kprobes/x86: Fix instruction patching corruption when copying more than one RIP-relative instruction

After copy_optimized_instructions() copies several instructions
to the working buffer it tries to fix up the real RIP address, but it
adjusts the RIP-relative instruction with an incorrect RIP address
for the 2nd and subsequent instructions due to a bug in the logic.

This will break the kernel pretty badly (with likely outcomes such as
a kernel freeze, a crash, or worse) because probed instructions can refer
to the wrong data.

For example putting kprobes on cpumask_next() typically hits 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 it gets jump-optimized, it gets
patched by the kprobes code like this:

 <cpumask_next>:
	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 that the first two MOV instructions were copied to a
trampoline buffer at 0xffffffffa000207a.

Here is the disassembled result of the trampoline, skipping
the optprobe template instructions:

	# Dump of assembly 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>

This dump shows that the second MOV accesses *(nr_cpu_ids+3) instead of
the original *nr_cpu_ids. This leads to a kernel freeze because
cpumask_next() always returns 0 and for_each_cpu() never ends.

Fix this by adding 'len' correctly to the real RIP address while
copying.

[ mingo: Improved the changelog. ]
Reported-by: Michael Rodin <michael@rodin.online>
Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Reviewed-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Cc: Arnaldo Carvalho de Melo <acme@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ravi Bangoria <ravi.bangoria@linux.ibm.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: stable@vger.kernel.org # v4.15+
Fixes: 63fef14f ("kprobes/x86: Make insn buffer always ROX and use text_poke()")
Link: http://lkml.kernel.org/r/153504457253.22602.1314289671019919596.stgit@devboxSigned-off-by: Ingo Molnar <mingo@kernel.org>