• Masami Hiramatsu's avatar
    kprobes/x86: Fix instruction patching corruption when copying more than one... · 43a1b0cb
    Masami Hiramatsu authored
    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: default avatarMichael Rodin <michael@rodin.online>
    Signed-off-by: default avatarMasami Hiramatsu <mhiramat@kernel.org>
    Reviewed-by: default avatarSteven 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: default avatarIngo Molnar <mingo@kernel.org>
    43a1b0cb
opt.c 13.3 KB