On Thu, 22 Dec 2022 13:01:55 +0000, Ard Biesheuvel <ardb@xxxxxxxxxx> wrote: > > On Tue, 20 Dec 2022 at 21:09, Marc Zyngier <maz@xxxxxxxxxx> wrote: > > > > A recent development on the EFI front has resulted in guests having > > their page tables baked in the firmware binary, and mapped into > > the IPA space as part as a read-only memslot. > > > > Not only this is legitimate, but it also results in added security, > > so thumbs up. However, this clashes mildly with our handling of a S1PTW > > as a write to correctly handle AF/DB updates to the S1 PTs, and results > > in the guest taking an abort it won't recover from (the PTs mapping the > > vectors will suffer freom the same problem...). > > > > So clearly our handling is... wrong. > > > > Instead, switch to a two-pronged approach: > > > > - On S1PTW translation fault, handle the fault as a read > > > > - On S1PTW permission fault, handle the fault as a write > > > > This is of no consequence to SW that *writes* to its PTs (the write > > will trigger a non-S1PTW fault), and SW that uses RO PTs will not > > use AF/DB anyway, as that'd be wrong. > > > > Only in the case described in c4ad98e4b72c ("KVM: arm64: Assume write > > fault on S1PTW permission fault on instruction fetch") do we end-up > > with two back-to-back faults (page being evicted and faulted back). > > I don't think this is a case worth optimising for. > > > > Fixes: c4ad98e4b72c ("KVM: arm64: Assume write fault on S1PTW permission fault on instruction fetch") > > Signed-off-by: Marc Zyngier <maz@xxxxxxxxxx> > > Cc: stable@xxxxxxxxxxxxxxx > > Reviewed-by: Ard Biesheuvel <ardb@xxxxxxxxxx> > > I have tested this patch on my TX2 with one of the EFI builds in > question, and everything works as before (I never observed the issue > itself) If you get the chance, could you try with non-4kB page sizes? Here, I could only reproduce it with 16kB pages. It was firing like clockwork on Cortex-A55 with that. > > Regression-tested-by: Ard Biesheuvel <ardb@xxxxxxxxxx> > > For the record, the EFI build in question targets QEMU/mach-virt and > switches to a set of read-only page tables in emulated NOR flash > straight out of reset, so it can create and populate the real page > tables with MMU and caches enabled. EFI does not use virtual memory or > paging so managing access flags or dirty bits in hardware is unlikely > to add any value, and it is not being used at the moment. And given > that this is emulated NOR flash, any ordinary write to it tears down > the r/o memslot altogether, and kicks the NOR flash emulation in QEMU > into programming mode, which is fully based on MMIO emulation and does > not use a memslot at all. IOW, even if we could figure out what store > the PTW was attempting to do, it is always going to be rejected since > the r/o page tables can only be modified by 'programming' the NOR > flash sector. Indeed, and this would be a pretty dodgy setup anyway. Thanks for having had a look, M. -- Without deviation from the norm, progress is not possible.