Date: Wed, 2 May 2018 07:29:51 -0700 From: Kees Cook <keescook@...omium.org> To: Thomas-Mich Richter <tmricht@...ux.ibm.com> Cc: Greg KH <gregkh@...uxfoundation.org>, 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 Wed, May 2, 2018 at 12:16 AM, Thomas-Mich Richter <tmricht@...ux.ibm.com> wrote: > 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. Okay, sounds good. "Make permissions less surprising" is a perfectly good reason to make the change. :) > I have not thought about the bind-mount case nor did I test this scenario. I think this case is fine too. Anyone doing this is likely doing some pretty special things with permissions already. So, FWIW: Reviewed-by: Kees Cook <keescook@...omium.org> -Kees -- Kees Cook Pixel Security
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