On Mon, Nov 30, 2015 at 2:24 PM, Russell King - ARM Linux <linux@xxxxxxxxxxxxxxxx> wrote: > On Mon, Nov 30, 2015 at 01:52:10PM -0800, Kees Cook wrote: >> On Wed, Nov 25, 2015 at 11:51 PM, Ingo Molnar <mingo@xxxxxxxxxx> wrote: >> > * Kees Cook <keescook@xxxxxxxxxxxx> wrote: >> >> +#ifdef CONFIG_DEBUG_RODATA >> > >> > Btw., could you please remove the Kconfig option altogether in an additional patch >> > and make read-only sections an always-on feature? It has been default-y for years >> > and all distros have it enabled. >> >> Yeah, this is something I've wanted to do for a while, but I would >> point out that only a few architectures have actually implemented it, >> and for arm and arm64 it was very recent: > > I don't think it can entirely be a kernel command line option. On ARM, > enabling DEBUG_RODATA has a substantial effect on the size of the kernel > image - we have to pad various sections to 1MB boundaries so we can > set the appropriate permissions. > > Forcing this layout on everyone won't work. > > What we can do is the half-way house: we can have the kernel command > line option which enables and disables the protections, but the layout > of the kernel image would still need to be controlled by DEBUG_RODATA. > I'm left wondering what the advantage of that would be: it'd end up > offering a suboptimal layout, additional memory usage but without the > benefits of memory protections. Right, I think it'll be there just as a debugging assist: something broke with DEBUG_RODATA, let's boot with rodata=off and see what happens. > The alternative is keeping the kernel in unlinked object form, and > laying out and linking the kernel at boot time, probably in PIC > assembly code. That's possible but I think is undesirable. > > So all in all, I'm in favour of keeping things as they are on ARM. I've looked at the implementation in ARM again, and I think I see how it can be improved slightly. I think I named things incorrectly when I implemented, and I'll be sending a patch to fix that up. In the end, though, I agree: the thing that is CONFIG_DEBUG_RODATA may change its name, but on some architectures, there is a cost to using it, so it needs to remain a CONFIG. -Kees -- Kees Cook Chrome OS & Brillo Security -- To unsubscribe from this list: send the line "unsubscribe linux-arch" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
