powerpc/kprobes: Fix validation of prefixed instructions across page boundary

When checking if the probed instruction is the suffix of a prefixed
instruction, we access the instruction at the previous word. If the
probed instruction is the very first word of a module, we can end up
trying to access an invalid page.

Fix this by skipping the check for all instructions at the beginning of
a page. Prefixed instructions cannot cross a 64-byte boundary and as
such, we don't expect to encounter a suffix as the very first word in a
page for kernel text. Even if there are prefixed instructions crossing
a page boundary (from a module, for instance), the instruction will be
illegal, so preventing probing on the suffix of such prefix instructions
isn't worthwhile.

Fixes: b4657f76 ("powerpc/kprobes: Don't allow breakpoints on suffixes")
Cc: stable@vger.kernel.org # v5.8+
Reported-by: NChristophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: NNaveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
Signed-off-by: NMichael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/0df9a032a05576a2fa8e97d1b769af2ff0eafbd6.1621416666.git.naveen.n.rao@linux.vnet.ibm.com

arch/powerpc/kernel/kprobes.c

@@ -108,7 +108,6 @@ 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));
 
 	if ((unsigned long)p->addr & 0x03) {
 		printk("Attempt to register kprobe at an unaligned address\n");
@@ -116,7 +115,8 @@ 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)) {
+	} else if ((unsigned long)p->addr & ~PAGE_MASK &&
+		   ppc_inst_prefixed(ppc_inst_read((struct ppc_inst *)(p->addr - 1)))) {
 		printk("Cannot register a kprobe on the second word of prefixed instruction\n");
 		ret = -EINVAL;
 	}