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

On Tue, May 18, 2021 at 08:43:39PM +0200, Christophe Leroy wrote:

Le 06/05/2020 à 05:40, Jordan Niethe a écrit :

quoted

Do not allow inserting breakpoints on the suffix of a prefix instruction
in kprobes.

Signed-off-by: Jordan Niethe <redacted>
---
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.

quoted

  	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

quoted

+		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