c000afe0 c66d0ad0 100d3dd6 fffffff3 00000000
>> [   33.150699] GPR16: 00000000 00000041 00000000 ca4d3d70 00000000 00000000 0000416d 00000000
>> [   33.150699] GPR24: 00000004 c53b6128 00000000 0000e268 00000000 c07c0000 c07bb6fc ca4d3c60
>> [   33.188015] NIP [c000ec34] kprobe_handler+0x128/0x290
>> [   33.192989] LR [c000ecd8] kprobe_handler+0x1cc/0x290
>> [   33.197854] Call Trace:
>> [   33.200340] [ca4d3c30] [c000b09c] program_check_exception+0xbc/0x6fc
>> [   33.206590] [ca4d3c50] [c000e43c] ret_from_except_full+0x0/0x4
>> [   33.212392] --- interrupt: 700 at 0xe268
>> [   33.270401] Instruction dump:
>> [   33.273335] 913e0008 81220000 38600001 3929ffff 91220000 80010024 bb410008 7c0803a6
>> [   33.280992] 38210020 4e800020 38600000 4e800020 <813b0000> 6d2a7fe0 2f8a0008 419e0154
>> [   33.288841] ---[ end trace 5b9152d4cdadd06d ]---
>>
>> Check MSR and convert regs->nip to virtual address if the trap
>> happened with MSR_IR cleared.
>>
>> Reported-by: Larry Finger <Larry.Finger@lwfinger.net>
>> Fixes: 6cc89bad60a6 ("powerpc/kprobes: Invoke handlers directly")
>> Cc: stable@kernel.vger.org
>> Cc: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
>> Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
>>
>> ---
>> The bug might have existed even before that commit from Naveen.
>>
>> Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
>> ---
>>   arch/powerpc/kernel/kprobes.c | 4 ++++
>>   1 file changed, 4 insertions(+)
>>
>> diff --git a/arch/powerpc/kernel/kprobes.c b/arch/powerpc/kernel/kprobes.c
>> index 2d27ec4feee4..f8b848aa65bd 100644
>> --- a/arch/powerpc/kernel/kprobes.c
>> +++ b/arch/powerpc/kernel/kprobes.c
>> @@ -23,6 +23,7 @@
>>   #include <asm/cacheflush.h>
>>   #include <asm/sstep.h>
>>   #include <asm/sections.h>
>> +#include <asm/io.h>
>>   #include <linux/uaccess.h>
>>   
>>   DEFINE_PER_CPU(struct kprobe *, current_kprobe) = NULL;
>> @@ -264,6 +265,9 @@ int kprobe_handler(struct pt_regs *regs)
>>   	if (user_mode(regs))
>>   		return 0;
>>   
>> +	if (!(regs->msr & MSR_IR))
>> +		addr = phys_to_virt(regs->nip);
>> +
>>   	/*
>>   	 * We don't want to be preempted for the entire
>>   	 * duration of kprobe processing
>> -- 
>> 2.25.0
>>
> 
>