On Fri, Aug 24, 2018 at 7:29 PM, <nadav.a...@gmail.com> wrote: > > > On August 24, 2018 5:58:43 PM PDT, Linus Torvalds > <torva...@linux-foundation.org> wrote: >>Adding a few people to the cc. >> >>On Fri, Aug 24, 2018 at 1:24 PM Nadav Amit <nadav.a...@gmail.com> >>wrote: >>> > >>> > Can you actually find something that changes the fixmaps after boot >>> > (again, ignoring kmap)? >>> >>> At least the alternatives mechanism appears to do so. >>> >>> IIUC the following path is possible when adding a module: >>> >>> jump_label_add_module() >>> ->__jump_label_update() >>> ->arch_jump_label_transform() >>> ->__jump_label_transform() >>> ->text_poke_bp() >>> ->text_poke() >>> ->set_fixmap() >> >>Yeah, that looks a bit iffy. >> >>But making the tlb flush global wouldn't help. This is running on a >>local core, and if there are other CPU's that can do this at the same >>time, then they'd just fight about the same mapping. >> >>Honestly, I think it's ok just because I *hope* this is all serialized >>anyway (jump_label_lock? But what about other users of text_poke?). > > The users should hold text_mutex. > >> >>But I'd be a lot happier about it if it either used an explicit lock >>to make sure, or used per-cpu fixmap entries. > > My concern is that despite the lock, one core would do a speculative page > walk and cache a translation that soon after would become stale. > >> >>And the tlb flush is done *after* the address is used, which is bogus >>anyway. > > It seems to me that it is intended to remove the mapping that might be a > security issue. > > But anyhow, set_fixmap and clear_fixmap perform a local TLB flush, (in > __set_pte_vaddr()) so locally things should be fine. > >> >>> And a similar path can happen when static_key_enable/disable() is >>called. >> >>Same comments. >> >>How about replacing that >> >> local_irq_save(flags); >> ... do critical things here ... >> local_irq_restore(flags); >> >>in text_poke() with >> >> static DEFINE_SPINLOCK(poke_lock); >> >> spin_lock_irqsave(&poke_lock, flags); >> ... do critical things here ... >> spin_unlock_irqrestore(&poke_lock, flags); >> >>and moving the local_flush_tlb() to after the set_fixmaps, but before >>the access through the virtual address. >> >>But changing things to do a global tlb flush would just be wrong. > > As I noted, I think that locking and local flushes as they are right now are > fine (besides the redundant flush). > > My concern is merely that speculative page walks on other cores would cache > stale entries. > >
This is almost certainly a bug, or even two bugs. Bug 1: why on Earth do we flush in __set_pte_vaddr()? We should flush when *clearing* or when modifying an existing fixmap entry. Right now, if we do text_poke() after boot, then the TLB entry will stick around and will be a nice exploit target. Bug 2: what you're describing. It's racy. Couldn't text_poke() use kmap_atomic()? Or, even better, just change CR3?
