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

Masami Hiramatsu
2021-03-24T01:40:58Z

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?

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>