kprobes/x86: Prohibit probing on exception masking instructions

Since MOV SS and POP SS instructions will delay the exceptions until the
next instruction is executed, single-stepping on it by kprobes must be
prohibited.

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 it is used 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 <[email protected]>
Signed-off-by: Thomas Gleixner <[email protected]>
Cc: Ricardo Neri <[email protected]>
Cc: Francis Deslauriers <[email protected]>
Cc: Oleg Nesterov <[email protected]>
Cc: Alexei Starovoitov <[email protected]>
Cc: Steven Rostedt <[email protected]>
Cc: Andy Lutomirski <[email protected]>
Cc: "H . Peter Anvin" <[email protected]>
Cc: Yonghong Song <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: "David S . Miller" <[email protected]>
Link: https://lkml.kernel.org/r/152587069574.17316.3311695234863248641.stgit@devbox

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 */

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)) {