[PATCH v7 15/15] arm64: mm: Unmap kernel data/bss entirely from the linear map

[Ard Biesheuvel]

From: Ard Biesheuvel <ardb@xxxxxxxxxx>

The linear aliases of the kernel text and rodata are also mapped
read-only in the linear map. Given that the contents of these regions
are mostly identical to the version in the loadable image, mapping them
read-only and leaving their contents visible is a reasonable hardening
measure.

Data and bss, however, are now also mapped read-only but the contents of
these regions are more likely to contain data that we'd rather not leak.
So let's unmap these entirely in the linear map when the kernel is
running normally.

When going into hibernation or waking up from it, these regions need to
be mapped, so map the region initially, and toggle the valid bit so
map/unmap the region as needed.

Doing so is required because pages covering the kernel image are marked
as PageReserved, and therefore disregarded for snapshotting by the
hibernate logic unless they are mapped.

Signed-off-by: Ard Biesheuvel <ardb@xxxxxxxxxx>
---
 arch/arm64/mm/mmu.c | 45 ++++++++++++++++++--
 1 file changed, 41 insertions(+), 4 deletions(-)

diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
index 7b18dc2f1721..07a6fa210171 100644
--- a/arch/arm64/mm/mmu.c
+++ b/arch/arm64/mm/mmu.c
@@ -24,6 +24,7 @@
 #include <linux/mm.h>
 #include <linux/vmalloc.h>
 #include <linux/set_memory.h>
+#include <linux/suspend.h>
 #include <linux/kfence.h>
 #include <linux/pkeys.h>
 #include <linux/mm_inline.h>
@@ -1056,6 +1057,29 @@ static void __init __map_memblock(phys_addr_t start, phys_addr_t end,
 				 end - start, prot, early_pgtable_alloc, flags);
 }
 
+static void mark_linear_data_alias_valid(bool valid)
+{
+	set_memory_valid((unsigned long)lm_alias(__init_end),
+			 (unsigned long)(__bss_stop - __init_end) / PAGE_SIZE,
+			 valid);
+}
+
+static int arm64_hibernate_pm_notify(struct notifier_block *nb,
+				     unsigned long mode, void *unused)
+{
+	switch (mode) {
+	default:
+		break;
+	case PM_POST_HIBERNATION:
+		mark_linear_data_alias_valid(false);
+		break;
+	case PM_HIBERNATION_PREPARE:
+		mark_linear_data_alias_valid(true);
+		break;
+	}
+	return 0;
+}
+
 void __init mark_linear_text_alias_ro(void)
 {
 	/*
@@ -1064,6 +1088,21 @@ void __init mark_linear_text_alias_ro(void)
 	update_mapping_prot(__pa_symbol(_text), (unsigned long)lm_alias(_text),
 			    (unsigned long)__init_begin - (unsigned long)_text,
 			    PAGE_KERNEL_RO);
+
+	/*
+	 * Register a PM notifier to remap the linear alias of data/bss as
+	 * valid read-only before hibernation. This is needed because the
+	 * snapshot logic disregards PageReserved pages (such as the ones
+	 * covering the kernel image) unless they are mapped in the linear
+	 * map.
+	 */
+	if (IS_ENABLED(CONFIG_HIBERNATION)) {
+		static struct notifier_block nb = {
+			.notifier_call = arm64_hibernate_pm_notify
+		};
+
+		register_pm_notifier(&nb);
+	}
 }
 
 #ifdef CONFIG_KFENCE
@@ -1193,10 +1232,8 @@ static void __init map_mem(void)
 			       flags);
 	}
 
-	/* Map the kernel data/bss read-only in the linear map */
-	__map_memblock(init_end, kernel_end, PAGE_KERNEL_RO, flags);
-	flush_tlb_kernel_range((unsigned long)lm_alias(__init_end),
-			       (unsigned long)lm_alias(__bss_stop));
+	/* Map the kernel data/bss as invalid in the linear map */
+	mark_linear_data_alias_valid(false);
 }
 
 void mark_rodata_ro(void)
-- 
2.54.0.823.g6e5bcc1fc9-goog