Re: [RFC] PCI: sysfs: add bypass for config read admin check

[Bjorn Helgaas <helgaas@kernel.org>]

[+cc Kees]

On Wed, Apr 06, 2022 at 04:11:31PM +0900, David Stevens wrote:
> From: David Stevens <stevensd@chromium.org>
> 
> Add a moduleparam that can be set to bypass the check that limits users
> without CAP_SYS_ADMIN to only being able to read the first 64 bytes of
> the config space. This allows systems without problematic hardware to be
> configured to allow users without CAP_SYS_ADMIN to read PCI
> capabilities.

Can you expand this a bit to explain the purpose of this?  I guess it
makes "lspci -v" work without having to be root?  How much of a
problem is that?  Is there some specific use case that needs this
change?  Maybe there's some way to address that without having to add
a new parameter that bypasses CAP_SYS_ADMIN.

> Signed-off-by: David Stevens <stevensd@chromium.org>
> ---
>  drivers/pci/pci-sysfs.c | 10 +++++++++-
>  1 file changed, 9 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/pci/pci-sysfs.c b/drivers/pci/pci-sysfs.c
> index 602f0fb0b007..162423b3c052 100644
> --- a/drivers/pci/pci-sysfs.c
> +++ b/drivers/pci/pci-sysfs.c
> @@ -28,10 +28,17 @@
>  #include <linux/pm_runtime.h>
>  #include <linux/msi.h>
>  #include <linux/of.h>
> +#include <linux/moduleparam.h>
>  #include "pci.h"
>  
>  static int sysfs_initialized;	/* = 0 */
>  
> +static bool allow_unsafe_config_reads;
> +module_param_named(allow_unsafe_config_reads,
> +		   allow_unsafe_config_reads, bool, 0644);
> +MODULE_PARM_DESC(allow_unsafe_config_reads,
> +		 "Enable full read access to config space without CAP_SYS_ADMIN.");
> +
>  /* show configuration fields */
>  #define pci_config_attr(field, format_string)				\
>  static ssize_t								\
> @@ -696,7 +703,8 @@ static ssize_t pci_read_config(struct file *filp, struct kobject *kobj,
>  	u8 *data = (u8 *) buf;
>  
>  	/* Several chips lock up trying to read undefined config space */
> -	if (file_ns_capable(filp, &init_user_ns, CAP_SYS_ADMIN))
> +	if (allow_unsafe_config_reads ||
> +	    file_ns_capable(filp, &init_user_ns, CAP_SYS_ADMIN))
>  		size = dev->cfg_size;
>  	else if (dev->hdr_type == PCI_HEADER_TYPE_CARDBUS)
>  		size = 128;
> -- 
> 2.35.1.1094.g7c7d902a7c-goog
>