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

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Message-Id: <1582037375.4mkd6m1m5m.naveen@linux.ibm.com>
Date:   Tue, 18 Feb 2020 20:25:00 +0530
From:   "Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com>
To:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Christophe Leroy <christophe.leroy@....fr>,
        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

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?


- Naveen