Re: [RFC PATCH 1/2] x86/kprobes: Prohibit kprobing on INT and UD

[Jinghao Jia]

On 1/27/24 19:19, Masami Hiramatsu (Google) wrote:
> On Fri, 26 Jan 2024 22:41:23 -0600
> Jinghao Jia <jinghao7@xxxxxxxxxxxx> wrote:
> 
>> Both INTs (INT n, INT1, INT3, INTO) and UDs (UD0, UD1, UD2) serve
>> special purposes in the kernel, e.g., INT3 is used by KGDB and UD2 is
>> involved in LLVM-KCFI instrumentation. At the same time, attaching
>> kprobes on these instructions (particularly UDs) will pollute the stack
>> trace dumped in the kernel ring buffer, since the exception is triggered
>> in the copy buffer rather than the original location.
>>
>> Check for INTs and UDs in can_probe and reject any kprobes trying to
>> attach to these instructions.
>>
> 
> Thanks for implement this check!
> 

You are very welcome :)

> 
>> Suggested-by: Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>
>> Signed-off-by: Jinghao Jia <jinghao7@xxxxxxxxxxxx>
>> ---
>>  arch/x86/kernel/kprobes/core.c | 33 ++++++++++++++++++++++++++-------
>>  1 file changed, 26 insertions(+), 7 deletions(-)
>>
>> diff --git a/arch/x86/kernel/kprobes/core.c b/arch/x86/kernel/kprobes/core.c
>> index e8babebad7b8..792b38d22126 100644
>> --- a/arch/x86/kernel/kprobes/core.c
>> +++ b/arch/x86/kernel/kprobes/core.c
>> @@ -252,6 +252,22 @@ unsigned long recover_probed_instruction(kprobe_opcode_t *buf, unsigned long add
>>  	return __recover_probed_insn(buf, addr);
>>  }
>>  
>> +static inline int is_exception_insn(struct insn *insn)
>> +{
>> +	if (insn->opcode.bytes[0] == 0x0f) {
>> +		/* UD0 / UD1 / UD2 */
>> +		return insn->opcode.bytes[1] == 0xff ||
>> +		       insn->opcode.bytes[1] == 0xb9 ||
>> +		       insn->opcode.bytes[1] == 0x0b;
>> +	} else {
> 
> If "else" block just return, you don't need this "else".
> 
> bool func()
> {
> 	if (cond)
> 		return ...
> 
> 	return ...
> }
> 
> Is preferrable because this puts "return val" always at the end of non-void
> function.
> 

I will fix this in the v2.

>> +		/* INT3 / INT n / INTO / INT1 */
>> +		return insn->opcode.bytes[0] == 0xcc ||
>> +		       insn->opcode.bytes[0] == 0xcd ||
>> +		       insn->opcode.bytes[0] == 0xce ||
>> +		       insn->opcode.bytes[0] == 0xf1;
>> +	}
>> +}
>> +
>>  /* Check if paddr is at an instruction boundary */
>>  static int can_probe(unsigned long paddr)
>>  {
>> @@ -294,6 +310,16 @@ static int can_probe(unsigned long paddr)
>>  #endif
>>  		addr += insn.length;
>>  	}
>> +	__addr = recover_probed_instruction(buf, addr);
>> +	if (!__addr)
>> +		return 0;
>> +
>> +	if (insn_decode_kernel(&insn, (void *)__addr) < 0)
>> +		return 0;
>> +
>> +	if (is_exception_insn(&insn))
>> +		return 0;
>> +
> 
> Please don't put this outside of decoding loop. You should put these in
> the loop which decodes the instruction from the beginning of the function.
> Since the x86 instrcution is variable length, can_probe() needs to check
> whether that the address is instruction boundary and decodable.
> 
> Thank you,

If my understanding is correct then this is trying to decode the kprobe
target instruction, given that it is after the main decoding loop.  Here I
hoisted the decoding logic out of the if(IS_ENABLED(CONFIG_CFI_CLANG))
block so that we do not need to decode the same instruction twice.  I left
the main decoding loop unchanged so it is still decoding the function from
the start and should handle instruction boundaries. Are there any caveats
that I missed?

--Jinghao

> 
>>  	if (IS_ENABLED(CONFIG_CFI_CLANG)) {
>>  		/*
>>  		 * The compiler generates the following instruction sequence
>> @@ -308,13 +334,6 @@ static int can_probe(unsigned long paddr)
>>  		 * Also, these movl and addl are used for showing expected
>>  		 * type. So those must not be touched.
>>  		 */
>> -		__addr = recover_probed_instruction(buf, addr);
>> -		if (!__addr)
>> -			return 0;
>> -
>> -		if (insn_decode_kernel(&insn, (void *)__addr) < 0)
>> -			return 0;
>> -
>>  		if (insn.opcode.value == 0xBA)
>>  			offset = 12;
>>  		else if (insn.opcode.value == 0x3)
>> -- 
>> 2.43.0
>>
> 
>

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