[PATCH RFC v2 16/29] mm: asi: Map kernel text and static data as nonsensitive

[Brendan Jackman <jackmanb@google.com>]

Basically we need to map the kernel code and all its static variables.
Per-CPU variables need to be treated specially as described in the
comments. The cpu_entry_area is similar - this needs to be
nonsensitive so that the CPU can access the GDT etc when handling
a page fault.

Under 5-level paging, most of the kernel memory comes under a single PGD
entry (see Documentation/x86/x86_64/mm.rst. Basically, the mapping is
for this big region is the same as under 4-level, just wrapped in an
outer PGD entry). For that region, the "clone" logic is moved down one
step of the paging hierarchy.

Note that the p4d_alloc in asi_clone_p4d won't actually be used in
practice; the relevant PGD entry will always have been populated by
prior asi_map calls so this code would "work" if we just wrote
p4d_offset (but asi_clone_p4d would be broken if viewed in isolation).

The vmemmap area is not under this single PGD, it has its own 2-PGD
area, so we still use asi_clone_pgd for that one.

Signed-off-by: Brendan Jackman <jackmanb@google.com>
---
 arch/x86/mm/asi.c                 | 105 +++++++++++++++++++++++++++++++++++++-
 include/asm-generic/vmlinux.lds.h |  11 ++++
 2 files changed, 115 insertions(+), 1 deletion(-)

diff --git a/arch/x86/mm/asi.c b/arch/x86/mm/asi.c
index b951f2100b8bdea5738ded16166255deb29faf57..bc2cf0475a0e7344a66d81453f55034b2fc77eef 100644
--- a/arch/x86/mm/asi.c
+++ b/arch/x86/mm/asi.c
@@ -7,7 +7,6 @@
 #include <linux/init.h>
 #include <linux/pgtable.h>
 
-#include <asm/asi.h>
 #include <asm/cmdline.h>
 #include <asm/cpufeature.h>
 #include <asm/page.h>
@@ -186,8 +185,68 @@ void __init asi_check_boottime_disable(void)
 		pr_info("ASI enablement ignored due to incomplete implementation.\n");
 }
 
+/*
+ * Map data by sharing sub-PGD pagetables with the unrestricted mapping. This is
+ * more efficient than asi_map, but only works when you know the whole top-level
+ * page needs to be mapped in the restricted tables. Note that the size of the
+ * mappings this creates differs between 4 and 5-level paging.
+ */
+static void asi_clone_pgd(pgd_t *dst_table, pgd_t *src_table, size_t addr)
+{
+	pgd_t *src = pgd_offset_pgd(src_table, addr);
+	pgd_t *dst = pgd_offset_pgd(dst_table, addr);
+
+	if (!pgd_val(*dst))
+		set_pgd(dst, *src);
+	else
+		WARN_ON_ONCE(pgd_val(*dst) != pgd_val(*src));
+}
+
+/*
+ * For 4-level paging this is exactly the same as asi_clone_pgd. For 5-level
+ * paging it clones one level lower. So this always creates a mapping of the
+ * same size.
+ */
+static void asi_clone_p4d(pgd_t *dst_table, pgd_t *src_table, size_t addr)
+{
+	pgd_t *src_pgd = pgd_offset_pgd(src_table, addr);
+	pgd_t *dst_pgd = pgd_offset_pgd(dst_table, addr);
+	p4d_t *src_p4d = p4d_alloc(&init_mm, src_pgd, addr);
+	p4d_t *dst_p4d = p4d_alloc(&init_mm, dst_pgd, addr);
+
+	if (!p4d_val(*dst_p4d))
+		set_p4d(dst_p4d, *src_p4d);
+	else
+		WARN_ON_ONCE(p4d_val(*dst_p4d) != p4d_val(*src_p4d));
+}
+
+/*
+ * percpu_addr is where the linker put the percpu variable. asi_map_percpu finds
+ * the place where the percpu allocator copied the data during boot.
+ *
+ * This is necessary even when the page allocator defaults to
+ * global-nonsensitive, because the percpu allocator uses the memblock allocator
+ * for early allocations.
+ */
+static int asi_map_percpu(struct asi *asi, void *percpu_addr, size_t len)
+{
+	int cpu, err;
+	void *ptr;
+
+	for_each_possible_cpu(cpu) {
+		ptr = per_cpu_ptr(percpu_addr, cpu);
+		err = asi_map(asi, ptr, len);
+		if (err)
+			return err;
+	}
+
+	return 0;
+}
+
 static int __init asi_global_init(void)
 {
+	int err;
+
 	if (!boot_cpu_has(X86_FEATURE_ASI))
 		return 0;
 
@@ -207,6 +266,46 @@ static int __init asi_global_init(void)
 				  VMALLOC_START, VMALLOC_END,
 				  "ASI Global Non-sensitive vmalloc");
 
+	/* Map all kernel text and static data */
+	err = asi_map(ASI_GLOBAL_NONSENSITIVE, (void *)__START_KERNEL,
+		      (size_t)_end - __START_KERNEL);
+	if (WARN_ON(err))
+		return err;
+	err = asi_map(ASI_GLOBAL_NONSENSITIVE, (void *)FIXADDR_START,
+		      FIXADDR_SIZE);
+	if (WARN_ON(err))
+		return err;
+	/* Map all static percpu data */
+	err = asi_map_percpu(
+		ASI_GLOBAL_NONSENSITIVE,
+		__per_cpu_start, __per_cpu_end - __per_cpu_start);
+	if (WARN_ON(err))
+		return err;
+
+	/*
+	 * The next areas are mapped using shared sub-P4D paging structures
+	 * (asi_clone_p4d instead of asi_map), since we know the whole P4D will
+	 * be mapped.
+	 */
+	asi_clone_p4d(asi_global_nonsensitive_pgd, init_mm.pgd,
+		      CPU_ENTRY_AREA_BASE);
+#ifdef CONFIG_X86_ESPFIX64
+	asi_clone_p4d(asi_global_nonsensitive_pgd, init_mm.pgd,
+		      ESPFIX_BASE_ADDR);
+#endif
+	/*
+	 * The vmemmap area actually _must_ be cloned via shared paging
+	 * structures, since mappings can potentially change dynamically when
+	 * hugetlbfs pages are created or broken down.
+	 *
+	 * We always clone 2 PGDs, this is a corrolary of the sizes of struct
+	 * page, a page, and the physical address space.
+	 */
+	WARN_ON(sizeof(struct page) * MAXMEM / PAGE_SIZE != 2 * (1UL << PGDIR_SHIFT));
+	asi_clone_pgd(asi_global_nonsensitive_pgd, init_mm.pgd, VMEMMAP_START);
+	asi_clone_pgd(asi_global_nonsensitive_pgd, init_mm.pgd,
+		      VMEMMAP_START + (1UL << PGDIR_SHIFT));
+
 	return 0;
 }
 subsys_initcall(asi_global_init)
@@ -599,6 +698,10 @@ static bool follow_physaddr(
  * Map the given range into the ASI page tables. The source of the mapping is
  * the regular unrestricted page tables. Can be used to map any kernel memory.
  *
+ * In contrast to some internal ASI logic (asi_clone_pgd and asi_clone_p4d) this
+ * never shares pagetables between restricted and unrestricted address spaces,
+ * instead it creates wholly new equivalent mappings.
+ *
  * The caller MUST ensure that the source mapping will not change during this
  * function. For dynamic kernel memory, this is generally ensured by mapping the
  * memory within the allocator.
diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
index eeadbaeccf88b73af40efe5221760a7cb37058d2..18f6c0448baf5dfbd0721ba9a6d89000fa86f061 100644
--- a/include/asm-generic/vmlinux.lds.h
+++ b/include/asm-generic/vmlinux.lds.h
@@ -1022,6 +1022,16 @@
 	COMMON_DISCARDS							\
 	}
 
+/*
+ * ASI maps certain sections with certain sensitivity levels, so they need to
+ * have a page-aligned size.
+ */
+#ifdef CONFIG_MITIGATION_ADDRESS_SPACE_ISOLATION
+#define ASI_ALIGN() ALIGN(PAGE_SIZE)
+#else
+#define ASI_ALIGN() .
+#endif
+
 /**
  * PERCPU_INPUT - the percpu input sections
  * @cacheline: cacheline size
@@ -1043,6 +1053,7 @@
 	*(.data..percpu)						\
 	*(.data..percpu..shared_aligned)				\
 	PERCPU_DECRYPTED_SECTION					\
+	. = ASI_ALIGN();						\
 	__per_cpu_end = .;
 
 /**

-- 
2.47.1.613.gc27f4b7a9f-goog