4.19-stable review patch. If anyone has any objections, please let me know.
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From: Masami Hiramatsu mhiramat@kernel.org
commit 43a1b0cb4cd6dbfd3cd9c10da663368394d299d8 upstream.
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: 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@kernel.org Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org
--- arch/x86/kernel/kprobes/opt.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)
--- a/arch/x86/kernel/kprobes/opt.c +++ b/arch/x86/kernel/kprobes/opt.c @@ -189,7 +189,7 @@ static int copy_optimized_instructions(u 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;