linux-kernel - Re: [PATCH v5 1/7] fs: introduce kernel_pread

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Date:   Wed, 13 May 2020 14:39:13 -0400
From:   Mimi Zohar <zohar@...nel.org>
To:     Scott Branden <scott.branden@...adcom.com>,
        Luis Chamberlain <mcgrof@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        David Brown <david.brown@...aro.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Shuah Khan <shuah@...nel.org>, bjorn.andersson@...aro.org,
        Shuah Khan <skhan@...uxfoundation.org>,
        Arnd Bergmann <arnd@...db.de>
Cc:     "Rafael J . Wysocki" <rafael@...nel.org>,
        linux-kernel@...r.kernel.org, linux-arm-msm@...r.kernel.org,
        linux-fsdevel@...r.kernel.org,
        BCM Kernel Feedback <bcm-kernel-feedback-list@...adcom.com>,
        Olof Johansson <olof@...om.net>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Dan Carpenter <dan.carpenter@...cle.com>,
        Colin Ian King <colin.king@...onical.com>,
        Kees Cook <keescook@...omium.org>,
        Takashi Iwai <tiwai@...e.de>, linux-kselftest@...r.kernel.org,
        Andy Gross <agross@...nel.org>,
        linux-security-module <linux-security-module@...r.kernel.org>,
        linux-integrity <linux-integrity@...r.kernel.org>
Subject: Re: [PATCH v5 1/7] fs: introduce kernel_pread_file* support

[Cc'ing linux-security-module, linux-integrity]

On Thu, 2020-05-07 at 17:27 -0700, Scott Branden wrote:
> Add kernel_pread_file* support to kernel to allow for partial read
> of files with an offset into the file.  Existing kernel_read_file
> functions call new kernel_pread_file functions with offset=0 and
> flags=KERNEL_PREAD_FLAG_WHOLE.
> 
> Signed-off-by: Scott Branden <scott.branden@...adcom.com>
> ---

<snip>

> @@ -941,14 +955,16 @@ int kernel_read_file(struct file *file, void **buf, loff_t *size,
>  
>  		if (bytes == 0)
>  			break;
> +
> +		buf_pos += bytes;
>  	}
>  
> -	if (pos != i_size) {
> +	if (pos != read_end) {
>  		ret = -EIO;
>  		goto out_free;
>  	}
>  
> -	ret = security_kernel_post_read_file(file, *buf, i_size, id);
> +	ret = security_kernel_post_read_file(file, *buf, alloc_size, id);
>  	if (!ret)
>  		*size = pos;

Prior to the patch set that introduced this security hook, firmware
would be read twice, once for measuring/appraising the firmware and
again reading the file contents into memory.  Partial reads will break
both IMA's measuring the file and appraising the file signatures.

Mimi