Subject: Re: [PATCH v2 5/8] fwctl: FWCTL_RPC to execute a Remote Procedure Call to device firmware

From: Jonathan Cameron <[email-masked]>
Date: Fri, 26 Jul 2024 16:30:09 +0100

On Mon, 24 Jun 2024 19:47:29 -0300
Jason Gunthorpe <[email-masked]> wrote:

> Add the FWCTL_RPC ioctl which allows a request/response RPC call to device
> firmware. Drivers implementing this call must follow the security
> guidelines under Documentation/userspace-api/fwctl.rst
> 
> The core code provides some memory management helpers to get the messages
> copied from and back to userspace. The driver is responsible for
> allocating the output message memory and delivering the message to the
> device.
> 
> Signed-off-by: Jason Gunthorpe <[email-masked]>
A few minor things inline.

> ---
>  drivers/fwctl/main.c       | 62 +++++++++++++++++++++++++++++++++++
>  include/linux/fwctl.h      |  5 +++
>  include/uapi/fwctl/fwctl.h | 66 ++++++++++++++++++++++++++++++++++++++
>  3 files changed, 133 insertions(+)
> 
> diff --git a/drivers/fwctl/main.c b/drivers/fwctl/main.c
> index f1dec0b590aee4..9506b993a1a56d 100644
> --- a/drivers/fwctl/main.c
> +++ b/drivers/fwctl/main.c
> @@ -8,16 +8,20 @@
>  #include <linux/slab.h>
>  #include <linux/container_of.h>
>  #include <linux/fs.h>
> +#include <linux/sizes.h>
>  
>  #include <uapi/fwctl/fwctl.h>
>  
>  enum {
>  	FWCTL_MAX_DEVICES = 256,
> +	MAX_RPC_LEN = SZ_2M,
>  };

In what way is that usefully handled as an enum?
I'd just use #defines

>  static dev_t fwctl_dev;
>  static DEFINE_IDA(fwctl_ida);
> +static unsigned long fwctl_tainted;
>  
>  DEFINE_FREE(kfree_errptr, void *, if (!IS_ERR_OR_NULL(_T)) kfree(_T));
> +DEFINE_FREE(kvfree_errptr, void *, if (!IS_ERR_OR_NULL(_T)) kvfree(_T));
kvfree define free already defined as this since 6.9

>  
>  struct fwctl_ucmd {
>  	struct fwctl_uctx *uctx;
> @@ -75,9 +79,66 @@ static int fwctl_cmd_info(struct fwctl_ucmd *ucmd)
>  	return ucmd_respond(ucmd, sizeof(*cmd));
>  }
>  
> +static int fwctl_cmd_rpc(struct fwctl_ucmd *ucmd)
> +{
> +	struct fwctl_device *fwctl = ucmd->uctx->fwctl;
> +	struct fwctl_rpc *cmd = ucmd->cmd;
> +	size_t out_len;
> +
> +	if (cmd->in_len > MAX_RPC_LEN || cmd->out_len > MAX_RPC_LEN)
> +		return -EMSGSIZE;
> +
> +	switch (cmd->scope) {
> +	case FWCTL_RPC_CONFIGURATION:
> +	case FWCTL_RPC_DEBUG_READ_ONLY:
> +		break;
> +
> +	case FWCTL_RPC_DEBUG_WRITE_FULL:
> +		if (!capable(CAP_SYS_RAWIO))
> +			return -EPERM;
> +		fallthrough;
> +	case FWCTL_RPC_DEBUG_WRITE:
> +		if (!test_and_set_bit(0, &fwctl_tainted)) {
> +			dev_warn(
> +				&fwctl->dev,
> +				"%s(%d): has requested full access to the physical device device",
> +				current->comm, task_pid_nr(current));
> +			add_taint(TAINT_FWCTL, LOCKDEP_STILL_OK);
> +		}
> +		break;
> +	default:
> +		return -EOPNOTSUPP;
> +	};
> +
> +	void *inbuf __free(kvfree) =
> +		kvzalloc(cmd->in_len, GFP_KERNEL | GFP_KERNEL_ACCOUNT);

As before
#define GFP_KERNEL_ACCOUNT (GFP_KERNEL | __GFP_ACCOUNT)
so don't need both.

> +	if (!inbuf)
> +		return -ENOMEM;
> +	if (copy_from_user(inbuf, u64_to_user_ptr(cmd->in), cmd->in_len))
> +		return -EFAULT;
> +
> +	out_len = cmd->out_len;
> +	void *outbuf __free(kvfree_errptr) = fwctl->ops->fw_rpc(

> +		ucmd->uctx, cmd->scope, inbuf, cmd->in_len, &out_len);
> +	if (IS_ERR(outbuf))
> +		return PTR_ERR(outbuf);
> +	if (outbuf == inbuf) {
> +		/* The driver can re-use inbuf as outbuf */
> +		inbuf = NULL;
I wish no_free_ptr() didn't have __must_check. Can do something ugly
like
		outbuf = no_free_ptr(inbuf);
probably but maybe just setting it NULL is simpler.

> +	}
> +
> +	if (copy_to_user(u64_to_user_ptr(cmd->out), outbuf,
> +			 min(cmd->out_len, out_len)))
> +		return -EFAULT;
> +
> +	cmd->out_len = out_len;
> +	return ucmd_respond(ucmd, sizeof(*cmd));
> +}

> diff --git a/include/uapi/fwctl/fwctl.h b/include/uapi/fwctl/fwctl.h
> index 39db9f09f8068e..8bde0d4416fd55 100644
> --- a/include/uapi/fwctl/fwctl.h
> +++ b/include/uapi/fwctl/fwctl.h
> @@ -67,4 +67,70 @@ struct fwctl_info {
>  };
>  #define FWCTL_INFO _IO(FWCTL_TYPE, FWCTL_CMD_INFO)
>  
> +/**
> + * enum fwctl_rpc_scope - Scope of access for the RPC
> + */
> +enum fwctl_rpc_scope {
...

> +	/**
> +	 * @FWCTL_RPC_DEBUG_READ_ONLY: Read only access to debug information
> +	 *
> +	 * Readable debug information. Debug information is compatible with
> +	 * kernel lockdown, and does not disclose any sensitive information. For
> +	 * instance exposing any encryption secrets from this information is
> +	 * forbidden.
> +	 */
> +	FWCTL_RPC_DEBUG_READ_ONLY = 1,
> +	/**
> +	 * @FWCTL_RPC_DEBUG_WRITE: Writable access to lockdown compatible debug information

Write access
probably rather than writeable.

> +	 *
> +	 * Allows write access to data in the device which may leave a fully
> +	 * supported state. This is intended to permit intensive and possibly
> +	 * invasive debugging. This scope will taint the kernel.
> +	 */

> +};