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

[Greg KH <gregkh@linuxfoundation.org>]

On Fri, Dec 17, 2021 at 03:55:44PM -0800, Dave Hansen wrote:
> On 12/17/21 3:30 PM, Greg KH wrote:
> >>>
> >>> 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.
> > Or, if you _have_ to have the file present, return an error instead of a
> > fake value.
> > 
> > But really, only create the file if the system supports it, that's the
> > rule for sysfs and it makes it very easy to do so (see the is_visible
> > callback for the attribute group for how to do it.)
> 
> Thanks for the suggestion!  That's a lot nicer.  It's also dirt simple
> since we only have one attribute.  Updated patch is attached.
> 
> I'll send this out as a real v2 soon if no other issues pop up.

> 
> 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.
> 
> To fix this, hide the entire nodeX/x86/ attribute group on
> systems without SGX support using the ->is_visible attribute
> group callback.
> 
> Fixes: 50468e431335 ("x86/sgx: Add an attribute for the amount of SGX memory in a NUMA node")
> 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, 8 insertions(+)
> 
> 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 15:48:10.358932505 -0800
> @@ -910,6 +910,13 @@ static ssize_t sgx_total_bytes_show(stru
>  }
>  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: */
> +	return !nodes_empty(sgx_numa_mask);

That's a very odd umode_t return value :)

Did you test this?  What was the mode of the sysfs file that is created?

thanks,

greg k-h