Messages in this thread | | | Subject | Re: [PATCH 3/5] ARM: stacktrace: Allow stack trace saving for non-current tasks | From | Li Huafei <> | Date | Tue, 26 Jul 2022 20:08:14 +0800 |
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On 2022/7/26 17:49, Russell King (Oracle) wrote: > On Tue, Jul 26, 2022 at 05:12:39PM +0800, Li Huafei wrote: >> >> >> On 2022/7/18 17:07, Linus Walleij wrote: >>> On Tue, Jul 12, 2022 at 4:18 AM Li Huafei <lihuafei1@huawei.com> wrote: >>> >>>> The current ARM implementation of save_stack_trace_tsk() does not allow >>>> saving stack trace for non-current tasks, which may limit the scenarios >>>> in which stack_trace_save_tsk() can be used. Like other architectures, >>>> or like ARM's unwind_backtrace(), we can leave it up to the caller to >>>> ensure that the task that needs to be unwound is not running. >>>> >>>> Signed-off-by: Li Huafei <lihuafei1@huawei.com> >>> >>> That sounds good, but: >>> >>>> if (tsk != current) { >>>> -#ifdef CONFIG_SMP >>>> - /* >>>> - * What guarantees do we have here that 'tsk' is not >>>> - * running on another CPU? For now, ignore it as we >>>> - * can't guarantee we won't explode. >>>> - */ >>>> - return; >>>> -#else >>>> + /* task blocked in __switch_to */ >>> >>> The commit text is not consistent with the comment you are removing. >>> >>> The commit is talking about "non-current" tasks which is one thing, >>> but the code is avoiding any tasks under SMP because they may be >>> running on another CPU. So you need to update the commit >>> message to say something like "non-current or running on another CPU". >>> >>> If this condition will be checked at call sites in following patches, >>> then mention >>> that in the commit as well, so we know the end result is that we do >>> not break it, >> >> The generic code stack_trace_save_tsk() does not have this check, and by >> 'caller' I mean the caller of stack_trace_save_tsk(), expecting the >> 'caller' to ensure that the task is not running. So in effect this check >> has been dropped and there is no more guarantee. Sorry for not >> clarifying the change here. > > Can you prove in every case that the thread we're being asked to unwind > is not running? I don't think you can. > > There are things like proc_pid_stack() in procfs and the stack traces > in sysrq-t which have attempted to unwind everything whether it's > running or not. > > So no, there is no guarantee that the thread is blocked in > __switch_to(). >
Yes, I agree.
>> But can we assume that the user should know that the stacktrace is >> unreliable for a task that is running on another CPU? If not, I should >> remove this patch and keep the check. > > It's not about "unreliable" stack traces, it's about the unwinder > killing the kernel. > > The hint is this: > > frame.fp = thread_saved_fp(tsk); > frame.sp = thread_saved_sp(tsk); > frame.lr = 0; /* recovered from the stack */ > frame.pc = thread_saved_pc(tsk); > > These access the context saved by the scheduler when the task is > sleeping. When the thread is running, these saved values will be > the state when the thread last slept. However, with the thread > running, the stack could now contain any data what so ever, and > could change at any moment. >
I get it. For example, since the data on the stack is changing, '*(unsigned long *)fp' could access any illegal address and crash the kernel.
> Whether the unwind-table unwinder is truely safe in such a > situation is unknown - we try to ensure that it won't do anything > stupid, but proving that is a hard task, and we've recently had > issues with the unwinder even without that. > > So, allowing this feels like we're opening the door to DoS attacks > from userspace, where userspace sits there reading /proc/*/stack of > some thread running on a different CPU waiting for the kernel to > oops itself, possibly holding a lock, resulting in the system > dying. > > These decisions need to be made by architecture code not generic > code, particularly where the method of unwinding is architecture > specific and thus may have criteria defining when its safe to do so. >
Thank you for your comments, I'll remove the patch and keep the check.
Thanks, Huafei
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