05, 2018 at 03:23:10PM -0800, Sai Praneeth Prakhya wrote: > > From: Sai Praneeth <sai.praneeth.prakhya@xxxxxxxxx> > > > > Presently, efi_runtime_services() are executed by firmware in process > > context. To execute efi_runtime_service(), kernel switches the page > > directory from swapper_pgd to efi_pgd. However, efi_pgd doesn't have > > any user space mappings. A potential issue could be, for instance, an > > NMI interrupt (like perf) trying to profile some user data while in efi_pgd. > > It might be worth pointing out that this could result in disaster (e.g. > if the frame pointer happens to point at MMIO in the EFI runtime services > mappings). > Sorry! I didn't get it. I would like to add this point, so could you please explain it more? > [...] > > > pstore writes could potentially be invoked in interrupt context and it > > uses set_variable<>() and query_variable_info<>() to store logs. If we > > invoke efi_runtime_services() through efi_rts_wq while in atomic() > > kernel issues a warning ("scheduling wile in atomic") and prints stack > > trace. One way to overcome this is to not make the caller process wait > > for the worker thread to finish. This approach breaks pstore i.e. the > > log messages aren't written to efi variables. Hence, pstore calls > > efi_runtime_services() without using efi_rts_wq or in other words > > efi_rts_wq will be used unconditionally for all the > > efi_runtime_services() except set_variable<>() and > > query_variable_info<>() > > Can't NMIs still come in during this? > > ... or do we assume that since something has already gone wrong, this doesn't > matter? > I think they can come. AFAIK, pstore (if enabled) runs only when we crashed. Since, we are still executing stuff to log messages and NMI's can't be masked, there is still a possibility for NMI's to occur (please correct me if I am wrong). But, as you said earlier, I guess it doesn't matter because anyways we are going down. > Otherwise, this doesn't seem to break basic stuff on arm64 platforms. I can boot > up, read the EFI RTC, and reboot. I ahd lockdep and KASAN enabled, I received > no splats from either. > > FWIW: > > Tested-by: Mark Rutland <mark.rutland@xxxxxxx [arm64] > > Thanks, > Mark. -- To unsubscribe from this list: send the line "unsubscribe linux-efi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html