On Tue, Dec 8, 2015 at 6:19 AM, Borislav Petkov <bp@xxxxxxxxx> wrote: > On Tue, Dec 08, 2015 at 12:25:57PM +0000, Matt Fleming wrote: >> On Mon, 07 Dec, at 11:10:43PM, Kosuke Tatsukawa wrote: >> > >> > Thank you pointing that out. >> > >> > linux-4.4-rc3 booted without a problem on a real server even with XD >> > turned off by the firmware. I didn't notice this before because I was > > The aforementioned patch reenables NX. > >> Borislav, what do you think about stripping PAGE_NX from 'page_flags' >> in kernel_map_pages_in_pgd() if NX isn't supported, rather than >> returning EINVAL? At least that way EFI runtime services would still >> work. > > I guess we can - I mean, I don't see what can go wrong more when > allowing the kernel to execute even NX UEFI regions. Maybe easier > generation of "gadgets" in the ROP sense ... > > On a related node, I'm very sceptical of the existence of this "noexec" > chicken bit, if you ask me. It is a really bad idea, security-wise, to > disable NX. Is there even a valid use case to disable NX? > > Because if not, I'd vote for removing that chicken bit or at least > taining the kernel with > > add_taint(TAINT_USER_MORON, ... ); If we add this for not-nx, I would like to add it for not-rodata too. > Kees, has this NX disabling practice come up in the past, per chance... ? I've never seen anyone actually use it. I was asked to include it out of fear of some kind of rogue imagined CPU configuration that mixed NX and non-NX capable CPUs in a single machine where the forced NX re-enablement code would cause problems. As you might imagine, I'm not aware of this case ever being an issue. ;) -Kees -- Kees Cook Chrome OS & Brillo Security -- 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
