Commit ee6a7354 authored May 09, 2018 by Masami Hiramatsu Committed by Thomas Gleixner May 13, 2018

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 <mhiramat@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Ricardo Neri <ricardo.neri-calderon@linux.intel.com>
Cc: Francis Deslauriers <francis.deslauriers@efficios.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: "H . Peter Anvin" <hpa@zytor.com>
Cc: Yonghong Song <yhs@fb.com>
Cc: Borislav Petkov <bp@suse.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: "David S . Miller" <davem@davemloft.net>
Link: https://lkml.kernel.org/r/152587069574.17316.3311695234863248641.stgit@devbox

parent a466ef76

arch/x86/include/asm/insn.h

+18 −0

Original line number	Diff line number	Diff line
		@@ -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

+4 −0

Original line number	Diff line number	Diff line
		@@ -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)) {

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