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

[Peter Zijlstra]

On Wed, Sep 07, 2022 at 09:55:21AM +0900, Masami Hiramatsu (Google) wrote:
> From: Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>
> 
> Since the CONFIG_RETHUNK and CONFIG_SLS will use INT3 for padding after
> RET instruction, kprobes always failes to check the probed instruction
> boundary by decoding the function body if the probed address is after
> such paddings (Note that some conditional code blocks will be placed
> after RET instruction, 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 There are INT3 just for padding in the function, it doesn't need
> to recover the original instruction.
> 
> To avoid this issue, if kprobe finds an INT3, it gets the address of
> next non-INT3 byte, and search a branch which jumps to the address.
> If there is the branch, these INT3 will be for padding, so it can be
> skipped.
> 
> Signed-off-by: Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>
> Suggested-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Fixes: 15e67227c49a ("x86: Undo return-thunk damage")

I take objection with this Fixes tag.. if anything it should be the SLS
commit that predates this.

  e463a09af2f0 ("x86: Add straight-line-speculation mitigation")