[PATCH v6 4/4] x86/mm/KASLR: Adjust the padding size for the direct mapping.

[Masayoshi Mizuma <msys.mizuma@gmail.com>]

From: Masayoshi Mizuma <m.mizuma@jp.fujitsu.com>

The system sometimes crashes while memory hot-adding on KASLR
enabled system. The crash happens because the regions pointed by
kaslr_regions[].base are overwritten by the hot-added memory.

It happens because of the padding size for kaslr_regions[].base isn't
enough for the system whose physical memory layout has huge space for
memory hotplug. kaslr_regions[].base points "actual installed
memory size + padding" or higher address. So, if the "actual + padding"
is lower address than the maximum memory address, which means the memory
address reachable by memory hot-add, kaslr_regions[].base is destroyed by
the overwritten.

  address
    ^
    |------- maximum memory address (Hotplug)
    |                                    ^
    |------- kaslr_regions[0].base       | Hotadd-able region
    |     ^                              |
    |     | padding                      |
    |     V                              V
    |------- actual memory address (Installed on boot)
    |

Fix it by getting the maximum memory address from SRAT and store
the value in boot_param, then set the padding size while KASLR
initializing if the default padding size isn't enough.

Acked-by: Baoquan He <bhe@redhat.com>
Signed-off-by: Masayoshi Mizuma <m.mizuma@jp.fujitsu.com>
---
 arch/x86/mm/kaslr.c | 63 ++++++++++++++++++++++++++++++++++-----------
 1 file changed, 48 insertions(+), 15 deletions(-)

diff --git a/arch/x86/mm/kaslr.c b/arch/x86/mm/kaslr.c
index dc6182eecefa..b3f2f468d20f 100644
--- a/arch/x86/mm/kaslr.c
+++ b/arch/x86/mm/kaslr.c
@@ -70,15 +70,58 @@ static inline bool kaslr_memory_enabled(void)
 	return kaslr_enabled() && !IS_ENABLED(CONFIG_KASAN);
 }
 
+/*
+ * Even though a huge virtual address space is reserved for the direct
+ * mapping of physical memory, e.g in 4-level paging mode, it's 64TB,
+ * rare system can own enough physical memory to use it up, most are
+ * even less than 1TB. So with KASLR enabled, we adapt the size of
+ * direct mapping area to the size of actual physical memory plus the
+ * configured padding CONFIG_RANDOMIZE_MEMORY_PHYSICAL_PADDING.
+ * The left part will be taken out to join memory randomization.
+ *
+ * And in case CONFIG_MEMORY_HOTPLUG is enabled and boot_params.max_addr
+ * has a physical memory address, that means the system memory can be
+ * expanded by memory hot-add to boot_params.max_addr. Set the size
+ * of direct mapping area to boot_params.max_addr so that we can avoid
+ * overwriting the hot-added memory.
+ */
+static inline unsigned long calc_direct_mapping_size(void)
+{
+	unsigned long size_tb, memory_tb;
+
+	memory_tb = DIV_ROUND_UP(max_pfn << PAGE_SHIFT, 1UL << TB_SHIFT) +
+		CONFIG_RANDOMIZE_MEMORY_PHYSICAL_PADDING;
+
+#ifdef CONFIG_MEMORY_HOTPLUG
+	if (boot_params.max_addr) {
+		unsigned long maximum_tb;
+
+		maximum_tb = DIV_ROUND_UP(boot_params.max_addr,
+				1UL << TB_SHIFT);
+
+		if (maximum_tb > memory_tb)
+			memory_tb = maximum_tb;
+	}
+#endif
+	size_tb = 1 << (MAX_PHYSMEM_BITS - TB_SHIFT);
+
+	/*
+	 * Adapt physical memory region size based on available memory
+	 */
+	if (memory_tb < size_tb)
+		size_tb = memory_tb;
+
+	return size_tb;
+}
+
 /* Initialize base and padding for each memory region randomized with KASLR */
 void __init kernel_randomize_memory(void)
 {
-	size_t i;
-	unsigned long vaddr_start, vaddr;
-	unsigned long rand, memory_tb;
-	struct rnd_state rand_state;
+	unsigned long vaddr_start, vaddr, rand;
 	unsigned long remain_entropy;
 	unsigned long vmemmap_size;
+	struct rnd_state rand_state;
+	size_t i;
 
 	vaddr_start = pgtable_l5_enabled() ? __PAGE_OFFSET_BASE_L5 : __PAGE_OFFSET_BASE_L4;
 	vaddr = vaddr_start;
@@ -95,20 +138,10 @@ void __init kernel_randomize_memory(void)
 	if (!kaslr_memory_enabled())
 		return;
 
-	kaslr_regions[0].size_tb = 1 << (MAX_PHYSMEM_BITS - TB_SHIFT);
+	kaslr_regions[0].size_tb = calc_direct_mapping_size();
 	kaslr_regions[1].size_tb = VMALLOC_SIZE_TB;
 
-	/*
-	 * Update Physical memory mapping to available and
-	 * add padding if needed (especially for memory hotplug support).
-	 */
 	BUG_ON(kaslr_regions[0].base != &page_offset_base);
-	memory_tb = DIV_ROUND_UP(max_pfn << PAGE_SHIFT, 1UL << TB_SHIFT) +
-		CONFIG_RANDOMIZE_MEMORY_PHYSICAL_PADDING;
-
-	/* Adapt phyiscal memory region size based on available memory */
-	if (memory_tb < kaslr_regions[0].size_tb)
-		kaslr_regions[0].size_tb = memory_tb;
 
 	/*
 	 * Calculate the vmemmap region size in TBs, aligned to a TB
-- 
2.20.1