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

[Greg KH <gregkh@linuxfoundation.org>]

On Fri, Dec 17, 2021 at 02:31:53PM -0800, Dave Hansen wrote:
> 
> 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);
>  }

Why is this file showing up if we do not have sgx_numa_nodes not
allocated?  It shouldn't even be there to access then.

don't return a fake number, just don't present the sysfs file at all.

thanks,

greg k-h