linux-kernel - Re: [PATCH v2] powerpc/kprobes: Fix trap address when trap happened in real mode

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Date:   Tue, 18 Feb 2020 20:37:23 +0100
From:   Christophe Leroy <christophe.leroy@....fr>
To:     "Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Larry Finger <Larry.Finger@...inger.net>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Paul Mackerras <paulus@...ba.org>
Cc:     Anil S Keshavamurthy <anil.s.keshavamurthy@...el.com>,
        "David S. Miller" <davem@...emloft.net>,
        linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        stable@...nel.vger.org
Subject: Re: [PATCH v2] powerpc/kprobes: Fix trap address when trap happened
 in real mode



Le 18/02/2020 à 15:55, Naveen N. Rao a écrit :
> Christophe Leroy wrote:
>> When a program check exception happens while MMU translation is
>> disabled, following Oops happens in kprobe_handler() in the following
>> code:
>>
>>         } 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 cleared, return 0 immediately.
>>
>> Reported-by: Larry Finger <Larry.Finger@...inger.net>
>> Fixes: 6cc89bad60a6 ("powerpc/kprobes: Invoke handlers directly")
>> Cc: stable@...r.kernel.org
>> Cc: Naveen N. Rao <naveen.n.rao@...ux.vnet.ibm.com>
>> Cc: Masami Hiramatsu <mhiramat@...nel.org>
>> Signed-off-by: Christophe Leroy <christophe.leroy@....fr>
>>
>> ---
>> 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@....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..673f349662e8 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))
>> +        return 0;
>> +
> 
> Should we also check for MSR_DR? Are there scenarios with ppc32 where 
> MSR_IR is on, but MSR_DR is off?

Yes indeed.

Christophe