x86/cpu: Use pinning mask for CR4 bits needing to be 0

The X86_CR4_FSGSBASE bit of CR4 should not change after boot[1]. Older
kernels should enforce this bit to zero, and newer kernels need to
enforce it depending on boot-time configuration (e.g. "nofsgsbase").
To support a pinned bit being either 1 or 0, use an explicit mask in
combination with the expected pinned bit values.

[1] https://lore.kernel.org/lkml/20200527103147.GI325280@hirez.programming.kicks-ass.net

Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: stable@vger.kernel.org
Link: https://lkml.kernel.org/r/202006082013.71E29A42@keescook

diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index 043d93c..95c090a 100644 (file)
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -347,6 +347,9 @@ out:
        cr4_clear_bits(X86_CR4_UMIP);
 }
 
+/* These bits should not change their value after CPU init is finished. */
+static const unsigned long cr4_pinned_mask =
+       X86_CR4_SMEP | X86_CR4_SMAP | X86_CR4_UMIP | X86_CR4_FSGSBASE;
 static DEFINE_STATIC_KEY_FALSE_RO(cr_pinning);
 static unsigned long cr4_pinned_bits __ro_after_init;
 
@@ -371,20 +374,20 @@ EXPORT_SYMBOL(native_write_cr0);
 
 void native_write_cr4(unsigned long val)
 {
-       unsigned long bits_missing = 0;
+       unsigned long bits_changed = 0;
 
 set_register:
        asm volatile("mov %0,%%cr4": "+r" (val), "+m" (cr4_pinned_bits));
 
        if (static_branch_likely(&cr_pinning)) {
-               if (unlikely((val & cr4_pinned_bits) != cr4_pinned_bits)) {
-                       bits_missing = ~val & cr4_pinned_bits;
-                       val |= bits_missing;
+               if (unlikely((val & cr4_pinned_mask) != cr4_pinned_bits)) {
+                       bits_changed = (val & cr4_pinned_mask) ^ cr4_pinned_bits;
+                       val = (val & ~cr4_pinned_mask) | cr4_pinned_bits;
                        goto set_register;
                }
-               /* Warn after we've set the missing bits. */
-               WARN_ONCE(bits_missing, "CR4 bits went missing: %lx!?\n",
-                         bits_missing);
+               /* Warn after we've corrected the changed bits. */
+               WARN_ONCE(bits_changed, "pinned CR4 bits changed: 0x%lx!?\n",
+                         bits_changed);
        }
 }
 #if IS_MODULE(CONFIG_LKDTM)
@@ -419,7 +422,7 @@ void cr4_init(void)
        if (boot_cpu_has(X86_FEATURE_PCID))
                cr4 |= X86_CR4_PCIDE;
        if (static_branch_likely(&cr_pinning))
-               cr4 |= cr4_pinned_bits;
+               cr4 = (cr4 & ~cr4_pinned_mask) | cr4_pinned_bits;
 
        __write_cr4(cr4);
 
@@ -434,10 +437,7 @@ void cr4_init(void)
  */
 static void __init setup_cr_pinning(void)
 {
-       unsigned long mask;
-
-       mask = (X86_CR4_SMEP | X86_CR4_SMAP | X86_CR4_UMIP);
-       cr4_pinned_bits = this_cpu_read(cpu_tlbstate.cr4) & mask;
+       cr4_pinned_bits = this_cpu_read(cpu_tlbstate.cr4) & cr4_pinned_mask;
        static_key_enable(&cr_pinning.key);
 }