Multi-antenna processing enables jammer mitigation through spatial filtering, provided that the receiver knows the spatial signature of the jammer interference. Estimating this signature is easy for barrage jammers that transmit continuously and with static signature, but difficult for more sophisticated jammers. Smart jammers may deliberately suspend transmission when the receiver tries to estimate their spatial signature, or they may use time-varying beamforming to continuously change their spatial signature. To deal with such smart jammers, we propose MASH, the first method that indiscriminately mitigates all types of jammers. Assume that the transmitter and receiver share a common secret. Based on this secret, the transmitter embeds (with a time-domain transform) its signal in a secret subspace of a higher-dimensional space. The receiver applies a reciprocal transform to the receive signal, which (i) raises the legitimate transmit signal from its secret subspace and (ii) provably transforms any jammer into a barrage jammer, making estimation and mitigation via multi-antenna processing straightforward. Focusing on the massive multi-user MIMO uplink, we present three MASH-based data detectors and show their jammer-resilience via extensive simulations. We also introduce strategies for multi-user communication without a global secret as well as methods that use computationally efficient embedding and raising transforms.