Re: x86: warning in unwind_get_return_address

[Dmitry Vyukov]

On Thu, Jan 5, 2017 at 6:03 PM, Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote:
> On Thu, Jan 05, 2017 at 09:17:00AM -0600, Josh Poimboeuf wrote:
>> On Thu, Jan 05, 2017 at 03:59:52PM +0100, Dmitry Vyukov wrote:
>> > On Thu, Jan 5, 2017 at 3:49 PM, Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote:
>> > > On Tue, Dec 27, 2016 at 05:38:59PM +0100, Dmitry Vyukov wrote:
>> > >> On Thu, Dec 22, 2016 at 6:17 AM, Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote:
>> > >> > On Wed, Dec 21, 2016 at 01:46:36PM +0100, Andrey Konovalov wrote:
>> > >> >> On Wed, Dec 21, 2016 at 12:36 AM, Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote:
>> > >> >> >
>> > >> >> > Thanks.  Looking at the stack trace, my guess is that an interrupt hit
>> > >> >> > while running in generated BPF code, and the unwinder got confused
>> > >> >> > because regs->ip points to the generated code.  I may need to disable
>> > >> >> > that warning until we figure out a better solution.
>> > >> >> >
>> > >> >> > Can you share your .config file?
>> > >> >>
>> > >> >> Sure, attached.
>> > >> >
>> > >> > Ok, I was able to recreate with your config.  The culprit was generated
>> > >> > code, as I suspected, though it wasn't BPF, it was a kprobe (created by
>> > >> > dccpprobe_init()).
>> > >> >
>> > >> > I'll make a patch to disable the warning.
>> > >>
>> > >> Hi,
>> > >>
>> > >> I am also seeing the following warnings:
>> > >>
>> > >> [  281.889259] WARNING: kernel stack regs at ffff8801c29a7ea8 in
>> > >> syz-executor8:1302 has bad 'bp' value ffff8801c29a7f28
>> > >> [  833.994878] WARNING: kernel stack regs at ffff8801c4e77ea8 in
>> > >> syz-executor1:13094 has bad 'bp' value ffff8801c4e77f28
>> > >>
>> > >> Can it also be caused by bpf/kprobe?
>> > >
>> > > This is a different warning.  I suspect it's due to unwinding the stack
>> > > of another CPU while it's running, which is still possible in a few
>> > > places.  I'm going to have to disable all these warnings for now.
>> >
>> >
>> > I also have the following diff locally. These loads trigger episodic
>> > KASAN warnings about stack-of-bounds reads on rcu stall warnings when
>> > it does backtrace of all cpus.
>> > If it looks correct to you, can you please also incorporate it into your patch?
>>
>> Ok, will do.
>>
>> BTW, I think there's an issue with your mail client.  Your last two
>> replies to me didn't have me on To/Cc.
>
> Would you mind testing if the following patch fixes it?  It's based on
> the latest linus/master.
>
>
> diff --git a/arch/x86/kernel/unwind_frame.c b/arch/x86/kernel/unwind_frame.c
> index 4443e49..05adf9a 100644
> --- a/arch/x86/kernel/unwind_frame.c
> +++ b/arch/x86/kernel/unwind_frame.c
> @@ -6,6 +6,21 @@
>
>  #define FRAME_HEADER_SIZE (sizeof(long) * 2)
>
> +/*
> + * This disables KASAN checking when reading a value from another task's stack,
> + * since the other task could be running on another CPU and could have poisoned
> + * the stack in the meantime.
> + */
> +#define UNWIND_READ_ONCE(state, x)                     \
> +({                                                     \
> +       unsigned long val;                              \
> +       if (state->task == current)                     \
> +               val = READ_ONCE(x);                     \
> +       else                                            \
> +               val = READ_ONCE_NOCHECK(x);             \
> +       val;                                            \
> +})
> +
>  static void unwind_dump(struct unwind_state *state, unsigned long *sp)
>  {
>         static bool dumped_before = false;
> @@ -48,7 +63,8 @@ unsigned long unwind_get_return_address(struct unwind_state *state)
>         if (state->regs && user_mode(state->regs))
>                 return 0;
>
> -       addr = ftrace_graph_ret_addr(state->task, &state->graph_idx, *addr_p,
> +       addr = UNWIND_READ_ONCE(state, *addr_p);
> +       addr = ftrace_graph_ret_addr(state->task, &state->graph_idx, addr,
>                                      addr_p);
>
>         return __kernel_text_address(addr) ? addr : 0;
> @@ -162,7 +178,7 @@ bool unwind_next_frame(struct unwind_state *state)
>         if (state->regs)
>                 next_bp = (unsigned long *)state->regs->bp;
>         else
> -               next_bp = (unsigned long *)*state->bp;
> +               next_bp = (unsigned long *)UNWIND_READ_ONCE(state, *state->bp);
>
>         /* is the next frame pointer an encoded pointer to pt_regs? */
>         regs = decode_frame_pointer(next_bp);
> @@ -207,6 +223,16 @@ bool unwind_next_frame(struct unwind_state *state)
>         return true;
>
>  bad_address:
> +       /*
> +        * When dumping a task other than current, the task might actually be
> +        * running on another CPU, in which case it could be modifying its
> +        * stack while we're reading it.  This is generally not a problem and
> +        * can be ignored as long as the caller understands that unwinding
> +        * another task will not always succeed.
> +        */
> +       if (state->task != current)
> +               goto the_end;
> +
>         if (state->regs) {
>                 printk_deferred_once(KERN_WARNING
>                         "WARNING: kernel stack regs at %p in %s:%d has bad 'bp' value %p\n",


Applied locally for testing.


What about this part?

diff --git a/arch/x86/include/asm/stacktrace.h
b/arch/x86/include/asm/stacktrace.h
index a3269c897ec5..d8d4fc66ffec 100644
--- a/arch/x86/include/asm/stacktrace.h
+++ b/arch/x86/include/asm/stacktrace.h
@@ -58,7 +58,7 @@ get_frame_pointer(struct task_struct *task, struct
pt_regs *regs)
        if (task == current)
                return __builtin_frame_address(0);

-       return (unsigned long *)((struct
inactive_task_frame*)task->thread.sp)->bp;
+       return (unsigned long *)READ_ONCE_NOCHECK(((struct
inactive_task_frame *)task->thread.sp)->bp);
 }
 #else
 static inline unsigned long *


I am hitting it as welL:

[<     inline     >] kasan_report_error mm/kasan/report.c:213
[<        none        >] kasan_report+0x42d/0x460 mm/kasan/report.c:307
[<        none        >] __asan_report_load8_noabort+0x14/0x20
mm/kasan/report.c:333
[<     inline     >] get_frame_pointer ./arch/x86/include/asm/stacktrace.h:61
[<        none        >] __unwind_start+0x38d/0x3a0
arch/x86/kernel/unwind_frame.c:247
[<     inline     >] unwind_start ./arch/x86/include/asm/unwind.h:40
[<        none        >] show_trace_log_lvl+0x1ed/0x2c0
arch/x86/kernel/dumpstack.c:61
[<        none        >] show_stack+0x2f/0x40 arch/x86/kernel/dumpstack.c:174
[<        none        >] sched_show_task+0x3e4/0x540 kernel/sched/core.c:5217