archive mirror
 help / color / mirror / Atom feed
From: Ondrej Mosnacek <>
To: Steven Rostedt <>
Cc: Linux kernel mailing list <>,
	Linus Torvalds <>,
	Ingo Molnar <>,
	Andrew Morton <>,
	Matthew Garrett <>,
	James Morris James Morris <>,
	LSM List <>,
	Linux API <>,
	Ben Hutchings <>,
	Al Viro <>,
	Stephen Smalley <>,
	SElinux list <>,
	Herton Krzesinski <>
Subject: Re: [PATCH 7/7 v2] tracing: Do not create tracefs files if tracefs lockdown is in effect
Date: Tue, 13 Apr 2021 10:13:04 +0200	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <>

On Sat, Oct 12, 2019 at 2:59 AM Steven Rostedt <> wrote:
> From: "Steven Rostedt (VMware)" <>
> If on boot up, lockdown is activated for tracefs, don't even bother creating
> the files. This can also prevent instances from being created if lockdown is
> in effect.
> Link:
> Suggested-by: Linus Torvalds <>
> Signed-off-by: Steven Rostedt (VMware) <>
> ---
>  fs/tracefs/inode.c | 4 ++++
>  1 file changed, 4 insertions(+)
> diff --git a/fs/tracefs/inode.c b/fs/tracefs/inode.c
> index eeeae0475da9..0caa151cae4e 100644
> --- a/fs/tracefs/inode.c
> +++ b/fs/tracefs/inode.c
> @@ -16,6 +16,7 @@
>  #include <linux/namei.h>
>  #include <linux/tracefs.h>
>  #include <linux/fsnotify.h>
> +#include <linux/security.h>
>  #include <linux/seq_file.h>
>  #include <linux/parser.h>
>  #include <linux/magic.h>
> @@ -390,6 +391,9 @@ struct dentry *tracefs_create_file(const char *name, umode_t mode,
>         struct dentry *dentry;
>         struct inode *inode;
> +       if (security_locked_down(LOCKDOWN_TRACEFS))
> +               return NULL;
> +
>         if (!(mode & S_IFMT))
>                 mode |= S_IFREG;
>         BUG_ON(!S_ISREG(mode));
> --
> 2.23.0

Hi all,

sorry for coming back to an old thread, but it turns out that this
patch doesn't play well with SELinux's implementation of the
security_locked_down() hook, which was added a few months later (so
not your fault :) in commit 59438b46471a ("security,lockdown,selinux:
implement SELinux lockdown").

What SELinux does is it checks if the current task's creds are allowed
the lockdown::integrity or lockdown::confidentiality permission in the
policy whenever security_locked_down() is called. The idea is to be
able to control at SELinux domain level which tasks can do these
sensitive operations (when the kernel is not actually locked down by
the Lockdown LSM).

With this patch + the SELinux lockdown mechanism in use, when a
userspace task loads a module that creates some tracefs nodes in its
initcall SELinux will check if the task has the
lockdown::confidentiality permission and if not, will report denials
in audit log and prevent the tracefs entries from being created. But
that is not a very logical behavior, since the task loading the module
is itself not (explicitly) doing anything that would breach
confidentiality. It just indirectly causes some tracefs nodes to be
created, but doesn't actually use them at that point.

Since it seems the other patches also added security_locked_down()
calls to the tracefs nodes' open functions, I guess reverting this
patch could be an acceptable way to fix this problem (please correct
me if there is something that this call catches, which the other ones
don't). However, even then I can understand that you (or someone else)
might want to keep this as an optimization, in which case we could
instead do this:
1. Add a new hook security_locked_down_permanently() (the name is open
for discussion), which would be intended for situations when we want
to avoid doing some pointless work when the kernel is in a "hard"
lockdown that can't be taken back (except perhaps in some rescue
2. This hook would be backed by the same implementation as
security_locked_down() in the Lockdown LSM and left unimplemented by
3. tracefs_create_file() would call this hook instead of security_locked_down().

This way it would work as before relative to the standard lockdown via
the Lockdown LSM and would be simply ignored by SELinux. I went over
all the security_locked_down() call in the kernel and I think this
alternative hook could also fit better in arch/powerpc/xmon/xmon.c,
where it seems to be called from interrupt context (so task creds are
irrelevant, anyway...) and mainly causes some values to be redacted.
(I also found a couple minor issues with how the hook is used in other
places, for which I plan to send patches later.)


Ondrej Mosnacek
Software Engineer, Linux Security - SELinux kernel
Red Hat, Inc.

  reply	other threads:[~2021-04-13  8:13 UTC|newest]

Thread overview: 15+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-10-12  0:57 [PATCH 0/7 v2] tracing: Fix tracefs lockdown and various clean ups Steven Rostedt
2019-10-12  0:57 ` [PATCH 1/7 v2] tracefs: Revert ccbd54ff54e8 ("tracefs: Restrict tracefs when the kernel is locked down") Steven Rostedt
2019-10-12 22:56   ` Linus Torvalds
2019-10-13  0:35     ` Steven Rostedt
2019-10-13  0:39       ` Steven Rostedt
2019-10-12  0:57 ` [PATCH 2/7 v2] ftrace: Get a reference counter for the trace_array on filter files Steven Rostedt
2019-10-12  0:57 ` [PATCH 3/7 v2] tracing: Get trace_array reference for available_tracers files Steven Rostedt
2019-10-12  0:57 ` [PATCH 4/7 v2] tracing: Have trace events system open call tracing_open_generic_tr() Steven Rostedt
2019-10-12  2:09   ` Steven Rostedt
2019-10-12  0:57 ` [PATCH 5/7 v2] tracing: Add tracing_check_open_get_tr() Steven Rostedt
2019-10-12  0:57 ` [PATCH 6/7 v2] tracing: Add some more locked_down checks Steven Rostedt
2019-10-12  0:57 ` [PATCH 7/7 v2] tracing: Do not create tracefs files if tracefs lockdown is in effect Steven Rostedt
2021-04-13  8:13   ` Ondrej Mosnacek [this message]
2021-05-07 12:00     ` Ondrej Mosnacek
2021-05-07 13:26     ` Steven Rostedt

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).