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Subject: Re: [PATCH] apparmor: don't try to replace stale label in ptrace
 access check
To:     Jann Horn <jannh@google.com>, apparmor@lists.ubuntu.com
Cc:     linux-kernel@vger.kernel.org, Kees Cook <keescook@chromium.org>
References: <20180913161209.63693-1-jannh@google.com>
From:   John Johansen <john.johansen@canonical.com>
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Organization: Canonical
Message-ID: <8721c54a-9a92-5efa-d7c2-27adda1d3353@canonical.com>
Date:   Thu, 13 Sep 2018 09:23:12 -0700
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On 09/13/2018 09:12 AM, Jann Horn wrote:
> As a comment above begin_current_label_crit_section() explains,
> begin_current_label_crit_section() must run in sleepable context because
> when label_is_stale() is true, aa_replace_current_label() runs, which uses
> prepare_creds(), which can sleep.
> Until now, the ptrace access check (which runs with a task lock held)
> violated this rule.
> 
> Also add a might_sleep() assertion to begin_current_label_crit_section(),
> because asserts are less likely to be ignored than comments.
> 
> Signed-off-by: Jann Horn <jannh@google.com>

oh ouch, thanks Jann nice catch. I've pulled it into apparmor-next

Acked-by: John Johansen <john.johansen@canonical.com>


> ---
> I have tested that just the might_sleep() yells a lot, and adding the
> rest of the patch stops the yelling.
> 
>  security/apparmor/include/cred.h | 2 ++
>  security/apparmor/lsm.c          | 4 ++--
>  2 files changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/security/apparmor/include/cred.h b/security/apparmor/include/cred.h
> index e287b7d0d4be..265ae6641a06 100644
> --- a/security/apparmor/include/cred.h
> +++ b/security/apparmor/include/cred.h
> @@ -151,6 +151,8 @@ static inline struct aa_label *begin_current_label_crit_section(void)
>  {
>  	struct aa_label *label = aa_current_raw_label();
>  
> +	might_sleep();
> +
>  	if (label_is_stale(label)) {
>  		label = aa_get_newest_label(label);
>  		if (aa_replace_current_label(label) == 0)
> diff --git a/security/apparmor/lsm.c b/security/apparmor/lsm.c
> index 8b8b70620bbe..e3f40c20b9b4 100644
> --- a/security/apparmor/lsm.c
> +++ b/security/apparmor/lsm.c
> @@ -114,13 +114,13 @@ static int apparmor_ptrace_access_check(struct task_struct *child,
>  	struct aa_label *tracer, *tracee;
>  	int error;
>  
> -	tracer = begin_current_label_crit_section();
> +	tracer = __begin_current_label_crit_section();
>  	tracee = aa_get_task_label(child);
>  	error = aa_may_ptrace(tracer, tracee,
>  			(mode & PTRACE_MODE_READ) ? AA_PTRACE_READ
>  						  : AA_PTRACE_TRACE);
>  	aa_put_label(tracee);
> -	end_current_label_crit_section(tracer);
> +	__end_current_label_crit_section(tracer);
>  
>  	return error;
>  }
>