linux-kernel - Re: [PATCH v15 08/11] fw_cfg: handle fw_cfg_read

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Date:   Tue, 27 Feb 2018 02:20:10 +0200
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Marc-André Lureau <marcandre.lureau@...hat.com>
Cc:     linux-kernel@...r.kernel.org, slp@...hat.com, bhe@...hat.com,
        somlo@....edu, xiaolong.ye@...el.com
Subject: Re: [PATCH v15 08/11] fw_cfg: handle fw_cfg_read_blob() error

On Thu, Feb 15, 2018 at 10:33:09PM +0100, Marc-André Lureau wrote:
> fw_cfg_read_blob() may fail, but does not return error. This may lead
> to undefined behaviours, such as a memcmp(sig, "QEMU") on uninitilized
> memory.

I don't think that's true - there's a memset there that
will initialize the memory. probe is likely the only
case where it returns a slightly incorrect data.

> Return an error if ACPI locking failed. Also, the following
> DMA read/write extension will add more error paths that should be
> handled appropriately.
> 
> Signed-off-by: Marc-André Lureau <marcandre.lureau@...hat.com>
> ---
>  drivers/firmware/qemu_fw_cfg.c | 32 ++++++++++++++++++++++----------
>  1 file changed, 22 insertions(+), 10 deletions(-)
> 
> diff --git a/drivers/firmware/qemu_fw_cfg.c b/drivers/firmware/qemu_fw_cfg.c
> index f6f90bef604c..5e6e5ac71dab 100644
> --- a/drivers/firmware/qemu_fw_cfg.c
> +++ b/drivers/firmware/qemu_fw_cfg.c
> @@ -59,8 +59,8 @@ static void fw_cfg_sel_endianness(u16 key)
>  }
>  
>  /* read chunk of given fw_cfg blob (caller responsible for sanity-check) */
> -static void fw_cfg_read_blob(u16 key,
> -			void *buf, loff_t pos, size_t count)
> +static ssize_t fw_cfg_read_blob(u16 key,
> +				void *buf, loff_t pos, size_t count)
>  {
>  	u32 glk = -1U;
>  	acpi_status status;
> @@ -73,7 +73,7 @@ static void fw_cfg_read_blob(u16 key,
>  		/* Should never get here */
>  		WARN(1, "fw_cfg_read_blob: Failed to lock ACPI!\n");
>  		memset(buf, 0, count);
> -		return;
> +		return -EINVAL;
>  	}
>  
>  	mutex_lock(&fw_cfg_dev_lock);

Wouldn't something like -EBUSY be more appropriate?

> @@ -84,6 +84,7 @@ static void fw_cfg_read_blob(u16 key,
>  	mutex_unlock(&fw_cfg_dev_lock);
>  
>  	acpi_release_global_lock(glk);
> +	return count;
>  }
>  
>  /* clean up fw_cfg device i/o */
> @@ -165,8 +166,9 @@ static int fw_cfg_do_platform_probe(struct platform_device *pdev)
>  	}
>  
>  	/* verify fw_cfg device signature */
> -	fw_cfg_read_blob(FW_CFG_SIGNATURE, sig, 0, FW_CFG_SIG_SIZE);
> -	if (memcmp(sig, "QEMU", FW_CFG_SIG_SIZE) != 0) {
> +	if (fw_cfg_read_blob(FW_CFG_SIGNATURE, sig,
> +				0, FW_CFG_SIG_SIZE) < 0 ||
> +		memcmp(sig, "QEMU", FW_CFG_SIG_SIZE) != 0) {
>  		fw_cfg_io_cleanup();
>  		return -ENODEV;
>  	}
> @@ -326,8 +328,7 @@ static ssize_t fw_cfg_sysfs_read_raw(struct file *filp, struct kobject *kobj,
>  	if (count > entry->size - pos)
>  		count = entry->size - pos;
>  
> -	fw_cfg_read_blob(entry->select, buf, pos, count);
> -	return count;
> +	return fw_cfg_read_blob(entry->select, buf, pos, count);
>  }
>  
>  static struct bin_attribute fw_cfg_sysfs_attr_raw = {
> @@ -483,7 +484,11 @@ static int fw_cfg_register_dir_entries(void)
>  	struct fw_cfg_file *dir;
>  	size_t dir_size;
>  
> -	fw_cfg_read_blob(FW_CFG_FILE_DIR, &files_count, 0, sizeof(files_count));
> +	ret = fw_cfg_read_blob(FW_CFG_FILE_DIR, &files_count,
> +			0, sizeof(files_count));
> +	if (ret < 0)
> +		return ret;
> +
>  	count = be32_to_cpu(files_count);
>  	dir_size = count * sizeof(struct fw_cfg_file);
>  
> @@ -491,7 +496,10 @@ static int fw_cfg_register_dir_entries(void)
>  	if (!dir)
>  		return -ENOMEM;
>  
> -	fw_cfg_read_blob(FW_CFG_FILE_DIR, dir, sizeof(files_count), dir_size);
> +	ret = fw_cfg_read_blob(FW_CFG_FILE_DIR, dir,
> +			sizeof(files_count), dir_size);
> +	if (ret < 0)
> +		goto end;
>  
>  	for (i = 0; i < count; i++) {
>  		ret = fw_cfg_register_file(&dir[i]);
> @@ -499,6 +507,7 @@ static int fw_cfg_register_dir_entries(void)
>  			break;
>  	}
>  
> +end:
>  	kfree(dir);
>  	return ret;
>  }
> @@ -539,7 +548,10 @@ static int fw_cfg_sysfs_probe(struct platform_device *pdev)
>  		goto err_probe;
>  
>  	/* get revision number, add matching top-level attribute */
> -	fw_cfg_read_blob(FW_CFG_ID, &rev, 0, sizeof(rev));
> +	err = fw_cfg_read_blob(FW_CFG_ID, &rev, 0, sizeof(rev));
> +	if (err < 0)
> +		goto err_probe;
> +
>  	fw_cfg_rev = le32_to_cpu(rev);
>  	err = sysfs_create_file(fw_cfg_top_ko, &fw_cfg_rev_attr.attr);
>  	if (err)

I think that this is the only case where it's not doing the right thing right now in
that it shows 0 as the revision to the users.  Is it worth failing probe
here?  We could just skip the attribute, could we not?

> -- 
> 2.16.1.73.g5832b7e9f2