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

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Date:   Fri, 24 Aug 2018 02:16:12 +0900
From:   Masami Hiramatsu <mhiramat@...nel.org>
To:     Ingo Molnar <mingo@...hat.com>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        Masami Hiramatsu <mhiramat@...nel.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: [BUGFIX PATCH -tip] kprobes/x86: Fix to copy RIP relative instruction correctly

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);
 		if (!ret || !can_boost(&insn, src + len))
 			return -EINVAL;
 		len += ret;