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

[Xin Li <xin@zytor.com>]

On 1/26/2024 8:41 PM, Jinghao Jia 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.
> 
> Suggested-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
> Signed-off-by: Jinghao Jia <jinghao7@illinois.edu>
> ---
>   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)

s/int/bool

> +{
> +	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 {
> +		/* 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;
> +
>   	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)

-- 
Thanks!
     Xin