Re: Why do kprobes and uprobes singlestep?

[Masami Hiramatsu]

On Wed, 24 Feb 2021 11:45:10 -0800
Andy Lutomirski <luto@xxxxxxxxxx> wrote:

> On Tue, Feb 23, 2021 at 5:18 PM Masami Hiramatsu <mhiramat@xxxxxxxxxx> wrote:
> >
> > On Tue, 23 Feb 2021 15:24:19 -0800
> > Andy Lutomirski <luto@xxxxxxxxxx> wrote:
> >
> > > A while back, I let myself be convinced that kprobes genuinely need to
> > > single-step the kernel on occasion, and I decided that this sucked but
> > > I could live with it.  it would, however, be Really Really Nice (tm)
> > > if we could have a rule that anyone running x86 Linux who single-steps
> > > the kernel (e.g. kgdb and nothing else) gets to keep all the pieces
> > > when the system falls apart around them.  Specifically, if we don't
> > > allow kernel single-stepping and if we suitably limit kernel
> > > instruction breakpoints (the latter isn't actually a major problem),
> > > then we don't really really need to use IRET to return to the kernel,
> > > and that means we can avoid some massive NMI nastiness.
> >
> > Would you mean using "pop regs + popf + ret" instead of IRET after
> > int3 handled for avoiding IRET releasing the NMI mask? Yeah, it is
> > possible. I don't complain about that.
> 
> Yes, more or less.
> 
> >
> > However, what is the relationship between the IRET and single-stepping?
> > I think we can do same thing in do_debug...
> 
> Because there is no way to single-step without using IRET.  POPF; RET
> will trap after RET and you won't make forward progress.

Ah, indeed. "POPF; RET" is not atomically exceute.

> > > But I was contemplating the code, and I'm no longer convinced.
> > > Uprobes seem to single-step user code for no discernable reason.
> > > (They want to trap after executing an out of line instruction, AFAICT.
> > > Surely INT3 or even CALL after the out-of-line insn would work as well
> > > or better.)  Why does kprobe single-step?  I spend a while staring at
> > > the code, and it was entirely unclear to me what the purpose of the
> > > single-step is.
> >
> > For kprobes, there are 2 major reasons for (still relaying on) single stepping.
> > One is to provide post_handler, another is executing the original code,
> > which is replaced by int3, without modifying code nor emulation.
> 
> I don't follow.  Suppose we execute out of line.  If we originally have:
> 
> INSN
> 
> we replace it with:
> 
> INT3
> 
> and we have, out of line:
> 
> INSN [but with displacement modified if it's RIP-relative]
> 
> right now, we single-step the out of line copy.  But couldn't we instead do:
> 
> INSN [but with displacement modified if it's RIP-relative]
> INT3

If the INSN is "jmp +127", it will skip the INT3. So those instructions
must be identified and emulated. We did it already in the arm64 (see commit
7ee31a3aa8f4 ("arm64: kprobes: Use BRK instead of single-step when executing
 instructions out-of-line")), because arm64 already emulated the branch
instructions. I have to check x86 insns can be emulated without side-effects.

> 
> or even
> 
> INSN [but with displacement modified if it's RIP-relative]
> JMP kprobe_post_handler

This needs a sequence of push-regs etc. ;)

> 
> and avoid single-stepping?
> 
> I guess I see the point for CALL, JMP and RET, but it seems like we
> could emulate those cases instead fairly easily.

OK, let's try to do it. I think it should be possible because even in the
current code, resume fixup code (adjust IP register) works only for a few
groups of instructions.

Thank you,

-- 
Masami Hiramatsu <mhiramat@xxxxxxxxxx>