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RRChangeProviderProperty() has two bugs in the PropModePrepend case:

1. new_value.size is set to 'len' (the new data length) instead of
   'total_len' (len + prop_value->size), losing track of the existing
   data. This also affects PropModeAppend.

2. The old_data offset is computed using prop_value->size instead of
   len. For PrependMode, the layout should be [new(len)][old(size)],
   so old_data should start at len * size_in_bytes.

These bugs are analogous to the ones fixed in RRChangeOutputProperty(),
see commit 541ab2ecd41d ("Xi/randr: fix handling of PropModeAppend/Prepend").

This vulnerability was discovered by:
  Anonymous working with TrendAI Zero Day Initiative

ZDI-CAN-31944