[Xen-devel] [PATCH v2] x86: use fixed read-only IDT

[Kees Cook <keescook@xxxxxxxxxxxx>]

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.

Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
Cc: Eric Northup <digitaleric@xxxxxxxxxx>
---
v2:
 - clarify commit and comments
---
 arch/x86/include/asm/fixmap.h |    4 +---
 arch/x86/kernel/cpu/intel.c   |   18 +-----------------
 arch/x86/kernel/traps.c       |    8 ++++++++
 arch/x86/xen/mmu.c            |    4 +---
 4 files changed, 11 insertions(+), 23 deletions(-)

diff --git a/arch/x86/include/asm/fixmap.h b/arch/x86/include/asm/fixmap.h
index a09c285..51b9e32 100644
--- a/arch/x86/include/asm/fixmap.h
+++ b/arch/x86/include/asm/fixmap.h
@@ -104,9 +104,7 @@ enum fixed_addresses {
        FIX_LI_PCIA,    /* Lithium PCI Bridge A */
        FIX_LI_PCIB,    /* Lithium PCI Bridge B */
 #endif
-#ifdef CONFIG_X86_F00F_BUG
-       FIX_F00F_IDT,   /* Virtual mapping for IDT */
-#endif
+       FIX_RO_IDT,     /* Virtual mapping for read-only IDT */
 #ifdef CONFIG_X86_CYCLONE_TIMER
        FIX_CYCLONE_TIMER, /*cyclone timer register*/
 #endif
diff --git a/arch/x86/kernel/cpu/intel.c b/arch/x86/kernel/cpu/intel.c
index 1905ce9..7170024 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;
                }
diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index 68bda7a..a2a9b78 100644
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -753,6 +753,14 @@ void __init trap_init(void)
 #endif
 
        /*
+        * Set the IDT descriptor to a fixed read-only location, so that the
+        * "sidt" instruction will not leak the location of the kernel, and
+        * to defend the IDT against arbitrary memory write vulnerabilities.
+        * It will be reloaded in cpu_init() */
+       __set_fixmap(FIX_RO_IDT, __pa_symbol(idt_table), PAGE_KERNEL_RO);
+       idt_descr.address = fix_to_virt(FIX_RO_IDT);
+
+       /*
         * Should be a barrier for any external CPU state:
         */
        cpu_init();
diff --git a/arch/x86/xen/mmu.c b/arch/x86/xen/mmu.c
index 6afbb2c..8bc4dec 100644
--- a/arch/x86/xen/mmu.c
+++ b/arch/x86/xen/mmu.c
@@ -2039,9 +2039,7 @@ static void xen_set_fixmap(unsigned idx, phys_addr_t 
phys, pgprot_t prot)
 
        switch (idx) {
        case FIX_BTMAP_END ... FIX_BTMAP_BEGIN:
-#ifdef CONFIG_X86_F00F_BUG
-       case FIX_F00F_IDT:
-#endif
+       case FIX_RO_IDT:
 #ifdef CONFIG_X86_32
        case FIX_WP_TEST:
        case FIX_VDSO:
-- 
1.7.9.5


-- 
Kees Cook
Chrome OS Security

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