Re: [PATCH 5/8] arm64: Recover kretprobe modified return address in stacktrace

Masami Hiramatsu
2021-10-14T09:59:29Z

On Wed, 13 Oct 2021 11:13:51 +0100
Mark Rutland <mark.rutland@arm.com> wrote:

> On Fri, Oct 08, 2021 at 09:28:58PM +0900, Masami Hiramatsu wrote:
> > Since the kretprobe replaces the function return address with
> > the kretprobe_trampoline on the stack, stack unwinder shows it
> > instead of the correct return address.
> >
> > This checks whether the next return address is the
> > __kretprobe_trampoline(), and if so, try to find the correct
> > return address from the kretprobe instance list.
> >
> > With this fix, now arm64 can enable
> > CONFIG_ARCH_CORRECT_STACKTRACE_ON_KRETPROBE, and pass the
> > kprobe self tests.
> >
> > Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
> > ---
> > arch/arm64/Kconfig | 1 +
> > arch/arm64/include/asm/stacktrace.h | 2 ++
> > arch/arm64/kernel/stacktrace.c | 3 +++
> > 3 files changed, 6 insertions(+)
> >
> > diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> > index 5c7ae4c3954b..edde5171ffb2 100644
> > --- a/arch/arm64/Kconfig
> > +++ b/arch/arm64/Kconfig
> > @@ -11,6 +11,7 @@ config ARM64
> > select ACPI_PPTT if ACPI
> > select ARCH_HAS_DEBUG_WX
> > select ARCH_BINFMT_ELF_STATE
> > + select ARCH_CORRECT_STACKTRACE_ON_KRETPROBE
> > select ARCH_ENABLE_HUGEPAGE_MIGRATION if HUGETLB_PAGE && MIGRATION
> > select ARCH_ENABLE_MEMORY_HOTPLUG
> > select ARCH_ENABLE_MEMORY_HOTREMOVE
> > diff --git a/arch/arm64/include/asm/stacktrace.h b/arch/arm64/include/asm/stacktrace.h
> > index 8aebc00c1718..8f997a602651 100644
> > --- a/arch/arm64/include/asm/stacktrace.h
> > +++ b/arch/arm64/include/asm/stacktrace.h
> > @@ -9,6 +9,7 @@
> > #include <linux/sched.h>
> > #include <linux/sched/task_stack.h>
> > #include <linux/types.h>
> > +#include <linux/llist.h>
> >
> > #include <asm/memory.h>
> > #include <asm/ptrace.h>
> > @@ -59,6 +60,7 @@ struct stackframe {
> > #ifdef CONFIG_FUNCTION_GRAPH_TRACER
> > int graph;
> > #endif
> > + struct llist_node *kr_cur;
>
> As with the fgraph bits above, please make this depedn on the relevant
> Kconfig, i.e.
>
> | #ifdef CONFIG_KRETPROBES
> | struct llist_node *kr_cur;
> | #endif
>
> > };

OK.

> >
> > extern int unwind_frame(struct task_struct *tsk, struct stackframe *frame);
> > diff --git a/arch/arm64/kernel/stacktrace.c b/arch/arm64/kernel/stacktrace.c
> > index 8982a2b78acf..f1eef5745542 100644
> > --- a/arch/arm64/kernel/stacktrace.c
> > +++ b/arch/arm64/kernel/stacktrace.c
> > @@ -129,6 +129,8 @@ int notrace unwind_frame(struct task_struct *tsk, struct stackframe *frame)
> > frame->pc = ret_stack->ret;
> > }
> > #endif /* CONFIG_FUNCTION_GRAPH_TRACER */
> > + if (is_kretprobe_trampoline(frame->pc))
> > + frame->pc = kretprobe_find_ret_addr(tsk, (void *)frame->fp, &frame->kr_cur);
>
> Please ifdef this, like the CONFIG_FUNCTION_GRAPH_TRACER bits above.
> i.e.
>
> | #ifdef CONFIG_KRETPROBES
> | if (is_kretprobe_trampoline(frame->pc))
> | frame->pc = kretprobe_find_ret_addr(tsk, (void *)frame->fp, &frame->kr_cur);
> | #endif

OK.

>
> What does kretprobe_find_ret_addr() do when it can't find the original
> address? I couldn't spot it in this series or in mainline.

If it couldn't find, it returns NULL.
Hmm, should we check it and keep frame->pc if the return value is NULL?
(anyway, it must not happen. If it happens, that task can not continue to run.)

> As a future thing, I'd like to be able to have monotonicity and
> completeness checks as part of the unwind, i.e. checking that we consume
> the kretprobe address *in-order*, and can identify whether we've skipped
> any, so that we can identify when unwinding has gone wrong. Does it do
> that today?

Good question. No today, but is easy to do since we have the loop cursor
(frame->kr_cur).

unsigned long kretprobe_find_ret_addr(struct task_struct *tsk, void *fp,
struct llist_node **cur)
{
...
do {
ret = __kretprobe_find_ret_addr(tsk, cur);
if (!ret)
break;
ri = container_of(*cur, struct kretprobe_instance, llist);
} while (ri->fp != fp);
...

Change this loop to;

ri = container_of(*cur, struct kretprobe_instance, llist);
prev_fp = ri->fp;
do {
ret = __kretprobe_find_ret_addr(tsk, cur);
if (WARN_ON_ONCE(!ret))
return ERR_PTR(-ENOENT);
ri = container_of(*cur, struct kretprobe_instance, llist);
} while (ri->fp == prev_fp);
if (ri->fp != fp)
return ERR_PTR(-EILSEQ);

Then, we can detect that wrong sequence from stacktrace side.

>
> It'd be nice if it could signal failure reliably (without causing a
> BUG() or similar), e.g. by returning an error code.

As above, -EILSEQ is OK?

>
> > frame->pc = ptrauth_strip_insn_pac(frame->pc);
> >
> > @@ -224,6 +226,7 @@ noinline notrace void arch_stack_walk(stack_trace_consume_fn consume_entry,
> > {
> > struct stackframe frame;
> >
> > + memset(&frame, 0, sizeof(frame));
>
> Please initialise stackframe::kr_cur in start_backtrace, where we
> initialize all the other fields in struct stackframe, i.e. just after
> the CONFIG_FUNCTION_GRAPH_TRACER bit, have:
>
> | #ifdef CONFIG_KRETPROBES
> | frame->kr_cur = NULL;
> | #endif

OK, let me update it.

Thank you!

>
> Thanks,
> Mark.
>
> > if (regs)
> > start_backtrace(&frame, regs->regs[29], regs->pc);
> > else if (task == current)
> >

--
Masami Hiramatsu <mhiramat@kernel.org>