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

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Date:   Thu, 6 Jan 2022 15:48:30 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Michal Suchánek <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 Wed, Jan 05, 2022 at 02:12:22PM +0100, Michal Suchánek wrote:
> Hello,
> 
> On Wed, Jan 05, 2022 at 12:51:31PM +0100, Greg Kroah-Hartman wrote:
> > 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?
> 
> It's because people use those LTSS kernels that don't have this change.
> 
> > > 
> > > 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?
> 
> The intent is to allow files that have permissions that are subset of
> 0444. The only one that makes sense and people complain about is 0400
> but if you had 0440 or 0004 it would be permitted as well.
> 
> > 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?
> 
> I don't understand that either but I am not really trying to challenge
> that part.

Fair enough.  I don't care about the locked-down mode stuff, so I'll go
queue this up now, thanks!

greg k-h