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

[Christophe Leroy]

Hi,

Le 14/02/2020 Ã 14:54, Masami Hiramatsu a ÃcritÂ:
> Hi,
>
> On Fri, 14 Feb 2020 12:47:49 +0000 (UTC)
> Christophe Leroy <christophe.leroy@xxxxxx> 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@xxxxxxxxxxxx>
> > Fixes: 6cc89bad60a6 ("powerpc/kprobes: Invoke handlers directly")
> > Cc: stable@xxxxxxxxxxxxxxx
> > Cc: Naveen N. Rao <naveen.n.rao@xxxxxxxxxxxxxxxxxx>
> > Signed-off-by: Christophe Leroy <christophe.leroy@xxxxxx>
> >
> > ---
> > The bug might have existed even before that commit from Naveen.
> >
> > Signed-off-by: Christophe Leroy <christophe.leroy@xxxxxx>
> > ---
> >   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