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

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