[PATCH] [v3] x86/sgx: Fix NULL pointer dereference on non-SGX systems

[Dave Hansen]

From: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>

== Problem ==

Nathan Chancellor reported an oops when aceessing the
'sgx_total_bytes' sysfs file:

	https://lore.kernel.org/all/YbzhBrimHGGpddDM@archlinux-ax161/

The sysfs output code accesses the sgx_numa_nodes[] array
unconditionally.  However, this array is allocated during SGX
initialization, which only occurs on systems where SGX is
supported.

If the sysfs file is accessed on systems without SGX support,
sgx_numa_nodes[] is NULL and an oops occurs.

== Solution ==

To fix this, hide the entire nodeX/x86/ attribute group on
systems without SGX support using the ->is_visible attribute
group callback.

Unfortunately, SGX is initialized via a device_initcall() which
occurs _after_ the ->is_visible() callback.  Instead of moving
SGX initialization earlier, call sysfs_update_group() during
SGX initialization to update the group visiblility.

This update requires moving the SGX sysfs code earlier in
sgx/main.c.  There are no code changes other than the addition of
arch_update_sysfs_visibility() and a minor whitespace fixup to
arch_node_attr_is_visible() which checkpatch caught.

Fixes: 50468e431335 ("x86/sgx: Add an attribute for the amount of SGX memory in a NUMA node")
Reported-by: Nathan Chancellor <nathan@xxxxxxxxxx>
Signed-off-by: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
CC: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
Cc: Jarkko Sakkinen <jarkko@xxxxxxxxxx>
Cc: linux-sgx@xxxxxxxxxxxxxxx
Cc: x86@xxxxxxxxxx
---

 b/arch/x86/kernel/cpu/sgx/main.c |   65 ++++++++++++++++++++++++++++-----------
 1 file changed, 47 insertions(+), 18 deletions(-)

diff -puN arch/x86/kernel/cpu/sgx/main.c~sgx-null-ptr arch/x86/kernel/cpu/sgx/main.c
--- a/arch/x86/kernel/cpu/sgx/main.c~sgx-null-ptr	2021-12-20 07:56:38.309584807 -0800
+++ b/arch/x86/kernel/cpu/sgx/main.c	2022-01-04 08:43:17.042821249 -0800
@@ -6,11 +6,13 @@
 #include <linux/highmem.h>
 #include <linux/kthread.h>
 #include <linux/miscdevice.h>
+#include <linux/node.h>
 #include <linux/pagemap.h>
 #include <linux/ratelimit.h>
 #include <linux/sched/mm.h>
 #include <linux/sched/signal.h>
 #include <linux/slab.h>
+#include <linux/sysfs.h>
 #include <asm/sgx.h>
 #include "driver.h"
 #include "encl.h"
@@ -780,6 +782,48 @@ static inline u64 __init sgx_calc_sectio
 	       ((high & GENMASK_ULL(19, 0)) << 32);
 }
 
+#ifdef CONFIG_NUMA
+static ssize_t sgx_total_bytes_show(struct device *dev, struct device_attribute *attr, char *buf)
+{
+	return sysfs_emit(buf, "%lu\n", sgx_numa_nodes[dev->id].size);
+}
+static DEVICE_ATTR_RO(sgx_total_bytes);
+
+static umode_t arch_node_attr_is_visible(struct kobject *kobj,
+		struct attribute *attr, int idx)
+{
+	/* Make all x86/ attributes invisible when SGX is not initialized: */
+	if (nodes_empty(sgx_numa_mask))
+		return 0;
+
+	return attr->mode;
+}
+
+static struct attribute *arch_node_dev_attrs[] = {
+	&dev_attr_sgx_total_bytes.attr,
+	NULL,
+};
+
+const struct attribute_group arch_node_dev_group = {
+	.name = "x86",
+	.attrs = arch_node_dev_attrs,
+	.is_visible = arch_node_attr_is_visible,
+};
+
+static void __init arch_update_sysfs_visibility(int nid)
+{
+	struct node *node = node_devices[nid];
+	int ret;
+
+	ret = sysfs_update_group(&node->dev.kobj, &arch_node_dev_group);
+
+	if (ret)
+		pr_err("sysfs update failed (%d), files may be invisible", ret);
+}
+#else /* !CONFIG_NUMA */
+static void __init arch_update_sysfs_visibility(int nid) {}
+#endif
+
 static bool __init sgx_page_cache_init(void)
 {
 	u32 eax, ebx, ecx, edx, type;
@@ -826,6 +870,9 @@ static bool __init sgx_page_cache_init(v
 			INIT_LIST_HEAD(&sgx_numa_nodes[nid].sgx_poison_page_list);
 			node_set(nid, sgx_numa_mask);
 			sgx_numa_nodes[nid].size = 0;
+
+			/* Make SGX-specific node sysfs files visible: */
+			arch_update_sysfs_visibility(nid);
 		}
 
 		sgx_epc_sections[i].node =  &sgx_numa_nodes[nid];
@@ -903,24 +950,6 @@ int sgx_set_attribute(unsigned long *all
 }
 EXPORT_SYMBOL_GPL(sgx_set_attribute);
 
-#ifdef CONFIG_NUMA
-static ssize_t sgx_total_bytes_show(struct device *dev, struct device_attribute *attr, char *buf)
-{
-	return sysfs_emit(buf, "%lu\n", sgx_numa_nodes[dev->id].size);
-}
-static DEVICE_ATTR_RO(sgx_total_bytes);
-
-static struct attribute *arch_node_dev_attrs[] = {
-	&dev_attr_sgx_total_bytes.attr,
-	NULL,
-};
-
-const struct attribute_group arch_node_dev_group = {
-	.name = "x86",
-	.attrs = arch_node_dev_attrs,
-};
-#endif /* CONFIG_NUMA */
-
 static int __init sgx_init(void)
 {
 	int ret;
_