Re: [PATCH] signal: Extend exec_id to 64bits

[Kees Cook]

On Wed, Apr 01, 2020 at 03:47:44PM -0500, 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@xxxxxxxxxx
> Fixes: 2.3.23pre2
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>

Thanks for chasing this down. :)

Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>

-Kees

> ---
> 
> 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;
>  	}
>  
> -- 
> 2.20.1
> 

-- 
Kees Cook