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

[Masami Hiramatsu <mhiramat@kernel.org>]

On Thu, 23 Aug 2018 21:41:09 -0400
Steven Rostedt <rostedt@goodmis.org> wrote:

> On Fri, 24 Aug 2018 02:16:12 +0900
> Masami Hiramatsu <mhiramat@kernel.org> wrote:
> 
> > 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.
> 
> Ouch! Nice catch.
> 
> > 
> > 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 <mhiramat@kernel.org>
> > Cc: stable@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);
> >  		if (!ret || !can_boost(&insn, src + len))
> >  			return -EINVAL;
> >  		len += ret;
> 
> Looking at the change that broke this we have:
> 
> > -static int copy_optimized_instructions(u8 *dest, u8 *src)
> > +static int copy_optimized_instructions(u8 *dest, u8 *src, u8 *real)
> >  {
> >         struct insn insn;
> >         int len = 0, ret;
> >  
> >         while (len < RELATIVEJUMP_SIZE) {
> > -               ret = __copy_instruction(dest + len, src + len, &insn);
> > +               ret = __copy_instruction(dest + len, src + len, real, &insn);
> 
> Where "real" was added as a parameter to __copy_instruction. Note that
> we pass in "dest + len" but not "real + len" as you patch fixes.
> __copy_instruction was changed by the bad commit with:
> 
> > -int __copy_instruction(u8 *dest, u8 *src, struct insn *insn)
> > +int __copy_instruction(u8 *dest, u8 *src, u8 *real, struct insn *insn)
> >  {
> >         kprobe_opcode_t buf[MAX_INSN_SIZE];
> >         unsigned long recovered_insn =
> > @@ -387,11 +388,11 @@ int __copy_instruction(u8 *dest, u8 *src, struct insn *insn)
> >                  * have given.
> >                  */
> >                 newdisp = (u8 *) src + (s64) insn->displacement.value
> > -                         - (u8 *) dest;
> > +                         - (u8 *) real;
> 
> "real" replaces "dest", which was the first parameter to __copy_instruction.
> 
> >                         return 0;
> 
> And:
> 
> >  int arch_prepare_optimized_kprobe(struct optimized_kprobe *op,
> >                                   struct kprobe *__unused)
> >  {
> > -       u8 *buf;
> > -       int ret;
> > +       u8 *buf = NULL, *slot;
> > +       int ret, len;
> >         long rel;
> >  
> >         if (!can_optimize((unsigned long)op->kp.addr))
> >                 return -EILSEQ;
> >  
> > -       op->optinsn.insn = get_optinsn_slot();
> > -       if (!op->optinsn.insn)
> > +       buf = kzalloc(MAX_OPTINSN_SIZE, GFP_KERNEL);
> > +       if (!buf)
> >                 return -ENOMEM;
> >  
> > +       op->optinsn.insn = slot = get_optinsn_slot();
> > +       if (!slot) {
> > +               ret = -ENOMEM;
> > +               goto out;
> > +       }
> > +
> >         /*
> >          * Verify if the address gap is in 2GB range, because this uses
> >          * a relative jump.
> >          */
> > -       rel = (long)op->optinsn.insn - (long)op->kp.addr + RELATIVEJUMP_SIZE;
> > +       rel = (long)slot - (long)op->kp.addr + RELATIVEJUMP_SIZE;
> >         if (abs(rel) > 0x7fffffff) {
> > -               __arch_remove_optimized_kprobe(op, 0);
> > -               return -ERANGE;
> > +               ret = -ERANGE;
> > +               goto err;
> >         }
> >  
> > -       buf = (u8 *)op->optinsn.insn;
> 
> "slot" is equivalent to the old "buf".
> 
> > -       set_memory_rw((unsigned long)buf & PAGE_MASK, 1);
> > +       /* Copy arch-dep-instance from template */
> > +       memcpy(buf, &optprobe_template_entry, TMPL_END_IDX);
> >  
> >         /* Copy instructions into the out-of-line buffer */
> > -       ret = copy_optimized_instructions(buf + TMPL_END_IDX, op->kp.addr);
> > -       if (ret < 0) {
> > -               __arch_remove_optimized_kprobe(op, 0);
> > -               return ret;
> > -       }
> > +       ret = copy_optimized_instructions(buf + TMPL_END_IDX, op->kp.addr,
> > +                                         slot + TMPL_END_IDX);
> 
> We pass in "real" as "slot + TMPL_END_IDX" and "dest" as "buf +
> TMPL_END_IDX", thus to make it be equivalent to the code before this
> commit, "real" should have "+ len" added to it in order to be
> equivalent to what was there before.

Right! The broken commit splits trampoline buffer into 
"temporary" destination buffer and "real" trampoline buffer,
and use the "real" address for RIP-relative adjustment.
However, I forgot to introduce update the "real" address
in the copying loop.

> 
> That said...
> 
> Reviewed-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

Thanks!


-- 
Masami Hiramatsu <mhiramat@kernel.org>