* Masami Hiramatsu <mhira...@kernel.org> 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 <michael@rodin.online>
> Signed-off-by: Masami Hiramatsu <mhira...@kernel.org>
> Cc: sta...@vger.kernel.org
> ---
> 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 ...
I.e. even if slightly over 80 cols, this is more readable:
ret = __copy_instruction(dest + len, src + len, real + len,
&insn);
Thanks,
Ingo
