So my bachelor's thesis and master's topics bear some similarity. Now I am doing literature review for my master's thesis under same supervisor. Can I copy some relevant parts (about 10 pages long) from my old bachelor's? It's just the literature review, not the findings.
Obviously you should check the regulations at the institution that you are attending to see exactly what they say about this matter.
With that said...
Submission of a thesis often includes signing a declaration that the material contained within the thesis has not previously been submitted for consideration for another degree (or words to that effect).
By doing what you suggest, copying and pasting a sizeable chunk of a previous thesis, you would be violating this declaration.
Firstly: it is not plagiarism as long as you clearly state which parts are repeated from your earlier bachelor thesis. Of course, you would clearly state this, wouldn't you?
I did my PhD in Sweden, which includes a licentiate thesis, which is somewhat of a mid-PhD thesis. Large parts were verbatim identical. I included a preface in the PhD thesis to explain this, and there was no issue. I specified this down to the chapter level but no further.
However, your school may still have non-plagiarism reasons to prohibit repeating parts, such as indicated by Ian_Fins answer. Personally I would understand material contained within the thesis to be new research, not the literature review or other purely textual parts. Therefore, I believe that you will be able to reuse the material, provided you clearly indicate which parts are reused and which parts are new.
But: the only definite answer can be given by your advisor and others within your institution.