linux-kernel - Re: [PATCH v3 1/3] ptrace: Provide ___ptrace_may_access() that can be applied on arbitrary tasks

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Date:   Tue, 4 Sep 2018 10:26:43 -0700
From:   Tim Chen <tim.c.chen@...ux.intel.com>
To:     Jiri Kosina <jikos@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        "Woodhouse, David" <dwmw@...zon.co.uk>,
        Oleg Nesterov <oleg@...hat.com>,
        Casey Schaufler <casey.schaufler@...el.com>
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org
Subject: Re: [PATCH v3 1/3] ptrace: Provide ___ptrace_may_access() that can be
 applied on arbitrary tasks

On 09/04/2018 07:40 AM, Jiri Kosina wrote:
> From: Jiri Kosina <jkosina@...e.cz>
> 
> Current ptrace_may_access() implementation assumes that the 'source' task is
> always the caller (current).
> 
> Expose ___ptrace_may_access() that can be used to apply the check on arbitrary
> tasks.

Casey recently has proposed putting the decision making of whether to
do IBPB in the security module.

https://lwn.net/ml/kernel-hardening/20180815235355.14908-4-casey.schaufler@intel.com/

That will have the advantage of giving the administrator a more flexibility
of when to turn on IBPB.  The policy is very similar to what you have proposed here
but I think the security module is a more appropriate place for the security policy.

Thanks.

Tim

> 
> Originally-by: Tim Chen <tim.c.chen@...ux.intel.com>
> Signed-off-by: Jiri Kosina <jkosina@...e.cz>
> ---
> 
> Sorry for the resend, my pine is buggered and broke threading.
> 
>  include/linux/ptrace.h | 12 ++++++++++++
>  kernel/ptrace.c        | 13 ++++++++++---
>  2 files changed, 22 insertions(+), 3 deletions(-)
> 
> diff --git a/include/linux/ptrace.h b/include/linux/ptrace.h
> index 4f36431c380b..09744d4113fb 100644
> --- a/include/linux/ptrace.h
> +++ b/include/linux/ptrace.h
> @@ -87,6 +87,18 @@ extern void exit_ptrace(struct task_struct *tracer, struct list_head *dead);
>   */
>  extern bool ptrace_may_access(struct task_struct *task, unsigned int mode);
>  
> +/**
> + * ___ptrace_may_access - variant of ptrace_may_access that can be used
> + * between two arbitrary tasks
> + * @curr: source task
> + * @task: target task
> + * @mode: selects type of access and caller credentials
> + *
> + * Returns true on success, false on denial.
> + */
> +extern int ___ptrace_may_access(struct task_struct *curr, struct task_struct *task,
> +				unsigned int mode);
> +
>  static inline int ptrace_reparented(struct task_struct *child)
>  {
>  	return !same_thread_group(child->real_parent, child->parent);
> diff --git a/kernel/ptrace.c b/kernel/ptrace.c
> index 21fec73d45d4..07ff6685ebed 100644
> --- a/kernel/ptrace.c
> +++ b/kernel/ptrace.c
> @@ -268,9 +268,10 @@ static int ptrace_has_cap(struct user_namespace *ns, unsigned int mode)
>  }
>  
>  /* Returns 0 on success, -errno on denial. */
> -static int __ptrace_may_access(struct task_struct *task, unsigned int mode)
> +int ___ptrace_may_access(struct task_struct *curr, struct task_struct *task,
> +		unsigned int mode)
>  {
> -	const struct cred *cred = current_cred(), *tcred;
> +	const struct cred *cred, *tcred;
>  	struct mm_struct *mm;
>  	kuid_t caller_uid;
>  	kgid_t caller_gid;
> @@ -290,9 +291,10 @@ static int __ptrace_may_access(struct task_struct *task, unsigned int mode)
>  	 */
>  
>  	/* Don't let security modules deny introspection */
> -	if (same_thread_group(task, current))
> +	if (same_thread_group(task, curr))
>  		return 0;
>  	rcu_read_lock();
> +	cred =  __task_cred(curr);
>  	if (mode & PTRACE_MODE_FSCREDS) {
>  		caller_uid = cred->fsuid;
>  		caller_gid = cred->fsgid;
> @@ -331,6 +333,11 @@ static int __ptrace_may_access(struct task_struct *task, unsigned int mode)
>  	return security_ptrace_access_check(task, mode);
>  }
>  
> +static int __ptrace_may_access(struct task_struct *task, unsigned int mode)
> +{
> +	return ___ptrace_may_access(current, task, mode);
> +}
> +
>  bool ptrace_may_access(struct task_struct *task, unsigned int mode)
>  {
>  	int err;
>