linux-kernel - Re: [PATCH] debugfs: lockdown: Allow reading debugfs files that are not world readable

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Date:   Wed, 5 Jan 2022 12:51:31 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Michal Suchanek <msuchanek@...e.de>
Cc:     linux-kernel@...r.kernel.org,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Matthew Garrett <matthewgarrett@...gle.com>,
        James Morris <jmorris@...ei.org>,
        David Howells <dhowells@...hat.com>,
        Andy Shevchenko <andy.shevchenko@...il.com>,
        acpi4asus-user@...ts.sourceforge.net,
        platform-driver-x86@...r.kernel.org,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] debugfs: lockdown: Allow reading debugfs files that are
 not world readable

On Tue, Jan 04, 2022 at 06:05:05PM +0100, Michal Suchanek wrote:
> 
> When the kernel is locked down the kernel allows reading only debugfs
> files with mode 444. Mode 400 is also valid but is not allowed.
> 
> Make the 444 into a mask.
> 
> Fixes: 5496197f9b08 ("debugfs: Restrict debugfs when the kernel is locked down")
> Signed-off-by: Michal Suchanek <msuchanek@...e.de>
> ---
>  fs/debugfs/file.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)

Why has it taken so long for anyone to notice this (2 years!)?

Is that because no one uses the lockdown mode and tries to read debugfs
files?


> 
> diff --git a/fs/debugfs/file.c b/fs/debugfs/file.c
> index 7d162b0efbf0..950c63fa4d0b 100644
> --- a/fs/debugfs/file.c
> +++ b/fs/debugfs/file.c
> @@ -147,7 +147,7 @@ static int debugfs_locked_down(struct inode *inode,
>  			       struct file *filp,
>  			       const struct file_operations *real_fops)
>  {
> -	if ((inode->i_mode & 07777) == 0444 &&
> +	if ((inode->i_mode & 07777 & ~0444) == 0 &&

You are now allowing more than just 0400, is that intentional?

I never understood why files that were 0666 were not able to be read
here as well, why not allow that as well?  What was magic about 0444
files?

thanks,

greg k-h