On Mon, Aug 27, 2018 at 12:03:05PM +0900, Masami Hiramatsu wrote: > On Sun, 26 Aug 2018 11:09:58 +0200 > Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote: > > FWIW, before text_poke_bp(), text_poke() would only be used from > > stop_machine, so all the other CPUs would be stuck busy-waiting with > > IRQs disabled. These days, yeah, that's lots more dodgy, but yes > > text_mutex should be serializing all that. > > I'm still not sure that speculative page-table walk can be done > over the mutex. Also, if the fixmap area is for aliasing > pages (which always mapped to memory), what kind of > security issue can happen? So suppose CPU-A is doing the text_poke (let's say through text_poke_bp, such that other CPUs get to continue with whatever they're doing). While at that point, CPU-B gets an interrupt, and the CPU's branch-trace-buffer for the IRET points to / near our fixmap. Then the CPU could do a speculative TLB fill based on the BTB value, either directly or indirectly (through speculative driven fault-ahead) of whatever is in te fixmap at the time. Then CPU-A completes the text_poke and only does a local TLB invalidate on CPU-A, leaving CPU-B with an active translation. *FAIL*
