Re: [PATCH v3] powerpc/kprobes: Ignore traps that happened in real mode

From: Michael Ellerman <mpe@ellerman.id.au>
To: Christophe Leroy <christophe.leroy@c-s.fr>
Cc: "linuxppc-dev@lists.ozlabs.org" <linuxppc-dev@lists.ozlabs.org>
Subject: Re: [PATCH v3] powerpc/kprobes: Ignore traps that happened in real mode
Date: Tue, 24 Mar 2020 16:28:42 +1100	[thread overview]
Message-ID: <87o8smxqwl.fsf@mpe.ellerman.id.au> (raw)
In-Reply-To: <f28a0219-abf1-07d4-b98b-b19db4af0f12@c-s.fr>

Christophe Leroy <christophe.leroy@c-s.fr> writes:
> ping
>
>
> Le 18/02/2020 à 20:38, Christophe Leroy a écrit :
>> When a program check exception happens while MMU translation is
>> disabled, following Oops happens in kprobe_handler() in the following
>> code:
>
> Michael, we have several traps in assembly while MMU is still disabled 
> (TRACE_IRQFLAGS, KUAP DEBUG, syscall from kernel, machine check in RTAS, 
> ...).
> Without this fix, all of them trigger an Oops when CONFIG_KPROBE is set.

Only on 32-bit.

But I guess this fix is good, if someone really wants to handle kprobes
in real mode they can tell us and do the work to make it solid.

cheers

>> 		} else if (*addr != BREAKPOINT_INSTRUCTION) {
>> 
>> [   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 ]---
>> 
>> kprobe is not prepared to handle events in real mode and functions
>> running in real mode should have been blacklisted, so kprobe_handler()
>> can safely bail out telling 'this trap is not mine' for any trap that
>> happened while in real-mode.
>> 
>> If the trap happened with MSR_IR or MSR_DR cleared, return 0 immediately.
>> 
>> Reported-by: Larry Finger <Larry.Finger@lwfinger.net>
>> Fixes: 6cc89bad60a6 ("powerpc/kprobes: Invoke handlers directly")
>> Cc: stable@vger.kernel.org
>> Cc: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
>> Cc: Masami Hiramatsu <mhiramat@kernel.org>
>> Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
>> 
>> ---
>> v3: Also bail out if MSR_DR is cleared.
>> 
>> Resending v2 with a more appropriate name
>> 
>> v2: bailing out instead of converting real-time address to virtual and continuing.
>> 
>> 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 | 3 +++
>>   1 file changed, 3 insertions(+)
>> 
>> diff --git a/arch/powerpc/kernel/kprobes.c b/arch/powerpc/kernel/kprobes.c
>> index 2d27ec4feee4..9b340af02c38 100644
>> --- a/arch/powerpc/kernel/kprobes.c
>> +++ b/arch/powerpc/kernel/kprobes.c
>> @@ -264,6 +264,9 @@ int kprobe_handler(struct pt_regs *regs)
>>   	if (user_mode(regs))
>>   		return 0;
>>   
>> +	if (!(regs->msr & MSR_IR) || !(regs->msr & MSR_DR))
>> +		return 0;
>> +
>>   	/*
>>   	 * We don't want to be preempted for the entire
>>   	 * duration of kprobe processing
>> 