From: Naveen N. Rao <naveen.n.rao@xxxxxxxxxxxxxxxxxx> commit 82123a3d1d5a306fdf50c968a474cc60fe43a80f upstream. 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: b4657f7650ba ("powerpc/kprobes: Don't allow breakpoints on suffixes") Cc: stable@xxxxxxxxxxxxxxx # v5.8+ Reported-by: Christophe Leroy <christophe.leroy@xxxxxxxxxx> Signed-off-by: Naveen N. Rao <naveen.n.rao@xxxxxxxxxxxxxxxxxx> Signed-off-by: Michael Ellerman <mpe@xxxxxxxxxxxxxx> Link: https://lore.kernel.org/r/0df9a032a05576a2fa8e97d1b769af2ff0eafbd6.1621416666.git.naveen.n.rao@xxxxxxxxxxxxxxxxxx Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> --- arch/powerpc/kernel/kprobes.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-) --- 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; }