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| Started by | Russell King - ARM Linux <linux@armlinux.org.uk> |
|---|---|
| First post | 2016-08-11 01:10 +0200 |
| Last post | 2016-08-12 18:30 +0200 |
| Articles | 4 — 3 participants |
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Re: [PATCH 2/2] arm: apply more __ro_after_init Russell King - ARM Linux <linux@armlinux.org.uk> - 2016-08-11 01:10 +0200
Re: [PATCH 2/2] arm: apply more __ro_after_init Arnd Bergmann <arnd@arndb.de> - 2016-08-11 18:00 +0200
Re: [PATCH 2/2] arm: apply more __ro_after_init Kees Cook <keescook@chromium.org> - 2016-08-12 00:20 +0200
Re: [PATCH 2/2] arm: apply more __ro_after_init Russell King - ARM Linux <linux@armlinux.org.uk> - 2016-08-12 18:30 +0200
| From | Russell King - ARM Linux <linux@armlinux.org.uk> |
|---|---|
| Date | 2016-08-11 01:10 +0200 |
| Subject | Re: [PATCH 2/2] arm: apply more __ro_after_init |
| Message-ID | <s4KNj-3c9-27@gated-at.bofh.it> |
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. -- 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.
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| From | Arnd Bergmann <arnd@arndb.de> |
|---|---|
| Date | 2016-08-11 18:00 +0200 |
| Message-ID | <s50yK-5KE-23@gated-at.bofh.it> |
| In reply to | #1460088 |
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). Your other point is more important though: if one really cares about optimizing security here, they probably should disable kuser helpers completely anyway. Kees, is that something you have on your radar already? Arnd
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| From | Kees Cook <keescook@chromium.org> |
|---|---|
| Date | 2016-08-12 00:20 +0200 |
| Message-ID | <s56uu-1pD-23@gated-at.bofh.it> |
| In reply to | #1460638 |
On Thu, Aug 11, 2016 at 8:54 AM, Arnd Bergmann <arnd@arndb.de> 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). > > Your other point is more important though: if one really cares > about optimizing security here, they probably should disable > kuser helpers completely anyway. > > Kees, is that something you have on your radar already? It wasn't no. I will add it. :) -Kees -- Kees Cook Nexus Security
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| From | Russell King - ARM Linux <linux@armlinux.org.uk> |
|---|---|
| Date | 2016-08-12 18:30 +0200 |
| Message-ID | <s5nvk-403-15@gated-at.bofh.it> |
| In reply to | #1460638 |
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.
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