Kernel-hardening archive on lore.kernel.org
 help / color / Atom feed
From: Bernd Edlinger <bernd.edlinger@hotmail.de>
To: "Eric W. Biederman" <ebiederm@xmission.com>,
	Linus Torvalds <torvalds@linux-foundation.org>
Cc: Adam Zabrocki <pi3@pi3.com.pl>,
	linux-kernel@vger.kernel.org,
	kernel-hardening@lists.openwall.com, Jann Horn <jannh@google.com>,
	Oleg Nesterov <oleg@redhat.com>,
	Andy Lutomirski <luto@amacapital.net>,
	Kees Cook <keescook@chromium.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	stable@vger.kernel.org
Subject: Re: [PATCH] signal: Extend exec_id to 64bits
Date: Thu, 2 Apr 2020 09:22:28 +0200
Message-ID: <AM6PR03MB51701153BF3E55EB4285EADFE4C60@AM6PR03MB5170.eurprd03.prod.outlook.com> (raw)
In-Reply-To: <87zhbvlyq7.fsf_-_@x220.int.ebiederm.org>



On 4/1/20 10:47 PM, Eric W. Biederman wrote:
> 
> Replace the 32bit exec_id with a 64bit exec_id to make it impossible
> to wrap the exec_id counter.  With care an attacker can cause exec_id
> wrap and send arbitrary signals to a newly exec'd parent.  This
> bypasses the signal sending checks if the parent changes their
> credentials during exec.
> 
> The severity of this problem can been seen that in my limited testing
> of a 32bit exec_id it can take as little as 19s to exec 65536 times.
> Which means that it can take as little as 14 days to wrap a 32bit
> exec_id.  Adam Zabrocki has succeeded wrapping the self_exe_id in 7
> days.  Even my slower timing is in the uptime of a typical server.
> Which means self_exec_id is simply a speed bump today, and if exec
> gets noticably faster self_exec_id won't even be a speed bump.
> 
> Extending self_exec_id to 64bits introduces a problem on 32bit
> architectures where reading self_exec_id is no longer atomic and can
> take two read instructions.  Which means that is is possible to hit
> a window where the read value of exec_id does not match the written
> value.  So with very lucky timing after this change this still
> remains expoiltable.
> 
> I have updated the update of exec_id on exec to use WRITE_ONCE
> and the read of exec_id in do_notify_parent to use READ_ONCE
> to make it clear that there is no locking between these two
> locations.
> 
> Link: https://lore.kernel.org/kernel-hardening/20200324215049.GA3710@pi3.com.pl
> Fixes: 2.3.23pre2
> Cc: stable@vger.kernel.org
> Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>

Reviewed-by: Bernd Edlinger <bernd.edlinger@hotmail.de>


Thanks
Bernd.
> ---
> 
> Linus would you prefer to take this patch directly or I could put it in
> a brach and send you a pull request.
>  
>  fs/exec.c             | 2 +-
>  include/linux/sched.h | 4 ++--
>  kernel/signal.c       | 2 +-
>  3 files changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/fs/exec.c b/fs/exec.c
> index 0e46ec57fe0a..d55710a36056 100644
> --- a/fs/exec.c
> +++ b/fs/exec.c
> @@ -1413,7 +1413,7 @@ void setup_new_exec(struct linux_binprm * bprm)
>  
>  	/* An exec changes our domain. We are no longer part of the thread
>  	   group */
> -	current->self_exec_id++;
> +	WRITE_ONCE(current->self_exec_id, current->self_exec_id + 1);
>  	flush_signal_handlers(current, 0);
>  }
>  EXPORT_SYMBOL(setup_new_exec);
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index 04278493bf15..0323e4f0982a 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -939,8 +939,8 @@ struct task_struct {
>  	struct seccomp			seccomp;
>  
>  	/* Thread group tracking: */
> -	u32				parent_exec_id;
> -	u32				self_exec_id;
> +	u64				parent_exec_id;
> +	u64				self_exec_id;
>  
>  	/* Protection against (de-)allocation: mm, files, fs, tty, keyrings, mems_allowed, mempolicy: */
>  	spinlock_t			alloc_lock;
> diff --git a/kernel/signal.c b/kernel/signal.c
> index 9ad8dea93dbb..5383b562df85 100644
> --- a/kernel/signal.c
> +++ b/kernel/signal.c
> @@ -1926,7 +1926,7 @@ bool do_notify_parent(struct task_struct *tsk, int sig)
>  		 * This is only possible if parent == real_parent.
>  		 * Check if it has changed security domain.
>  		 */
> -		if (tsk->parent_exec_id != tsk->parent->self_exec_id)
> +		if (tsk->parent_exec_id != READ_ONCE(tsk->parent->self_exec_id))
>  			sig = SIGCHLD;
>  	}
>  
> 

      parent reply index

Thread overview: 19+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-03-24 21:50 Curiosity around 'exec_id' and some problems associated with it Adam Zabrocki
2020-03-29 22:43 ` Kees Cook
2020-03-30  8:34   ` Oleg Nesterov
2020-03-31  4:29   ` Adam Zabrocki
2020-04-01 20:47   ` [PATCH] signal: Extend exec_id to 64bits Eric W. Biederman
2020-04-01 20:55     ` Linus Torvalds
2020-04-01 21:03       ` Eric W. Biederman
2020-04-01 23:37     ` Jann Horn
2020-04-01 23:51       ` Linus Torvalds
2020-04-01 23:55         ` Linus Torvalds
2020-04-02  1:35           ` Jann Horn
2020-04-02  2:05             ` Linus Torvalds
2020-04-02 13:11               ` Eric W. Biederman
2020-04-02 18:06                 ` Linus Torvalds
2020-04-02  4:46     ` Jann Horn
2020-04-02 14:14       ` Eric W. Biederman
2020-04-03  2:11       ` Adam Zabrocki
2020-04-02  7:19     ` Kees Cook
2020-04-02  7:22     ` Bernd Edlinger [this message]

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:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

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

  git send-email \
    --in-reply-to=AM6PR03MB51701153BF3E55EB4285EADFE4C60@AM6PR03MB5170.eurprd03.prod.outlook.com \
    --to=bernd.edlinger@hotmail.de \
    --cc=akpm@linux-foundation.org \
    --cc=ebiederm@xmission.com \
    --cc=jannh@google.com \
    --cc=keescook@chromium.org \
    --cc=kernel-hardening@lists.openwall.com \
    --cc=linux-kernel@vger.kernel.org \
    --cc=luto@amacapital.net \
    --cc=oleg@redhat.com \
    --cc=pi3@pi3.com.pl \
    --cc=stable@vger.kernel.org \
    --cc=torvalds@linux-foundation.org \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

Kernel-hardening archive on lore.kernel.org

Archives are clonable:
	git clone --mirror https://lore.kernel.org/kernel-hardening/0 kernel-hardening/git/0.git

	# If you have public-inbox 1.1+ installed, you may
	# initialize and index your mirror using the following commands:
	public-inbox-init -V2 kernel-hardening kernel-hardening/ https://lore.kernel.org/kernel-hardening \
		kernel-hardening@lists.openwall.com
	public-inbox-index kernel-hardening

Example config snippet for mirrors

Newsgroup available over NNTP:
	nntp://nntp.lore.kernel.org/com.openwall.lists.kernel-hardening


AGPL code for this site: git clone https://public-inbox.org/public-inbox.git