linux-kernel - Re: [PATCH v3 1/7] kprobes: Make blacklist root user read only

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Date:   Sat, 28 Apr 2018 00:58:31 +0900
From:   Masami Hiramatsu <mhiramat@...nel.org>
To:     Ingo Molnar <mingo@...nel.org>
Cc:     linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
        Ingo Molnar <mingo@...hat.com>,
        "H . Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        Ananth N Mavinakayanahalli <ananth@...ibm.com>,
        Anil S Keshavamurthy <anil.s.keshavamurthy@...el.com>,
        "David S . Miller" <davem@...emloft.net>,
        Jon Medhurst <tixy@...aro.org>,
        Will Deacon <will.deacon@....com>,
        Arnd Bergmann <arnd@...db.de>,
        David Howells <dhowells@...hat.com>,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        "Tobin C . Harding" <me@...in.cc>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Thomas Richter <tmricht@...ux.ibm.com>,
        akpm@...ux-foundation.org, acme@...nel.org, rostedt@...dmis.org,
        brueckner@...ux.vnet.ibm.com, schwidefsky@...ibm.com,
        stable@...r.kernel.org
Subject: Re: [PATCH v3 1/7] kprobes: Make blacklist root user read only

On Fri, 27 Apr 2018 09:04:12 +0200
Ingo Molnar <mingo@...nel.org> wrote:

> 
> * Masami Hiramatsu <mhiramat@...nel.org> wrote:
> 
> > Since the blacklist file indicates a sensitive address
> > information to reader, it should be restricted to the
> > root user.
> > 
> > Suggested-by: Thomas Richter <tmricht@...ux.ibm.com>
> > Signed-off-by: Masami Hiramatsu <mhiramat@...nel.org>
> > ---
> >  kernel/kprobes.c |    2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/kernel/kprobes.c b/kernel/kprobes.c
> > index ea619021d901..51096eece801 100644
> > --- a/kernel/kprobes.c
> > +++ b/kernel/kprobes.c
> > @@ -2621,7 +2621,7 @@ static int __init debugfs_kprobe_init(void)
> >  	if (!file)
> >  		goto error;
> >  
> > -	file = debugfs_create_file("blacklist", 0444, dir, NULL,
> > +	file = debugfs_create_file("blacklist", 0400, dir, NULL,
> >  				&debugfs_kprobe_blacklist_ops);
> >  	if (!file)
> >  		goto error;
> 
> Note that in a typical Linux distro debugfs is already root-only:
> 
>   fomalhaut:~> ls -ld /sys/kernel/debug
>   drwx------ 28 root root 0 Apr 23 08:55 /sys/kernel/debug
> 
> but this change might make sense if debugfs is mounted in some other fashion.
> 
> But the patch looks incomplete, 'blacklist' is not the only word-readable file in 
> the kprobes hierarchy. The kprobes directory itself, and the 'list' file is 
> readable as well:
> 
>   [root@...alhaut ~]# ls -ld /sys/kernel/debug/kprobes
>   drwxr-xr-x 2 root root 0 Apr 23 08:55 /sys/kernel/debug/kprobes
> 
>   [root@...alhaut ~]# ls -l /sys/kernel/debug/kprobes/
> 
>   -r--r--r-- 1 root root 0 Apr 23 08:55 blacklist
>   -rw------- 1 root root 0 Apr 23 08:55 enabled
>   -r--r--r-- 1 root root 0 Apr 23 08:55 list
> 
> So not just the blacklist should be 400 but 'list' as well, and the main kprobes 
> directory as well.

OK, I'll mark it 0400 too.
For the kprobes directory, as Thomas pointed in the original thread,
that is currently debugfs API's limitation.

https://patchwork.kernel.org/patch/10364817/

We only have debugfs_create_dir() but it doesn't have "mode" flag.

struct dentry *debugfs_create_dir(const char *name, struct dentry *parent);

And doesn't care the parent mode bits.

Thanks,

-- 
Masami Hiramatsu <mhiramat@...nel.org>