On 01/18/2018 08:03 PM, Kevin Easton wrote: > On Thu, Jan 18, 2018 at 04:38:32PM -0800, Tim Chen wrote: >> On 01/18/2018 05:48 AM, Peter Zijlstra wrote: >>> >>> + /* >>> + * Avoid user/user BTB poisoning by flushing the branch >>> predictor >>> + * when switching between processes. This stops one process from >>> + * doing spectre-v2 attacks on another process's data. >>> + */ >>> + indirect_branch_prediction_barrier(); >>> + >> >> Some optimizations can be done here to avoid overhead in barrier call. >> >> For example, don't do the barrier if prev and next mm are >> the same. If the two process trust each other, or the new process >> already have rights to look into the previous process, >> the barrier could be skipped. > > Isn't it the other way around with the BTB poisoning? previous is > potentially attacking next, so the barrier can be avoided only if previous > is allowed to ptrace next? >
Yes, if the next process don't trust the previous process, then doing a prediction barrier before the context switch makes sense. Tim
