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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