Re: [PATCH -tip v4 10/12] x86/kprobes: Push a fake return address at kretprobe_trampoline

Masami Hiramatsu
2021-03-25T18:05:03Z

On Wed, 24 Mar 2021 10:40:58 +0900
Masami Hiramatsu <mhiramat@kernel.org> wrote:

> On Tue, 23 Mar 2021 23:30:07 +0100
> Peter Zijlstra <peterz@infradead.org> wrote:
>
> > On Mon, Mar 22, 2021 at 03:41:40PM +0900, Masami Hiramatsu wrote:
> > > ".global kretprobe_trampoline\n"
> > > ".type kretprobe_trampoline, @function\n"
> > > "kretprobe_trampoline:\n"
> > > #ifdef CONFIG_X86_64
> >
> > So what happens if we get an NMI here? That is, after the RET but before
> > the push? Then our IP points into the trampoline but we've not done that
> > push yet.
>
> Not only NMI, but also interrupts can happen. There is no cli/sti here.
>
> Anyway, thanks for pointing!
> I think in UNWIND_HINT_TYPE_REGS and UNWIND_HINT_TYPE_REGS_PARTIAL cases
> ORC unwinder also has to check the state->ip and if it is kretprobe_trampoline,
> it should be recovered.
> What about this?

Hmm, this seems to intoduce another issue on stacktrace from kprobes.

<...>-137 [003] d.Z. 17.250714: p_full_proxy_read_5: (full_proxy_read+0x5/0x80)
<...>-137 [003] d.Z. 17.250737: <stack trace>
=> kprobe_trace_func+0x1d0/0x2c0
=> kprobe_dispatcher+0x39/0x60
=> aggr_pre_handler+0x4f/0x90
=> kprobe_int3_handler+0x152/0x1a0
=> exc_int3+0x47/0x140
=> asm_exc_int3+0x31/0x40
=> 0
=> 0
=> 0
=> 0
=> 0
=> 0
=> 0

Let me check...

Thanks,

>
> diff --git a/arch/x86/include/asm/unwind.h b/arch/x86/include/asm/unwind.h
> index 332aa6174b10..36d3971c0a2c 100644
> --- a/arch/x86/include/asm/unwind.h
> +++ b/arch/x86/include/asm/unwind.h
> @@ -101,6 +101,15 @@ void unwind_module_init(struct module *mod, void *orc_ip, size_t orc_ip_size,
> void *orc, size_t orc_size) {}
> #endif
>
> +static inline
> +unsigned long unwind_recover_kretprobe(struct unwind_state *state,
> + unsigned long addr, unsigned long *addr_p)
> +{
> + return is_kretprobe_trampoline(addr) ?
> + kretprobe_find_ret_addr(state->task, addr_p, &state->kr_cur) :
> + addr;
> +}
> +
> /* Recover the return address modified by instrumentation (e.g. kretprobe) */
> static inline
> unsigned long unwind_recover_ret_addr(struct unwind_state *state,
> @@ -110,10 +119,7 @@ unsigned long unwind_recover_ret_addr(struct unwind_state *state,
>
> ret = ftrace_graph_ret_addr(state->task, &state->graph_idx,
> addr, addr_p);
> - if (is_kretprobe_trampoline(ret))
> - ret = kretprobe_find_ret_addr(state->task, addr_p,
> - &state->kr_cur);
> - return ret;
> + return unwind_recover_kretprobe(state, ret, addr_p);
> }
>
> /*
> diff --git a/arch/x86/kernel/unwind_orc.c b/arch/x86/kernel/unwind_orc.c
> index 839a0698342a..cb59aeca6a4a 100644
> --- a/arch/x86/kernel/unwind_orc.c
> +++ b/arch/x86/kernel/unwind_orc.c
> @@ -549,7 +549,15 @@ bool unwind_next_frame(struct unwind_state *state)
> (void *)orig_ip);
> goto err;
> }
> -
> + /*
> + * There is a small chance to interrupt at the entry of
> + * kretprobe_trampoline where the ORC info doesn't exist.
> + * That point is right after the RET to kretprobe_trampoline
> + * which was modified return address. So the @addr_p must
> + * be right before the regs->sp.
> + */
> + state->ip = unwind_recover_kretprobe(state, state->ip,
> + state->sp - sizeof(unsigned long));
> state->regs = (struct pt_regs *)sp;
> state->prev_regs = NULL;
> state->full_regs = true;
> @@ -562,6 +570,9 @@ bool unwind_next_frame(struct unwind_state *state)
> (void *)orig_ip);
> goto err;
> }
> + /* See UNWIND_HINT_TYPE_REGS case comment. */
> + state->ip = unwind_recover_kretprobe(state, state->ip,
> + state->sp - sizeof(unsigned long));
>
> if (state->full_regs)
> state->prev_regs = state->regs;
>
>
> --
> Masami Hiramatsu <mhiramat@kernel.org>

--
Masami Hiramatsu <mhiramat@kernel.org>