On Tue, Apr 22, 2014 at 10:26 AM, Borislav Petkov <b...@alien8.de> wrote: > On Tue, Apr 22, 2014 at 10:11:27AM -0700, H. Peter Anvin wrote: >> The fastpath interference is: >> >> 1. Testing for an LDT SS selector before IRET. This is actually easier >> than on 32 bits, because on 64 bits the SS:RSP on the stack is always valid. >> >> 2. Testing for an RSP inside the espfix region on exception entry, so we >> can switch back the stack. This has to be done very early in the >> exception entry since the espfix stack is so small. If NMI and #MC >> didn't use IST it wouldn't work at all (but neither would SYSCALL). >> >> #2 currently saves/restores %rdi, which is also saved further down. >> This is obviously wasteful. > > Btw, can we runtime-patch the fastpath interference chunk the moment we > see a 16-bit segment? I.e., connect it to the write_idt() path, i.e. in > the hunk you've removed in there and enable the espfix checks there the > moment we load a 16-bit segment. > > I know, I know, this is not so important right now but let me put it out > there just the same.
Or we could add a TIF_NEEDS_ESPFIX that gets set once you have a 16-bit LDT entry. But I think it makes sense to nail down everything else first. I suspect that a single test-and-branch in the iret path will be lost in the noise from iret itself. --Andy -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
