[PATCH 1/5] powerpc/kprobes: Fix validation of prefixed instructions across page boundary

["Naveen N. Rao" <naveen.n.rao@linux.vnet.ibm.com>]

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, preventing probing on such
instructions is not worthwhile.

Cc: stable@vger.kernel.org # v5.8+
Fixes: b4657f7650babc ("powerpc/kprobes: Don't allow breakpoints on suffixes")
Reported-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
---
 arch/powerpc/kernel/kprobes.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/powerpc/kernel/kprobes.c b/arch/powerpc/kernel/kprobes.c
index 01ab2163659e4b..f611d9eb3562d7 100644
--- a/arch/powerpc/kernel/kprobes.c
+++ b/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;
 	}
-- 
2.30.2