[PATCH] x86_64: Unmap NULL during early bootup

We should zap the low mappings, as soon as possible, so that we can catch
kernel bugs more effectively. Previously early boot had NULL mapped
and didn't trap on NULL references.

This patch introduces boot_level4_pgt, which will always have low identity
addresses mapped.  Druing boot, all the processors will use this as their
level4 pgt.  On BP, we will switch to init_level4_pgt as soon as we enter C
code and zap the low mappings as soon as we are done with the usage of
identity low mapped addresses.  On AP's we will zap the low mappings as
soon as we jump to C code.

Signed-off-by: Suresh Siddha <suresh.b.siddha@intel.com>
Signed-off-by: Ashok Raj <ashok.raj@intel.com>
Signed-off-by: Andi Kleen <ak@suse.de>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 17 insertions, 11 deletions

diff --git a/arch/x86_64/mm/init.c b/arch/x86_64/mm/init.c
index 2b1d6c382396..be483a1d7b54 100644
--- a/arch/x86_64/mm/init.c
+++ b/arch/x86_64/mm/init.c
@@ -312,12 +312,19 @@ void __init init_memory_mapping(unsigned long start, unsigned long end)
 
 extern struct x8664_pda cpu_pda[NR_CPUS];
 
-/* Assumes all CPUs still execute in init_mm */
-void zap_low_mappings(void)
+void __cpuinit zap_low_mappings(int cpu)
 {
-	pgd_t *pgd = pgd_offset_k(0UL);
-	pgd_clear(pgd);
-	flush_tlb_all();
+	if (cpu == 0) {
+		pgd_t *pgd = pgd_offset_k(0UL);
+		pgd_clear(pgd);
+	} else {
+		/*
+		 * For AP's, zap the low identity mappings by changing the cr3
+		 * to init_level4_pgt and doing local flush tlb all
+		 */
+		asm volatile("movq %0,%%cr3" :: "r" (__pa_symbol(&init_level4_pgt)));
+	}
+	__flush_tlb_all();
 }
 
 /* Compute zone sizes for the DMA and DMA32 zones in a node. */
@@ -474,14 +481,13 @@ void __init mem_init(void)
 		datasize >> 10,
 		initsize >> 10);
 
+#ifdef CONFIG_SMP
 	/*
-	 * Subtle. SMP is doing its boot stuff late (because it has to
-	 * fork idle threads) - but it also needs low mappings for the
-	 * protected-mode entry to work. We zap these entries only after
-	 * the WP-bit has been tested.
+	 * Sync boot_level4_pgt mappings with the init_level4_pgt
+	 * except for the low identity mappings which are already zapped
+	 * in init_level4_pgt. This sync-up is essential for AP's bringup
 	 */
-#ifndef CONFIG_SMP
-	zap_low_mappings();
+	memcpy(boot_level4_pgt+1, init_level4_pgt+1, (PTRS_PER_PGD-1)*sizeof(pgd_t));
 #endif
 }