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Message-Id: <8d3e702b-470a-542e-4e0d-6a3c58419f0f@linux.ibm.com>
Date: Wed, 2 May 2018 09:16:29 +0200
From: Thomas-Mich Richter <tmricht@...ux.ibm.com>
To: Kees Cook <keescook@...omium.org>, Greg KH <gregkh@...uxfoundation.org>
Cc: Kernel Hardening <kernel-hardening@...ts.openwall.com>,
brueckner@...ux.vnet.ibm.com,
Martin Schwidefsky <schwidefsky@...ibm.com>,
Heiko Carstens <heiko.carstens@...ibm.com>,
LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] inode: debugfs_create_dir uses mode permission from
parent
On 04/27/2018 04:58 PM, Kees Cook wrote:
> On Fri, Apr 27, 2018 at 6:49 AM, Greg KH <gregkh@...uxfoundation.org> 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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