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Subject[tip: x86/mm] x86/mm: Randomize per-cpu entry area
The following commit has been merged into the x86/mm branch of tip:

Commit-ID: d6d1a3923799d04f56cd644a9cb7ecf19de67949
Gitweb: https://git.kernel.org/tip/d6d1a3923799d04f56cd644a9cb7ecf19de67949
Author: Peter Zijlstra <peterz@infradead.org>
AuthorDate: Thu, 27 Oct 2022 14:54:41 -07:00
Committer: Dave Hansen <dave.hansen@linux.intel.com>
CommitterDate: Thu, 27 Oct 2022 15:01:53 -07:00

x86/mm: Randomize per-cpu entry area

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.

Reported-by: Seth Jenkins <sethjenkins@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
---
arch/x86/include/asm/cpu_entry_area.h | 4 +--
arch/x86/include/asm/pgtable_areas.h | 8 ++++-
arch/x86/kernel/hw_breakpoint.c | 2 +-
arch/x86/mm/cpu_entry_area.c | 46 +++++++++++++++++++++++---
4 files changed, 50 insertions(+), 10 deletions(-)

diff --git a/arch/x86/include/asm/cpu_entry_area.h b/arch/x86/include/asm/cpu_entry_area.h
index 75efc4c..462fc34 100644
--- a/arch/x86/include/asm/cpu_entry_area.h
+++ b/arch/x86/include/asm/cpu_entry_area.h
@@ -130,10 +130,6 @@ 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);
diff --git a/arch/x86/include/asm/pgtable_areas.h b/arch/x86/include/asm/pgtable_areas.h
index d34cce1..62e5ede 100644
--- a/arch/x86/include/asm/pgtable_areas.h
+++ b/arch/x86/include/asm/pgtable_areas.h
@@ -11,6 +11,12 @@

#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

#endif /* _ASM_X86_PGTABLE_AREAS_H */
diff --git a/arch/x86/kernel/hw_breakpoint.c b/arch/x86/kernel/hw_breakpoint.c
index 668a4a6..bbb0f73 100644
--- a/arch/x86/kernel/hw_breakpoint.c
+++ b/arch/x86/kernel/hw_breakpoint.c
@@ -266,7 +266,7 @@ static inline bool within_cpu_entry(unsigned long addr, unsigned long end)

/* CPU entry erea is always used for CPU entry */
if (within_area(addr, end, CPU_ENTRY_AREA_BASE,
- CPU_ENTRY_AREA_TOTAL_SIZE))
+ CPU_ENTRY_AREA_MAP_SIZE))
return true;

/*
diff --git a/arch/x86/mm/cpu_entry_area.c b/arch/x86/mm/cpu_entry_area.c
index d7081b1..dff9001 100644
--- a/arch/x86/mm/cpu_entry_area.c
+++ b/arch/x86/mm/cpu_entry_area.c
@@ -16,16 +16,53 @@ static DEFINE_PER_CPU_PAGE_ALIGNED(struct entry_stack_page, entry_stack_storage)
#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);
-#endif

-#ifdef CONFIG_X86_32
+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)
+{
+ unsigned int max_cea;
+ unsigned int i, j;
+
+ max_cea = (CPU_ENTRY_AREA_MAP_SIZE - PAGE_SIZE) / CPU_ENTRY_AREA_SIZE;
+
+ /* O(sodding terrible) */
+ for_each_possible_cpu(i) {
+ unsigned int cea;
+
+again:
+ cea = prandom_u32_max(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 */
DECLARE_PER_CPU_PAGE_ALIGNED(struct doublefault_stack, doublefault_stack);
+
+static __always_inline unsigned int cea_offset(unsigned int cpu)
+{
+ return cpu;
+}
+static inline void init_cea_offsets(void) { }
#endif

/* Is called from entry code, so must be noinstr */
noinstr 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;
@@ -211,7 +248,6 @@ static __init void setup_cpu_entry_area_ptes(void)

/* 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;
@@ -227,6 +263,8 @@ void __init setup_cpu_entry_areas(void)
{
unsigned int cpu;

+ init_cea_offsets();
+
setup_cpu_entry_area_ptes();

for_each_possible_cpu(cpu)
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