linux-kernel - Re: [BUGFIX PATCH -tip] kprobes/x86: Fix to copy RIP relative instruction correctly

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Message-ID: <20181204081335.GB67285@gmail.com>
Date:   Tue, 4 Dec 2018 09:13:35 +0100
From:   Ingo Molnar <mingo@...nel.org>
To:     Masami Hiramatsu <mhiramat@...nel.org>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ravi Bangoria <ravi.bangoria@...ux.ibm.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Michael Rodin <michael@...in.online>,
        linux-kernel@...r.kernel.org
Subject: Re: [BUGFIX PATCH -tip] kprobes/x86: Fix to copy RIP relative
 instruction correctly


* Masami Hiramatsu <mhiramat@...nel.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@...in.online>
> Signed-off-by: Masami Hiramatsu <mhiramat@...nel.org>
> Cc: stable@...r.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