  We present a lattice QCD determination of light quark masses with three
sea-quark flavours ($N_f = 2+1$). Bare quark masses are known from PCAC
relations in the framework of CLS lattice computations with a
non-perturbatively improved Wilson-Clover action and a tree-level Symanzik
improved gauge action. They are fully non-perturbatively improved, including
the recently computed Symanzik counter-term $b_{\rm A} - b_{\rm P}$. The mass
renormalisation at hadronic scales and the renormalisation group running over a
wide range of scales are known non-perturbatively in the Schr\"odinger
functional scheme. In the present paper we perform detailed extrapolations to
the physical point, obtaining (for the four-flavour theory) $m_{u/d}(2{\rm
GeV}) = 3.54(12)(9)$ MeV and $m_s(2{\rm GeV}) = 95.7(2.5)(2.4)$ MeV in the
$\bar{MS}$ scheme. For the mass ratio we have $m_s/m_{u/d} = 27.0(1.0)(0.4)$.
The RGI values in the three-flavour theory are $M_{u/d} = 4.70(15)(12)$ MeV and
$M_s = 127.0(3.1)(3.2)$ MeV.
