Messages in this thread | | | Subject | Re: [PATCH v2 0/4] x86: clear XD_DISABLED flag on Intel to regain NX | From | "H. Peter Anvin" <> | Date | Sat, 19 Jun 2010 11:08:13 -0700 |
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SMM is not affected; it doesn't use the kernel page tables.
"Kees Cook" <kees.cook@canonical.com> wrote:
>Hi, > >On Sat, Jun 19, 2010 at 08:16:42AM -0700, Arjan van de Ven wrote: >> On Sat, 19 Jun 2010 10:21:29 +0200 >> Andi Kleen <andi@firstfloor.org> wrote: >> >> > Kees Cook <kees.cook@canonical.com> writes: >> > >> > > This will clear the MSR_IA32_MISC_ENABLE_XD_DISABLE bit so that NX >> > > cannot be inappropriately controlled by the BIOS on Intel CPUs. If >> > > NX actually needs to be disabled, "noexec=off" can be used. >> > >> > The patch still seems like a bad idea to me. What happens if >> > the NX bit is broken for some reason and the BIOS is right >> > to disable it? >> >> >> overriding the bios like this is almost always a bad idea. >> (you're doing a blanket override, not a specific, verified override) > >I've seen other things in the BIOS ignored (IDE bus settings jumps to >mind), so I figured it wasn't strictly bad. From what I've been able to >gather, this setting is never correct. If there are situations where it >must be left alone, we could add those as exceptions. > >> you have no idea if the SMM code can deal with NX, etc etc. > >The pages don't get marked as actually NX until setup_nx() is called, at >which point "noexec=off" would have already been handled, so if that >happens, a system can still boot with that cmdline option. > >> the real answer is "fix your bios setting". > >Well, the "best" answer is "fix the bios", which is why I got Dell to >fix their BIOSes. Unfortunately, there are still systems with this >misconfigured. > >> Don't as owner of the machine turn something off in the bios that you >> actually want. > >Most people don't know/care, so if they do and it's a problem, I thought >using "noexec=off" would be sufficient while still allowing the bulk of >systems to end up with NX correctly enabled. > >-Kees > >-- >Kees Cook >Ubuntu Security Team
-- Sent from my mobile phone. Please pardon any lack of formatting.
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