[PATCH 1/2] kprobes: x86: Prohibit probing on exception masking instructions

Masami Hiramatsu
2018-05-09T12:58:42Z

Since MOV SS and POP SS instructions will delay the exceptions
until the next instruction is executed, we should not do
single-stepping on it by kprobes.

However, kprobes usually executes those instructions directly
on trampoline buffer (a.k.a. kprobe-booster), except for the
kprobes which has post_handler. Thus if kprobe user probes MOV SS
with post_handler, it will do single-stepping on the MOV SS.

This means it is safe that if we use it via ftrace or perf/bpf
since those don't use the post_handler.

Anyway, since the stack switching is a rare case, it is safer
just rejecting kprobes on such instructions.

Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
---
arch/x86/include/asm/insn.h | 18 ++++++++++++++++++
arch/x86/kernel/kprobes/core.c | 4 ++++
2 files changed, 22 insertions(+)

diff --git a/arch/x86/include/asm/insn.h b/arch/x86/include/asm/insn.h
index b3e32b010ab1..c2c01f84df75 100644
--- a/arch/x86/include/asm/insn.h
+++ b/arch/x86/include/asm/insn.h
@@ -208,4 +208,22 @@ static inline int insn_offset_immediate(struct insn *insn)
return insn_offset_displacement(insn) + insn->displacement.nbytes;
}

+#define POP_SS_OPCODE 0x1f
+#define MOV_SREG_OPCODE 0x8e
+
+/*
+ * Intel SDM Vol.3A 6.8.3 states;
+ * "Any single-step trap that would be delivered following the MOV to SS
+ * instruction or POP to SS instruction (because EFLAGS.TF is 1) is
+ * suppressed."
+ * This function returns true if @insn is MOV SS or POP SS. On these
+ * instructions, single stepping is suppressed.
+ */
+static inline int insn_masking_exception(struct insn *insn)
+{
+ return insn->opcode.bytes[0] == POP_SS_OPCODE ||
+ (insn->opcode.bytes[0] == MOV_SREG_OPCODE &&
+ X86_MODRM_REG(insn->modrm.bytes[0]) == 2);
+}
+
#endif /* _ASM_X86_INSN_H */
diff --git a/arch/x86/kernel/kprobes/core.c b/arch/x86/kernel/kprobes/core.c
index 0715f827607c..6f4d42377fe5 100644
--- a/arch/x86/kernel/kprobes/core.c
+++ b/arch/x86/kernel/kprobes/core.c
@@ -370,6 +370,10 @@ int __copy_instruction(u8 *dest, u8 *src, u8 *real, struct insn *insn)
if (insn->opcode.bytes[0] == BREAKPOINT_INSTRUCTION)
return 0;

+ /* We should not singlestep on the exception masking instructions */
+ if (insn_masking_exception(insn))
+ return 0;
+
#ifdef CONFIG_X86_64
/* Only x86_64 has RIP relative instructions */
if (insn_rip_relative(insn)) {