On Mon, 08 Feb, at 11:37:58AM, Andy Lutomirski wrote: > On Feb 4, 2016 5:58 AM, "Ard Biesheuvel" <ard.biesheuvel@xxxxxxxxxx> wrote: > > > > OK, since Sai has confirmed that Windows leaves interrupts enabled when > > calling the EFI variable store related runtime services, we should be able > > to do the same for Linux, or at least be slightly more confident that we > > won't have to back out this change later. > > Could this use a mutex instead of a spinlock? > > Can someone with a mixed mode setup read a variable in a loop and make > sure it doesn't crash and burn? It should work fine, but explicit > testing would be nice. (It's interesting mainly because doing a mixed > mode call with interrupts on can result in a non-IST CPL0 to CPL0 > exception delivery, which won't result in a stack switch. This could > easily trigger a stack overflow, logic bug, microcode bug, or > as-yet-unknown CPU "feature". I don't have physical hardware for testing mixed mode anymore (that was returned to Intel when I left) but testing with Qemu didn't turn up any problems when running with interrupts enabled. > Hmm. We should also audit the mixed mode entry code to make sure that > the high bits of RSP are explicitly clear before switching into compat > mode. If I had to make a guess about how CPUs behave, I'd guess > pessimistically: Intel CPUs clear the high bits of RSP when switching > into long mode due to interrupt delivery, and AMD CPUs leave them set > just to mess with us. Interesting thought. I'm not aware of anyone testing mixed mode with AMD CPUs, so that would be a good data point. > Also, a WARN_ON(in_interrupt()) somewhere might be a good sanity check. lockdep should catch this kind of stuff pretty quickly since we grab the spinlock in every code path. -- 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
