[tip:perf/urgent] kprobes/x86: Fix instruction patching corruption when copying more than one RIP-relative instruction

From: tip-bot for Masami Hiramatsu
Date: Tue Dec 04 2018 - 18:06:18 EST


Commit-ID: 43a1b0cb4cd6dbfd3cd9c10da663368394d299d8
Gitweb: https://git.kernel.org/tip/43a1b0cb4cd6dbfd3cd9c10da663368394d299d8
Author: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
AuthorDate: Fri, 24 Aug 2018 02:16:12 +0900
Committer: Ingo Molnar <mingo@xxxxxxxxxx>
CommitDate: Tue, 4 Dec 2018 09:35:20 +0100

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@xxxxxxxxxxxx>
Signed-off-by: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
Reviewed-by: Steven Rostedt (VMware) <rostedt@xxxxxxxxxxx>
Cc: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Ravi Bangoria <ravi.bangoria@xxxxxxxxxxxxx>
Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx # v4.15+
Fixes: 63fef14fc98a ("kprobes/x86: Make insn buffer always ROX and use text_poke()")
Link: http://lkml.kernel.org/r/153504457253.22602.1314289671019919596.stgit@devbox
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
---
arch/x86/kernel/kprobes/opt.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/kernel/kprobes/opt.c b/arch/x86/kernel/kprobes/opt.c
index 40b16b270656..6adf6e6c2933 100644
--- a/arch/x86/kernel/kprobes/opt.c
+++ b/arch/x86/kernel/kprobes/opt.c
@@ -189,7 +189,7 @@ 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;