x86: Use a read-only IDT alias on all CPUs

Make a copy of the IDT (as seen via the "sidt" instruction) read-only.
This primarily removes the IDT from being a target for arbitrary memory
write attacks, and has the added benefit of also not leaking the kernel
base offset, if it has been relocated.

We already did this on vendor == Intel and family == 5 because of the
F0 0F bug -- regardless of if a particular CPU had the F0 0F bug or
not.  Since the workaround was so cheap, there simply was no reason to
be very specific.  This patch extends the readonly alias to all CPUs,
but does not activate the #PF to #UD conversion code needed to deliver
the proper exception in the F0 0F case except on Intel family 5
processors.

Signed-off-by: Kees Cook <keescook@chromium.org>
Link: http://lkml.kernel.org/r/20130410192422.GA17344@www.outflux.net
Cc: Eric Northup <digitaleric@google.com>
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>

1 files changed, 1 insertions, 17 deletions

diff --git a/arch/x86/kernel/cpu/intel.c b/arch/x86/kernel/cpu/intel.c
index 1905ce98bee0..71700247a5d7 100644
--- a/arch/x86/kernel/cpu/intel.c
+++ b/arch/x86/kernel/cpu/intel.c
@@ -164,20 +164,6 @@ int __cpuinit ppro_with_ram_bug(void)
         return 0;
 }
 
-#ifdef CONFIG_X86_F00F_BUG
-static void __cpuinit trap_init_f00f_bug(void)
-{
-        __set_fixmap(FIX_F00F_IDT, __pa_symbol(idt_table), PAGE_KERNEL_RO);
-
-        /*
-         * Update the IDT descriptor and reload the IDT so that
-         * it uses the read-only mapped virtual address.
-         */
-        idt_descr.address = fix_to_virt(FIX_F00F_IDT);
-        load_idt(&idt_descr);
-}
-#endif
-
 static void __cpuinit intel_smp_check(struct cpuinfo_x86 *c)
 {
         /* calling is from identify_secondary_cpu() ? */
@@ -206,8 +192,7 @@ static void __cpuinit intel_workarounds(struct cpuinfo_x86 *c)
         /*
          * All current models of Pentium and Pentium with MMX technology CPUs
          * have the F0 0F bug, which lets nonprivileged users lock up the
-         * system.
-         * Note that the workaround only should be initialized once...
+         * system. Announce that the fault handler will be checking for it.
          */
         c->f00f_bug = 0;
         if (!paravirt_enabled() && c->x86 == 5) {
@@ -215,7 +200,6 @@ static void __cpuinit intel_workarounds(struct cpuinfo_x86 *c)
 
                 c->f00f_bug = 1;
                 if (!f00f_workaround_enabled) {
-                        trap_init_f00f_bug();
                         printk(KERN_NOTICE "Intel Pentium with F0 0F bug - workaround enabled.\n");
                         f00f_workaround_enabled = 1;
                 }