Reading the paper further, there is this: However, the BTB provides partial target addresses [28], so the attacker only needs to branch to an address where

Reading the paper further, there is this: However, the BTB provides partial target addresses [28], so the attacker only needs to branch to an address where the lower portion matches the desired kernel target. The upper bits of the BTB target are provided by the victim branch source, which will be in the kernel address range. The technique follows the one we used in Section 6.1 and Figure 5. So it seems to me that the actual difficulty from JS is less passing down the desired memory destinations (that's harder, yes, but I wonder if it's hard enough), but to generate (benign) branches to almost arbitrary addresses within the JS code, as it's probably neigh impossible to control for where those branches go. Still, who really knows if there isn't some jump table generator or whatever to allow an attacker to generate branch targets arbitrarily enough (remember that it's not necessary to branch to the full address to train the branch predictor). Because this would not be a vulnerability in any sense by itself. It would be yet another completely benign but unlucky piece of code that just allows the tire fire that Spectre is to be leveraged. I'm probably missing other relevant aspects. As for cache flushing, I think that's what the disclosure gadget does: "The disclosure gadget needs to use the two attacker-controlled registers to leak and transmit the secret via Flush+Reload", so that's also kernel code which we mispredict into. But I'm not totally sure.