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

[Masami Hiramatsu]

On Tue, 4 Dec 2018 09:13:35 +0100
Ingo Molnar <mingo@xxxxxxxxxx> wrote:

> 
> * Masami Hiramatsu <mhiramat@xxxxxxxxxx> 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@xxxxxxxxxxxx>
> > Signed-off-by: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
> > Cc: stable@xxxxxxxxxxxxxxx
> > ---
> >  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 <mhiramat@xxxxxxxxxx>