From: Masami Hiramatsu mhiramat@kernel.org
commit ee6a7354a3629f9b65bc18dbe393503e9440d6f5 upstream.
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@devbo... Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org
--- arch/x86/include/asm/insn.h | 18 ++++++++++++++++++ arch/x86/kernel/kprobes/core.c | 4 ++++ 2 files changed, 22 insertions(+)
--- a/arch/x86/include/asm/insn.h +++ b/arch/x86/include/asm/insn.h @@ -208,4 +208,22 @@ static inline int insn_offset_immediate( 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 */ --- a/arch/x86/kernel/kprobes/core.c +++ b/arch/x86/kernel/kprobes/core.c @@ -376,6 +376,10 @@ int __copy_instruction(u8 *dest, u8 *src return 0; memcpy(dest, insn.kaddr, length);
+ /* We should not singlestep on the exception masking instructions */ + if (insn_masking_exception(&insn)) + return 0; + #ifdef CONFIG_X86_64 if (insn_rip_relative(&insn)) { s64 newdisp;