From: Peter Zijlstra <peterz@infradead.org>
Date: Thu, 27 Oct 2022 14:54:41 -0700
Subject: x86/mm: Randomize per-cpu entry area
Git-commit: 97e3d26b5e5f371b3ee223d94dd123e6c442ba80
Patch-mainline: v6.2-rc1
References: bsc#1207845 CVE-2023-0597

Seth found that the CPU-entry-area; the piece of per-cpu data that is
mapped into the userspace page-tables for kPTI is not subject to any
randomization -- irrespective of kASLR settings.

On x86_64 a whole P4D (512 GB) of virtual address space is reserved for
this structure, which is plenty large enough to randomize things a
little.

As such, use a straight forward randomization scheme that avoids
duplicates to spread the existing CPUs over the available space.

  [ bp: Fix le build. ]

Reported-by: Seth Jenkins <sethjenkins@google.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
[mkoutny: v5.14 backport: init_cea_offsets() is called way before
	  prandom_init_early() initcall, prandom_u32_max() is not properly
	  seeded yet. Use KASLR seed and local state to generate CPU entry
	  areas offsets, this is based on the approach in
	  kernel_randomize_memory() and should provide same randomness
	  guarantees -- beware we don't get cryptographically secure random
	  offsets. This reduces effective entropy in exfiltrating *all* CPU
	  entry areas by log2(nr_cpus) bits, entropy for *any* CPU is
	  unaffected.
	  This was chosen instead of backporting f62384995e4c ("random: split
	  initialization into early step and later step") and crng related
	  reworks.]
[mkoutny: v5.3 backport: dropped hw_breakpoint hunk without 24ae0c91cbc5
	  ("x86/hw_breakpoint: Prevent data breakpoints on cpu_entry_area"),
	  adjusted context for missing doublefault_stack on 32b]
Acked-by: Michal Koutný <mkoutny@suse.com>
---
 arch/x86/include/asm/cpu_entry_area.h |   13 ++++-----
 arch/x86/mm/cpu_entry_area.c          |   48 ++++++++++++++++++++++++++++++++--
 2 files changed, 53 insertions(+), 8 deletions(-)

--- a/arch/x86/include/asm/cpu_entry_area.h
+++ b/arch/x86/include/asm/cpu_entry_area.h
@@ -111,10 +111,6 @@ struct cpu_entry_area {
 };
 
 #define CPU_ENTRY_AREA_SIZE		(sizeof(struct cpu_entry_area))
-#define CPU_ENTRY_AREA_ARRAY_SIZE	(CPU_ENTRY_AREA_SIZE * NR_CPUS)
-
-/* Total size includes the readonly IDT mapping page as well: */
-#define CPU_ENTRY_AREA_TOTAL_SIZE	(CPU_ENTRY_AREA_ARRAY_SIZE + PAGE_SIZE)
 
 DECLARE_PER_CPU(struct cpu_entry_area *, cpu_entry_area);
 DECLARE_PER_CPU(struct cea_exception_stacks *, cea_exception_stacks);
@@ -128,8 +124,13 @@ extern void cea_set_pte(void *cea_vaddr,
 
 #define CPU_ENTRY_AREA_RO_IDT_VADDR	((void *)CPU_ENTRY_AREA_RO_IDT)
 
-#define CPU_ENTRY_AREA_MAP_SIZE			\
-	(CPU_ENTRY_AREA_PER_CPU + CPU_ENTRY_AREA_ARRAY_SIZE - CPU_ENTRY_AREA_BASE)
+#ifdef CONFIG_X86_32
+#define CPU_ENTRY_AREA_MAP_SIZE		(CPU_ENTRY_AREA_PER_CPU +		\
+					 (CPU_ENTRY_AREA_SIZE * NR_CPUS) -	\
+					 CPU_ENTRY_AREA_BASE)
+#else
+#define CPU_ENTRY_AREA_MAP_SIZE		P4D_SIZE
+#endif
 
 extern struct cpu_entry_area *get_cpu_entry_area(int cpu);
 
--- a/arch/x86/mm/cpu_entry_area.c
+++ b/arch/x86/mm/cpu_entry_area.c
@@ -4,6 +4,7 @@
 #include <linux/percpu.h>
 #include <linux/kallsyms.h>
 #include <linux/kcore.h>
+#include <linux/prandom.h>
 
 #include <asm/cpu_entry_area.h>
 #include <asm/pgtable.h>
@@ -15,11 +16,53 @@ static DEFINE_PER_CPU_PAGE_ALIGNED(struc
 #ifdef CONFIG_X86_64
 static DEFINE_PER_CPU_PAGE_ALIGNED(struct exception_stacks, exception_stacks);
 DEFINE_PER_CPU(struct cea_exception_stacks*, cea_exception_stacks);
+
+static DEFINE_PER_CPU_READ_MOSTLY(unsigned long, _cea_offset);
+
+static __always_inline unsigned int cea_offset(unsigned int cpu)
+{
+	return per_cpu(_cea_offset, cpu);
+}
+
+static __init void init_cea_offsets(void)
+{
+	struct rnd_state rand_state;
+	unsigned int max_cea, rand;
+	unsigned int i, j;
+
+	max_cea = (CPU_ENTRY_AREA_MAP_SIZE - PAGE_SIZE) / CPU_ENTRY_AREA_SIZE;
+	prandom_seed_state(&rand_state, kaslr_get_random_long("CPU entry"));
+
+	/* O(sodding terrible) */
+	for_each_possible_cpu(i) {
+		unsigned int cea;
+
+again:
+		prandom_bytes_state(&rand_state, &rand, sizeof(rand));
+		cea = rand % max_cea;
+
+		for_each_possible_cpu(j) {
+			if (cea_offset(j) == cea)
+				goto again;
+
+			if (i == j)
+				break;
+		}
+
+		per_cpu(_cea_offset, i) = cea;
+	}
+}
+#else /* !X86_64 */
+static __always_inline unsigned int cea_offset(unsigned int cpu)
+{
+	return cpu;
+}
+static inline void init_cea_offsets(void) { }
 #endif
 
 struct cpu_entry_area *get_cpu_entry_area(int cpu)
 {
-	unsigned long va = CPU_ENTRY_AREA_PER_CPU + cpu * CPU_ENTRY_AREA_SIZE;
+	unsigned long va = CPU_ENTRY_AREA_PER_CPU + cea_offset(cpu) * CPU_ENTRY_AREA_SIZE;
 	BUILD_BUG_ON(sizeof(struct cpu_entry_area) % PAGE_SIZE != 0);
 
 	return (struct cpu_entry_area *) va;
@@ -180,7 +223,6 @@ static __init void setup_cpu_entry_area_
 
 	/* The +1 is for the readonly IDT: */
 	BUILD_BUG_ON((CPU_ENTRY_AREA_PAGES+1)*PAGE_SIZE != CPU_ENTRY_AREA_MAP_SIZE);
-	BUILD_BUG_ON(CPU_ENTRY_AREA_TOTAL_SIZE != CPU_ENTRY_AREA_MAP_SIZE);
 	BUG_ON(CPU_ENTRY_AREA_BASE & ~PMD_MASK);
 
 	start = CPU_ENTRY_AREA_BASE;
@@ -196,6 +238,8 @@ void __init setup_cpu_entry_areas(void)
 {
 	unsigned int cpu;
 
+	init_cea_offsets();
+
 	setup_cpu_entry_area_ptes();
 
 	for_each_possible_cpu(cpu)
