[PATCH 1/2] x86/kmsan: Fix CPU entry area metadata lookup

[Viorel Cernateanu via B4 Relay <devnull+vrilutza.gmail.com@kernel.org>]

From: Viorel Cernateanu <vrilutza@gmail.com>

arch_kmsan_get_meta_or_null() works out which CPU owns an address in the
CPU entry area as (addr - CPU_ENTRY_AREA_BASE) / CPU_ENTRY_AREA_SIZE.
That is wrong in two ways:

 - With KASLR, init_cea_offsets() puts each CPU's area in a random slot,
   so the result is a slot number, usually far above NR_CPUS, and
   get_cpu_entry_area() reads past the end of __per_cpu_offset[].

 - The per-CPU areas start one page higher, at CPU_ENTRY_AREA_PER_CPU.
   Even without KASLR, the last page of each area is attributed to the
   next CPU, so KMSAN loses that page, and for the last CPU the lookup
   uses a CPU that does not exist.

On an Ivy Bridge laptop, a KMSAN kernel with KASLR oopses at boot when
the hard lockup detector sets up its perf event. In QEMU, a module that
reads one byte of the current CPU's GDT alias hits the same bug:

  UBSAN: array-index-out-of-bounds in arch/x86/mm/cpu_entry_area.c:25:9
  index 2100006 is out of range for type 'unsigned long[64]'
  Oops: general protection fault, probably for non-canonical address
  RIP: 0010:get_cpu_entry_area+0x14/0x50
  Call Trace:
   kmsan_get_metadata+0xb2/0x150
   kmsan_get_shadow_origin_ptr+0x31/0xa0
   __msan_metadata_ptr_for_load_1+0x22/0x40
   init_module+0x139/0xff0 [cea_repro]

Find the CPU whose entry area contains the address instead.

Fixes: ce732a7520b0 ("x86: kmsan: handle CPU entry area")
Fixes: 97e3d26b5e5f ("x86/mm: Randomize per-cpu entry area")
Signed-off-by: Viorel Cernateanu <vrilutza@gmail.com>
---
 arch/x86/include/asm/kmsan.h | 27 +++++++++++++++++++--------
 1 file changed, 19 insertions(+), 8 deletions(-)

diff --git a/arch/x86/include/asm/kmsan.h b/arch/x86/include/asm/kmsan.h
index d91b37f5b4bb..a71e84d6abab 100644
--- a/arch/x86/include/asm/kmsan.h
+++ b/arch/x86/include/asm/kmsan.h
@@ -13,6 +13,7 @@
 
 #include <asm/cpu_entry_area.h>
 #include <asm/processor.h>
+#include <linux/cpumask.h>
 #include <linux/mmzone.h>
 
 DECLARE_PER_CPU(char[CPU_ENTRY_AREA_SIZE], cpu_entry_area_shadow);
@@ -32,18 +33,28 @@ static inline void *arch_kmsan_get_meta_or_null(void *addr, bool is_origin)
 	unsigned long addr64 = (unsigned long)addr;
 	char *metadata_array;
 	unsigned long off;
-	int cpu;
+	unsigned int cpu;
 
 	if ((addr64 < CPU_ENTRY_AREA_BASE) ||
 	    (addr64 >= (CPU_ENTRY_AREA_BASE + CPU_ENTRY_AREA_MAP_SIZE)))
 		return NULL;
-	cpu = (addr64 - CPU_ENTRY_AREA_BASE) / CPU_ENTRY_AREA_SIZE;
-	off = addr64 - (unsigned long)get_cpu_entry_area(cpu);
-	if ((off < 0) || (off >= CPU_ENTRY_AREA_SIZE))
-		return NULL;
-	metadata_array = is_origin ? cpu_entry_area_origin :
-				     cpu_entry_area_shadow;
-	return &per_cpu(metadata_array[off], cpu);
+
+	/*
+	 * The per-CPU entry areas are not necessarily laid out in CPU order
+	 * (see init_cea_offsets()), so find the area containing @addr instead
+	 * of computing the CPU from the offset.
+	 */
+	for (cpu = 0; cpu < nr_cpu_ids; cpu++) {
+		if (!cpu_possible(cpu))
+			continue;
+		off = addr64 - (unsigned long)get_cpu_entry_area(cpu);
+		if (off >= CPU_ENTRY_AREA_SIZE)
+			continue;
+		metadata_array = is_origin ? cpu_entry_area_origin :
+					     cpu_entry_area_shadow;
+		return &per_cpu(metadata_array[off], cpu);
+	}
+	return NULL;
 }
 
 /*

-- 
2.53.0