Did Sharon Stone use a body double in Basic Instinct's infamous interrogation scene, a.k.a. the "leg crossing" scene, or was that all her?
Yes, that was Sharon Stone in that scene, there was no body double.
However, Stone has claimed that she was "tricked" into doing that scene, and was told by Paul Verhoeven (the director) that she would not be seen exposing herself on-camera in the final cut. As she tells it:
She says, "When we did it, it was going to be an innuendo and the director said, 'We're seeing the white of your underwear, I need you to take them off.' And I'm like, 'I don't want you to see anything and he's like, 'No, no you're not going to.' So I gave him the underwear, put them in the pocket of his shirt, and he said, 'Now watch on the monitor...' src
Her expectation was that Verhoeven would shoot and edit the scene in such a way that it was "clear" she wasn't wearing underwear, but nothing explicit would be shown. The director, clearly, didn't do that.
Stone does admit that, in the end, she wasn't angry that the scene stayed in, only that Verhoeven didn't tell her and show her the scene in private first; rather, she only found out while watching the final cut with others in the room.
Note that Stone has never been terribly shy about doing nude scenes -- the part in Basic Instinct was offered to her largely because she had done a spread in Playboy earlier that year, and she has appeared at least topless in a number of other equally high profile films (Casino, Total Recall, The Quick and the Dead), as well as the less-successful sequel Basic Instinct 2.