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

[Dave Hansen <dave.hansen@linux.intel.com>]

From: Dave Hansen <dave.hansen@linux.intel.com>

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.

Add a check to ensure that SGX has been initialized to the point
where sgx_numa_nodes[] is allocated, before accessing it.

Reported-by: Nathan Chancellor <nathan@kernel.org>
CC: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Jarkko Sakkinen <jarkko@kernel.org>
Cc: linux-sgx@vger.kernel.org
Cc: x86@kernel.org
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
---

 b/arch/x86/kernel/cpu/sgx/main.c |    8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

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	2021-12-17 13:38:00.217312383 -0800
+++ b/arch/x86/kernel/cpu/sgx/main.c	2021-12-17 14:00:36.293044390 -0800
@@ -906,7 +906,13 @@ 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);
+	unsigned long node_bytes = 0;
+
+	/* Avoid acccessing sgx_numa_nodes[] when it is not allocated: */
+	if (!nodes_empty(sgx_numa_mask))
+		node_bytes = sgx_numa_nodes[dev->id].size;
+
+	return sysfs_emit(buf, "%lu\n", node_bytes);
 }
 static DEVICE_ATTR_RO(sgx_total_bytes);
 
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