• David Howells's avatar
    KEYS: Fix race between updating and finding a negative key · 363b02da
    David Howells authored
    Consolidate KEY_FLAG_INSTANTIATED, KEY_FLAG_NEGATIVE and the rejection
    error into one field such that:
    
     (1) The instantiation state can be modified/read atomically.
    
     (2) The error can be accessed atomically with the state.
    
     (3) The error isn't stored unioned with the payload pointers.
    
    This deals with the problem that the state is spread over three different
    objects (two bits and a separate variable) and reading or updating them
    atomically isn't practical, given that not only can uninstantiated keys
    change into instantiated or rejected keys, but rejected keys can also turn
    into instantiated keys - and someone accessing the key might not be using
    any locking.
    
    The main side effect of this problem is that what was held in the payload
    may change, depending on the state.  For instance, you might observe the
    key to be in the rejected state.  You then read the cached error, but if
    the key semaphore wasn't locked, the key might've become instantiated
    between the two reads - and you might now have something in hand that isn't
    actually an error code.
    
    The state is now KEY_IS_UNINSTANTIATED, KEY_IS_POSITIVE or a negative error
    code if the key is negatively instantiated.  The key_is_instantiated()
    function is replaced with key_is_positive() to avoid confusion as negative
    keys are also 'instantiated'.
    
    Additionally, barriering is included:
    
     (1) Order payload-set before state-set during instantiation.
    
     (2) Order state-read before payload-read when using the key.
    
    Further separate barriering is necessary if RCU is being used to access the
    payload content after reading the payload pointers.
    
    Fixes: 146aa8b1 ("KEYS: Merge the type-specific data with the payload data")
    Cc: stable@vger.kernel.org # v4.4+
    Reported-by: 's avatarEric Biggers <ebiggers@google.com>
    Signed-off-by: 's avatarDavid Howells <dhowells@redhat.com>
    Reviewed-by: 's avatarEric Biggers <ebiggers@google.com>
    363b02da
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