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

From: Masami Hiramatsu <[email-masked]>
Date: Wed, 19 Feb 2020 10:01:39 +0900

On Tue, 18 Feb 2020 19:38:27 +0000 (UTC)
Christophe Leroy <[email-masked]> 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 or MSR_DR cleared, return 0 immediately.
> 

Looks good to me.

Reviewed-by: Masami Hiramatsu <[email-masked]>

Thank you!

> Reported-by: Larry Finger <[email-masked]>
> Fixes: 6cc89bad60a6 ("powerpc/kprobes: Invoke handlers directly")
> Cc: [email-masked]
> Cc: Naveen N. Rao <[email-masked]>
> Cc: Masami Hiramatsu <[email-masked]>
> Signed-off-by: Christophe Leroy <[email-masked]>
> 
> ---
> 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 <[email-masked]>
> ---
>  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
> -- 
> 2.25.0
> 

-- 
Masami Hiramatsu <[email-masked]>