Re: [PATCH v2 1/2] x86/kprobes: Fix kprobes instruction boudary check with CONFIG_RETHUNK

Josh Poimboeuf
2022-09-08T05:09:11Z

On Thu, Sep 08, 2022 at 10:34:43AM +0900, Masami Hiramatsu (Google) wrote:
> From: Masami Hiramatsu (Google) <mhiramat@kernel.org>
>
> Since the CONFIG_RETHUNK and CONFIG_SLS will use INT3 for stopping
> speculative execution after RET instruction, kprobes always failes to
> check the probed instruction boundary by decoding the function body if
> the probed address is after such sequence. (Note that some conditional
> code blocks will be placed after function return, if compiler decides
> it is not on the hot path.)
>
> This is because kprobes expects someone (e.g. kgdb) puts the INT3 as
> a software breakpoint and it will replace the original instruction.
> But these INT3 are not such purpose, it doesn't need to recover the
> original instruction.
>
> To avoid this issue, memorize the branch target address during decoding
> and if there is INT3, restart decoding from unchecked target address.

Hm, is kprobes conflicting with kgdb actually a realistic concern?
Seems like a dangerous combination

Either way, this feels overengineered. Sort of like implementing
objtool in the kernel.

And it's incomplete: for a switch statement jump table (or C goto jump
table like in BPF), you can't detect the potential targets of the
indirect branch.

Wouldn't it be much simpler to just encode the knowledge that

if (CONFIG_RETHUNK && !X86_FEATURE_RETHUNK)
// all rets are followed by four INT3s
else if (CONFIG_SLS)
// all rets are followed by one INT3

?

--
Josh