Re: [PATCH v8 27/30] powerpc/kprobes: Don't allow breakpoints on suffixes

[Gabriel Paubert <paubert@iram.es>]

On Tue, May 18, 2021 at 08:43:39PM +0200, Christophe Leroy wrote:
> 
> 
> Le 06/05/2020 à 05:40, Jordan Niethe a écrit :
> > Do not allow inserting breakpoints on the suffix of a prefix instruction
> > in kprobes.
> > 
> > Signed-off-by: Jordan Niethe <jniethe5@gmail.com>
> > ---
> > v8: Add this back from v3
> > ---
> >   arch/powerpc/kernel/kprobes.c | 13 +++++++++++++
> >   1 file changed, 13 insertions(+)
> > 
> > diff --git a/arch/powerpc/kernel/kprobes.c b/arch/powerpc/kernel/kprobes.c
> > index 33d54b091c70..227510df8c55 100644
> > --- a/arch/powerpc/kernel/kprobes.c
> > +++ b/arch/powerpc/kernel/kprobes.c
> > @@ -106,7 +106,9 @@ kprobe_opcode_t *kprobe_lookup_name(const char *name, unsigned int offset)
> >   int arch_prepare_kprobe(struct kprobe *p)
> >   {
> >   	int ret = 0;
> > +	struct kprobe *prev;
> >   	struct ppc_inst insn = ppc_inst_read((struct ppc_inst *)p->addr);
> > +	struct ppc_inst prefix = ppc_inst_read((struct ppc_inst *)(p->addr - 1));
> 
> What if p->addr is the first word of a page and the previous page is not mapped ?

IIRC prefixed instructions can't straddle 64 byte boundaries (or was it
128 bytes?), much less page boundaries.

> 
> >   	if ((unsigned long)p->addr & 0x03) {
> >   		printk("Attempt to register kprobe at an unaligned address\n");
> > @@ -114,6 +116,17 @@ int arch_prepare_kprobe(struct kprobe *p)
> >   	} else if (IS_MTMSRD(insn) || IS_RFID(insn) || IS_RFI(insn)) {
> >   		printk("Cannot register a kprobe on rfi/rfid or mtmsr[d]\n");
> >   		ret = -EINVAL;
> > +	} else if (ppc_inst_prefixed(prefix)) {
> 
> If p->addr - 2 contains a valid prefixed instruction, then p->addr - 1
> contains the suffix of that prefixed instruction. Are we sure a suffix can
> never ever be misinterpreted as the prefix of a prefixed instruction ?
> 

Prefixes are easy to decode, the 6 MSB are 0b000001 (from memory).

After some digging on the 'net: "All prefixes have the major opcode 1. A
prefix will never be a valid word instruction. A suffix may be an
existing word instruction or a new instruction."

IOW, detecting prefixes is trivial. It's not x86...

	Gabriel

> 
> > +		printk("Cannot register a kprobe on the second word of prefixed instruction\n");
> > +		ret = -EINVAL;
> > +	}
> > +	preempt_disable();
> > +	prev = get_kprobe(p->addr - 1);
> > +	preempt_enable_no_resched();
> > +	if (prev &&
> > +	    ppc_inst_prefixed(ppc_inst_read((struct ppc_inst *)prev->ainsn.insn))) {
> > +		printk("Cannot register a kprobe on the second word of prefixed instruction\n");
> > +		ret = -EINVAL;
> >   	}
> >   	/* insn must be on a special executable page on ppc64.  This is
> >