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SubjectRe: [PATCH v2 06/35] nds32: MMU fault handling and page table management
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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