On Fri, 24 Aug 2018 21:23:26 -0700 Andy Lutomirski <luto@xxxxxxxxxx> wrote: > On Fri, Aug 24, 2018 at 7:29 PM, <nadav.amit@xxxxxxxxx> wrote: > > > > > > On August 24, 2018 5:58:43 PM PDT, Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote: > >>Adding a few people to the cc. > >> > >>On Fri, Aug 24, 2018 at 1:24 PM Nadav Amit <nadav.amit@xxxxxxxxx> > >>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? No, since kmap_atomic() is only for x86_32 and highmem support kernel. In x86-64, it seems that returns just a page address. That is not good for text_poke, since it needs to make a writable alias for RO code page. Hmm, maybe, can we mimic copy_oldmem_page(), it uses ioremap_cache? Thank you, -- Masami Hiramatsu <mhiramat@xxxxxxxxxx>
