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    SubjectRe: [RFC PATCH 8/9] LSM: x86/sgx: Introduce ->enclave_load() hook for Intel SGX
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    >>> What ensures that the mapping referenced by src can't be changed
    >>> to an entirely different one (with a different vm_file) between
    >>> the time of check (here) and the time of use?
    >>
    >> Nothing. Holding mmap_sem across copy_from_user() would suffice,
    >> correct?
    >
    > I don't believe you can do that; copy_from_user() could stall
    > indefinitely. Not sure how to do what you want here or if it requires
    > changing the interface.

    Holding mmap_sem for *read* is OK since you can handle page faults
    underneath it. Holding it for write is not.

    But, holding it for read also locks out the writers which might be
    messing with vm_file or other parts of the VMA.

    Holding it for read for a long time is OK. It's obviously not ideal,
    but it is something we do widely today.

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