There was a quota, like in the US in the 1930s. In the USSR, about 2% of population was Jews, so university was allowed to admit 2% Jews every year. But because of culture of education and desire to avoid conscription to Red Army[1] and non-uniform population density, it was very difficult for Jews to get admitted in the good universities in Moscow and Leningrad (the two largest cities with most prestigious universities).

1 - The Soviet military was bad for everyone, but Jews tended to get hurt more. Students accepted by a university would not be conscripted and would be listed as reserve officers, having never spent time in uniform

Not true at all. How that explain then world known physicists and mathematicians from USSR with Jewish names?