On 1/27/24 19:19, Masami Hiramatsu (Google) wrote:

On Fri, 26 Jan 2024 22:41:23 -0600
Jinghao Jia [off-list ref] wrote:

quoted

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 :)

quoted

Suggested-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Signed-off-by: Jinghao Jia <redacted>
---
 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.

quoted

+		/* 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

quoted

 	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