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    SubjectRe: A question about PROT_NONE on ARM and ARM26
    Scott Wood wrote:
    > > However, plain ldr and str instructions will access the page, but
    > > get_user/put_user doesn't use them, and copy_from_user/copy_to_user
    > > are carefully crafted to ensure that we hit the necessary permission
    > > checks for each page it touches on the first access.
    >
    > What if CONFIG_PREEMPT is enabled, and you get preempted after that
    > first access, and another thread unmaps the page before you're
    > finished with it?

    The code in uaccess.S:__arch_copy_{from,to}_user doesn't disable
    pre-emption, and neither does its caller.

    Pages can be unmapped just due to background paging. I.e. it's a
    normal occurrence, it doesn't require anything contrived.

    So I think you're right: that looks like a bug.

    The ARM uaccess code was written before CONFIG_PREEMPT was added, and
    this couldn't happen then. It could panic a kernel now. I wonder why
    it hasn't been noticed. Maybe nobody turns on CONFIG_PREEMPT on ARM?

    -- Jamie
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