On Tue, 4 Dec 2018 09:13:35 +0100 Ingo Molnar <[email protected]> wrote:
> > * Masami Hiramatsu <[email protected]> wrote: > > > Since copy_optimized_instructions() misses to update real RIP > > address while copying several instructions to working buffer, > > it adjusts RIP-relative instruction with wrong RIP address for > > the 2nd and subsequent instructions. > > > > This may break the kernel (like kernel freeze) because > > probed instruction can refer a wrong data. For example, > > putting kprobes on cpumask_next hit this bug. > > > > cpumask_next is normally like below if CONFIG_CPUMASK_OFFSTACK=y > > (in this case nr_cpumask_bits is an alias of nr_cpu_ids) > > > > <cpumask_next>: > > 48 89 f0 mov %rsi,%rax > > 8b 35 7b fb e2 00 mov 0xe2fb7b(%rip),%esi > > # ffffffff82db9e64 <nr_cpu_ids> > > 55 push %rbp > > ... > > > > If we put a kprobe on it and optimized with jump, it becomes > > like this. > > > > e9 95 7d 07 1e jmpq 0xffffffffa000207a > > 7b fb jnp 0xffffffff81f8a2e2 <cpumask_next+2> > > e2 00 loop 0xffffffff81f8a2e9 <cpumask_next+9> > > 55 push %rbp > > > > This shows first 2 "mov" instructions are copied to trampoline > > buffer at 0xffffffffa000207a. Here is the disassembled result. > > (skipped optprobe template instructions) > > > > Dump of assembler code from 0xffffffffa000207a to 0xffffffffa00020ea: > > 54 push %rsp > > ... > > 48 83 c4 08 add $0x8,%rsp > > 9d popfq > > 48 89 f0 mov %rsi,%rax > > 8b 35 82 7d db e2 mov -0x1d24827e(%rip),%esi > > # 0xffffffff82db9e67 <nr_cpu_ids+3> > > > > As it shows, the 2nd mov accesses *(nr_cpu_ids+3) instead of > > *nr_cpu_ids. This leads a kernel freeze because cpumask_next() > > always returns 0 and for_each_cpu() never ended. > > > > Fixing this by adding len correctly to real RIP address while > > copying. > > > > Fixes: 63fef14fc98a ("kprobes/x86: Make insn buffer always ROX and use > > text_poke()") > > Reported-by: Michael Rodin <[email protected]> > > Signed-off-by: Masami Hiramatsu <[email protected]> > > Cc: [email protected] > > --- > > arch/x86/kernel/kprobes/opt.c | 3 ++- > > 1 file changed, 2 insertions(+), 1 deletion(-) > > > > diff --git a/arch/x86/kernel/kprobes/opt.c b/arch/x86/kernel/kprobes/opt.c > > index eaf02f2e7300..e92672b8b490 100644 > > --- a/arch/x86/kernel/kprobes/opt.c > > +++ b/arch/x86/kernel/kprobes/opt.c > > @@ -189,7 +189,8 @@ static int copy_optimized_instructions(u8 *dest, u8 > > *src, u8 *real) > > int len = 0, ret; > > > > while (len < RELATIVEJUMP_SIZE) { > > - ret = __copy_instruction(dest + len, src + len, real, &insn); > > + ret = __copy_instruction(dest + len, src + len, real + len, > > + &insn); > > I applied this, except that I unbroke the line: please ignore checkpatch > in such cases where the cure is worse than the disease ... Thanks Ingo! > > I.e. even if slightly over 80 cols, this is more readable: > > ret = __copy_instruction(dest + len, src + len, real + len, > &insn); Got it. BTW, this ugry "+ len" repeat would be avoided. I would better make a new inline function to wrap it. Thank you, -- Masami Hiramatsu <[email protected]>
