On Thu, Aug 11, 2016 at 05:54:08PM +0200, Arnd Bergmann wrote: > On Thursday, August 11, 2016 12:06:45 AM CEST Russell King - ARM Linux wrote: > > On Wed, Aug 10, 2016 at 09:41:23PM +0200, Arnd Bergmann wrote: > > > It might be better to start by making the fixed mapping readonly, > > > as KASLR doesn't protect that one at all, and change the TLS > > > code accordingly. > > > > I think that's impossible, because we gave userspace permission to > > read 0xffff0ff0 directly without using __kuser_get_tls. You're > > talking about potentially breaking userspace. > > > > If you disable kuser helpers, then the page becomes read-only and > > invisible to userspace anyway. So, everything is being done there > > which can be done - if you have kuser helpers enabled, then you > > lose some opportunities for these security improvements. > > What I meant was writing to the page through the linear mapping > rather than the virtual mapping at 0xffff0000 so we can leave that > one read-only (I did not consider whether that might cause cache > aliasing problems when reading from the other address). Kees original patch was about moving the vector pages into the read-only area after init, so the linear mapping of them becomes read-only as well. So that won't work. We need at least one read-write mapping for FIQ, and for context switching for kuser helpers. > Your other point is more important though: if one really cares > about optimizing security here, they probably should disable > kuser helpers completely anyway. We could probably predicate moving the vectors page into the RO section when kuser helpers are enabled. > Kees, is that something you have on your radar already? I believe Android already disable kuser helpers as of a few years ago. -- RMK's Patch system: http://www.armlinux.org.uk/developer/patches/ FTTC broadband for 0.8mile line: currently at 9.6Mbps down 400kbps up according to speedtest.net. -- 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
