[PATCH] Yama: Check for pid death before checking ancestry

[James Morris]

On Wed, 16 Jan 2019, Kees Cook wrote:

> It's possible that a pid has died before we take the rcu lock, in which
> case we can't walk the ancestry list as it may be detached. Instead, check
> for death first before doing the walk.
> 
> Reported-by: syzbot+a9ac39bf55329e206219 at syzkaller.appspotmail.com
> Fixes: 2d514487faf1 ("security: Yama LSM")
> Cc: stable at vger.kernel.org
> Suggested-by: Oleg Nesterov <oleg at redhat.com>
> Signed-off-by: Kees Cook <keescook at chromium.org>
> ---
> James, can you please send this to Linus in your -fixes tree?

Done.

> ---
>  security/yama/yama_lsm.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/security/yama/yama_lsm.c b/security/yama/yama_lsm.c
> index ffda91a4a1aa..02514fe558b4 100644
> --- a/security/yama/yama_lsm.c
> +++ b/security/yama/yama_lsm.c
> @@ -368,7 +368,9 @@ static int yama_ptrace_access_check(struct task_struct *child,
>  			break;
>  		case YAMA_SCOPE_RELATIONAL:
>  			rcu_read_lock();
> -			if (!task_is_descendant(current, child) &&
> +			if (!pid_alive(child))
> +				rc = -EPERM;
> +			if (!rc && !task_is_descendant(current, child) &&
>  			    !ptracer_exception_found(current, child) &&
>  			    !ns_capable(__task_cred(child)->user_ns, CAP_SYS_PTRACE))
>  				rc = -EPERM;
> 

-- 
James Morris
<jmorris at namei.org>