Messages in this thread | | | Date | Mon, 27 Nov 2017 13:51:36 +0000 | From | Mark Rutland <> | Subject | Re: [PATCH v2 06/35] nds32: MMU fault handling and page table management |
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Hi,
On Mon, Nov 27, 2017 at 08:27:53PM +0800, Greentime Hu wrote: > +void do_page_fault(unsigned long entry, unsigned long addr, > + unsigned int error_code, struct pt_regs *regs) > +{
> + /* > + * As per x86, we may deadlock here. However, since the kernel only > + * validly references user space from well defined areas of the code, > + * we can bug out early if this is from code which shouldn't. > + */ > + if (unlikely(!down_read_trylock(&mm->mmap_sem))) { > + if (!user_mode(regs) && > + !search_exception_tables(instruction_pointer(regs))) > + goto no_context; > +retry: > + down_read(&mm->mmap_sem); > + } else { > + /* > + * The above down_read_trylock() might have succeeded in which > + * case, we'll have missed the might_sleep() from down_read(). > + */ > + might_sleep(); > + if (IS_ENABLED(CONFIG_DEBUG_VM)) { > + if (!user_mode(regs) && > + !search_exception_tables(instruction_pointer(regs))) > + goto no_context; > + } > + }
> + fault = handle_mm_fault(vma, addr, flags); > + > + /* > + * If we need to retry but a fatal signal is pending, handle the > + * signal first. We do not need to release the mmap_sem because it > + * would already be released in __lock_page_or_retry in mm/filemap.c. > + */ > + if ((fault & VM_FAULT_RETRY) && fatal_signal_pending(current)) > + return;
I believe you can get stuck in a livelock here (with an unkillable task), if a uaccess primitive tries to access a region protected by a userfaultfd. Please see:
https://lkml.kernel.org/r/1499782590-31366-1-git-send-email-mark.rutland@arm.com
... for details and a test case.
Thanks, Mark.
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