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Date: Wed, 2 May 2018 09:16:29 +0200
From: Thomas-Mich Richter <>
To: Kees Cook <>, Greg KH <>
Cc: Kernel Hardening <>,,
        Martin Schwidefsky <>,
        Heiko Carstens <>,
        LKML <>
Subject: Re: [PATCH v2] inode: debugfs_create_dir uses mode permission from

On 04/27/2018 04:58 PM, Kees Cook wrote:
> On Fri, Apr 27, 2018 at 6:49 AM, Greg KH <> wrote:
>> I'm going to add Kees and the kernel-hardning list here, as I'd like
>> their opinions for the patch below.
>> Kees, do you have any problems with this patch?  I know you worked on
>> making debugfs more "secure" from non-root users, this should still keep
>> the intial mount permissions all fine, right?  Anything I'm not
>> considering here?
> This appears correct to me. I'd like to see some stronger rationale
> for why this is needed, just so I have a "design" to compare the
> implementation against. :)
> Normally, the top-level directory permissions should block all the
> subdirectories too. The only time I think of this being needed is if
> someone is explicitly bind-mounting a subdirectory to another location
> (e.g. Chrome OS does this for the i915 subdirectory). In that case,
> I'd expect them to tweak permissions too. Thomas, what's your
> use-case?
> -Kees

There is no 'real use case'. I wrote the patch because of discussions
regarding file permissions for files located deeply in the
directory tree, for example

  -r--r--r-- 1 root root 0 Apr 27 14:23 /sys/kernel/debug/kprobes/blacklist

which gives the impression it is world readable.
This happened to me in recent discussions when I wrote patches to fix some
of the address randomized output of /sys files which broke the perf tool.

During discussion people often forgot that the root /sys/kernel/debug is rwx for
root only and blocks non root access to subdirectories and files. They simply
looked at the file permissions.

I have not thougth about the bind-mount case nor did I test this scenario.

Thomas Richter, Dept 3303, IBM s390 Linux Development, Boeblingen, Germany
Vorsitzende des Aufsichtsrats: Martina Koederitz 
Geschäftsführung: Dirk Wittkopp
Sitz der Gesellschaft: Böblingen / Registergericht: Amtsgericht Stuttgart, HRB 243294

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