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

[Nathan Chancellor]

On Tue, Jan 04, 2022 at 09:15:27AM -0800, Dave Hansen wrote:
> 
> 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

Tested-by: Nathan Chancellor <nathan@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;
> _