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From: John Johansen <john.johansen@canonical.com>
To: Paul Moore <paul@paul-moore.com>,
	Casey Schaufler <casey@schaufler-ca.com>
Cc: selinux@vger.kernel.org, linux-security-module@vger.kernel.org,
	linux-audit@redhat.com
Subject: Re: [RFC PATCH 2/4] selinux: clarify task subjective and objective credentials
Date: Tue, 9 Mar 2021 19:05:54 -0800
Message-ID: <b27662cf-4bcf-ec23-92f5-49a5b2f8c119@canonical.com> (raw)
In-Reply-To: <161377735153.87807.7492842242100187888.stgit@sifl>

On 2/19/21 3:29 PM, Paul Moore wrote:
> SELinux has a function, task_sid(), which returns the task's
> objective credentials, but unfortunately is used in a few places
> where the subjective task credentials should be used.  Most notably
> in the new security_task_getsecid_subj() LSM hook.
> 
> This patch fixes this and attempts to make things more obvious by
> introducing a new function, task_sid_subj(), and renaming the
> existing task_sid() function to task_sid_obj().
> 
> Signed-off-by: Paul Moore <paul@paul-moore.com>

I have a couple of questions below but the rest looks good

> ---
>  security/selinux/hooks.c |   85 +++++++++++++++++++++++++++-------------------
>  1 file changed, 49 insertions(+), 36 deletions(-)
> 
> diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
> index f311541c4972e..1c53000d28e37 100644
> --- a/security/selinux/hooks.c
> +++ b/security/selinux/hooks.c
> @@ -229,10 +229,23 @@ static inline u32 cred_sid(const struct cred *cred)
>  	return tsec->sid;
>  }
>  
> +/*
> + * get the subjective security ID of a task
> + */
> +static inline u32 task_sid_subj(const struct task_struct *task)
> +{
> +	u32 sid;
> +
> +	rcu_read_lock();
> +	sid = cred_sid(rcu_dereference(task->cred));
> +	rcu_read_unlock();
> +	return sid;
> +}
> +
>  /*
>   * get the objective security ID of a task
>   */
> -static inline u32 task_sid(const struct task_struct *task)
> +static inline u32 task_sid_obj(const struct task_struct *task)
>  {
>  	u32 sid;
>  
> @@ -2034,11 +2047,8 @@ static inline u32 open_file_to_av(struct file *file)
>  
>  static int selinux_binder_set_context_mgr(struct task_struct *mgr)
>  {
> -	u32 mysid = current_sid();
> -	u32 mgrsid = task_sid(mgr);
> -
>  	return avc_has_perm(&selinux_state,
> -			    mysid, mgrsid, SECCLASS_BINDER,
> +			    current_sid(), task_sid_obj(mgr), SECCLASS_BINDER,
>  			    BINDER__SET_CONTEXT_MGR, NULL);
>  }
>  
> @@ -2046,8 +2056,8 @@ static int selinux_binder_transaction(struct task_struct *from,
>  				      struct task_struct *to)
>  {
>  	u32 mysid = current_sid();
> -	u32 fromsid = task_sid(from);
> -	u32 tosid = task_sid(to);
> +	u32 fromsid = task_sid_subj(from);

fromsid potentially gets used as both the subject and the object the following
permission checks. It makes sense to use the same cred for both checks but
what I am not sure about yet is whether its actually safe to use the subject
sid when the task isn't current.

ie. I am still trying to determine if there is a race here between the transaction
request and the permission check.

> +	u32 tosid = task_sid_subj(to);
its not clear to me that using the subj for to is correct

>  	int rc;
>  
>  	if (mysid != fromsid) {
> @@ -2066,11 +2076,9 @@ static int selinux_binder_transaction(struct task_struct *from,
>  static int selinux_binder_transfer_binder(struct task_struct *from,
>  					  struct task_struct *to)
>  {
> -	u32 fromsid = task_sid(from);
> -	u32 tosid = task_sid(to);
> -
>  	return avc_has_perm(&selinux_state,
> -			    fromsid, tosid, SECCLASS_BINDER, BINDER__TRANSFER,
> +			    task_sid_subj(from), task_sid_obj(to),
> +			    SECCLASS_BINDER, BINDER__TRANSFER,
>  			    NULL);
>  }
>  
> @@ -2078,7 +2086,7 @@ static int selinux_binder_transfer_file(struct task_struct *from,
>  					struct task_struct *to,
>  					struct file *file)
>  {
> -	u32 sid = task_sid(to);
> +	u32 sid = task_sid_subj(to);

same question about safety here

>  	struct file_security_struct *fsec = selinux_file(file);
>  	struct dentry *dentry = file->f_path.dentry;
>  	struct inode_security_struct *isec;
> @@ -2114,10 +2122,10 @@ static int selinux_binder_transfer_file(struct task_struct *from,
>  }
>  
>  static int selinux_ptrace_access_check(struct task_struct *child,
> -				     unsigned int mode)
> +				       unsigned int mode)
>  {
>  	u32 sid = current_sid();
> -	u32 csid = task_sid(child);
> +	u32 csid = task_sid_obj(child);
>  
>  	if (mode & PTRACE_MODE_READ)
>  		return avc_has_perm(&selinux_state,
> @@ -2130,15 +2138,15 @@ static int selinux_ptrace_access_check(struct task_struct *child,
>  static int selinux_ptrace_traceme(struct task_struct *parent)
>  {
>  	return avc_has_perm(&selinux_state,
> -			    task_sid(parent), current_sid(), SECCLASS_PROCESS,
> -			    PROCESS__PTRACE, NULL);
> +			    task_sid_subj(parent), task_sid_obj(current),
> +			    SECCLASS_PROCESS, PROCESS__PTRACE, NULL);
>  }
>  
>  static int selinux_capget(struct task_struct *target, kernel_cap_t *effective,
>  			  kernel_cap_t *inheritable, kernel_cap_t *permitted)
>  {
>  	return avc_has_perm(&selinux_state,
> -			    current_sid(), task_sid(target), SECCLASS_PROCESS,
> +			    current_sid(), task_sid_obj(target), SECCLASS_PROCESS,
>  			    PROCESS__GETCAP, NULL);
>  }
>  
> @@ -2263,7 +2271,7 @@ static u32 ptrace_parent_sid(void)
>  	rcu_read_lock();
>  	tracer = ptrace_parent(current);
>  	if (tracer)
> -		sid = task_sid(tracer);
> +		sid = task_sid_obj(tracer);
>  	rcu_read_unlock();
>  
>  	return sid;
> @@ -3916,7 +3924,7 @@ static int selinux_file_send_sigiotask(struct task_struct *tsk,
>  				       struct fown_struct *fown, int signum)
>  {
>  	struct file *file;
> -	u32 sid = task_sid(tsk);
> +	u32 sid = task_sid_obj(tsk);
>  	u32 perm;
>  	struct file_security_struct *fsec;
>  
> @@ -4135,47 +4143,52 @@ static int selinux_kernel_load_data(enum kernel_load_data_id id, bool contents)
>  static int selinux_task_setpgid(struct task_struct *p, pid_t pgid)
>  {
>  	return avc_has_perm(&selinux_state,
> -			    current_sid(), task_sid(p), SECCLASS_PROCESS,
> +			    current_sid(), task_sid_obj(p), SECCLASS_PROCESS,
>  			    PROCESS__SETPGID, NULL);
>  }
>  
>  static int selinux_task_getpgid(struct task_struct *p)
>  {
>  	return avc_has_perm(&selinux_state,
> -			    current_sid(), task_sid(p), SECCLASS_PROCESS,
> +			    current_sid(), task_sid_obj(p), SECCLASS_PROCESS,
>  			    PROCESS__GETPGID, NULL);
>  }
>  
>  static int selinux_task_getsid(struct task_struct *p)
>  {
>  	return avc_has_perm(&selinux_state,
> -			    current_sid(), task_sid(p), SECCLASS_PROCESS,
> +			    current_sid(), task_sid_obj(p), SECCLASS_PROCESS,
>  			    PROCESS__GETSESSION, NULL);
>  }
>  
> -static void selinux_task_getsecid(struct task_struct *p, u32 *secid)
> +static void selinux_task_getsecid_subj(struct task_struct *p, u32 *secid)
> +{
> +	*secid = task_sid_subj(p);
> +}
> +
> +static void selinux_task_getsecid_obj(struct task_struct *p, u32 *secid)
>  {
> -	*secid = task_sid(p);
> +	*secid = task_sid_obj(p);
>  }
>  
>  static int selinux_task_setnice(struct task_struct *p, int nice)
>  {
>  	return avc_has_perm(&selinux_state,
> -			    current_sid(), task_sid(p), SECCLASS_PROCESS,
> +			    current_sid(), task_sid_obj(p), SECCLASS_PROCESS,
>  			    PROCESS__SETSCHED, NULL);
>  }
>  
>  static int selinux_task_setioprio(struct task_struct *p, int ioprio)
>  {
>  	return avc_has_perm(&selinux_state,
> -			    current_sid(), task_sid(p), SECCLASS_PROCESS,
> +			    current_sid(), task_sid_obj(p), SECCLASS_PROCESS,
>  			    PROCESS__SETSCHED, NULL);
>  }
>  
>  static int selinux_task_getioprio(struct task_struct *p)
>  {
>  	return avc_has_perm(&selinux_state,
> -			    current_sid(), task_sid(p), SECCLASS_PROCESS,
> +			    current_sid(), task_sid_obj(p), SECCLASS_PROCESS,
>  			    PROCESS__GETSCHED, NULL);
>  }
>  
> @@ -4206,7 +4219,7 @@ static int selinux_task_setrlimit(struct task_struct *p, unsigned int resource,
>  	   upon context transitions.  See selinux_bprm_committing_creds. */
>  	if (old_rlim->rlim_max != new_rlim->rlim_max)
>  		return avc_has_perm(&selinux_state,
> -				    current_sid(), task_sid(p),
> +				    current_sid(), task_sid_obj(p),
>  				    SECCLASS_PROCESS, PROCESS__SETRLIMIT, NULL);
>  
>  	return 0;
> @@ -4215,21 +4228,21 @@ static int selinux_task_setrlimit(struct task_struct *p, unsigned int resource,
>  static int selinux_task_setscheduler(struct task_struct *p)
>  {
>  	return avc_has_perm(&selinux_state,
> -			    current_sid(), task_sid(p), SECCLASS_PROCESS,
> +			    current_sid(), task_sid_obj(p), SECCLASS_PROCESS,
>  			    PROCESS__SETSCHED, NULL);
>  }
>  
>  static int selinux_task_getscheduler(struct task_struct *p)
>  {
>  	return avc_has_perm(&selinux_state,
> -			    current_sid(), task_sid(p), SECCLASS_PROCESS,
> +			    current_sid(), task_sid_obj(p), SECCLASS_PROCESS,
>  			    PROCESS__GETSCHED, NULL);
>  }
>  
>  static int selinux_task_movememory(struct task_struct *p)
>  {
>  	return avc_has_perm(&selinux_state,
> -			    current_sid(), task_sid(p), SECCLASS_PROCESS,
> +			    current_sid(), task_sid_obj(p), SECCLASS_PROCESS,
>  			    PROCESS__SETSCHED, NULL);
>  }
>  
> @@ -4248,14 +4261,14 @@ static int selinux_task_kill(struct task_struct *p, struct kernel_siginfo *info,
>  	else
>  		secid = cred_sid(cred);
>  	return avc_has_perm(&selinux_state,
> -			    secid, task_sid(p), SECCLASS_PROCESS, perm, NULL);
> +			    secid, task_sid_obj(p), SECCLASS_PROCESS, perm, NULL);
>  }
>  
>  static void selinux_task_to_inode(struct task_struct *p,
>  				  struct inode *inode)
>  {
>  	struct inode_security_struct *isec = selinux_inode(inode);
> -	u32 sid = task_sid(p);
> +	u32 sid = task_sid_obj(p);
>  
>  	spin_lock(&isec->lock);
>  	isec->sclass = inode_mode_to_security_class(inode->i_mode);
> @@ -6148,7 +6161,7 @@ static int selinux_msg_queue_msgrcv(struct kern_ipc_perm *msq, struct msg_msg *m
>  	struct ipc_security_struct *isec;
>  	struct msg_security_struct *msec;
>  	struct common_audit_data ad;
> -	u32 sid = task_sid(target);
> +	u32 sid = task_sid_subj(target);
>  	int rc;
>  
>  	isec = selinux_ipc(msq);
> @@ -7143,8 +7156,8 @@ static struct security_hook_list selinux_hooks[] __lsm_ro_after_init = {
>  	LSM_HOOK_INIT(task_setpgid, selinux_task_setpgid),
>  	LSM_HOOK_INIT(task_getpgid, selinux_task_getpgid),
>  	LSM_HOOK_INIT(task_getsid, selinux_task_getsid),
> -	LSM_HOOK_INIT(task_getsecid_subj, selinux_task_getsecid),
> -	LSM_HOOK_INIT(task_getsecid_obj, selinux_task_getsecid),
> +	LSM_HOOK_INIT(task_getsecid_subj, selinux_task_getsecid_subj),
> +	LSM_HOOK_INIT(task_getsecid_obj, selinux_task_getsecid_obj),
>  	LSM_HOOK_INIT(task_setnice, selinux_task_setnice),
>  	LSM_HOOK_INIT(task_setioprio, selinux_task_setioprio),
>  	LSM_HOOK_INIT(task_getioprio, selinux_task_getioprio),
> 

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  parent reply index

Thread overview: 39+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-02-19 23:28 [RFC PATCH 0/4] Split security_task_getsecid() into subj and obj variants Paul Moore
2021-02-19 23:29 ` [RFC PATCH 1/4] lsm: separate security_task_getsecid() into subjective and objective variants Paul Moore
2021-02-20  2:55   ` James Morris
2021-02-20 14:44     ` Paul Moore
2021-03-04 10:04       ` Jeffrey Vander Stoep
2021-03-04 23:43         ` Paul Moore
2021-03-10  8:21           ` Jeffrey Vander Stoep
2021-03-11  1:56             ` Paul Moore
2021-02-21 12:51   ` John Johansen
2021-02-21 22:09     ` Paul Moore
2021-03-04  0:44     ` Paul Moore
2021-03-10  0:28       ` Paul Moore
2021-03-10  3:09         ` John Johansen
2021-02-24 16:49   ` Mimi Zohar
2021-03-08 19:25   ` Richard Guy Briggs
2021-03-10  0:23     ` Paul Moore
2021-03-10  1:03   ` John Johansen
2021-03-11  1:55     ` Paul Moore
2021-02-19 23:29 ` [RFC PATCH 2/4] selinux: clarify task subjective and objective credentials Paul Moore
2021-02-21 12:55   ` John Johansen
2021-03-08 19:26   ` Richard Guy Briggs
2021-03-10  3:05   ` John Johansen [this message]
2021-03-11  4:32     ` Paul Moore
2021-03-17 22:56       ` Paul Moore
2021-02-19 23:29 ` [RFC PATCH 3/4] smack: differentiate between subjective and objective task credentials Paul Moore
2021-02-21 12:56   ` John Johansen
2021-03-08 19:26   ` Richard Guy Briggs
2021-03-10  1:04   ` John Johansen
2021-02-19 23:29 ` [RFC PATCH 4/4] apparmor: " Paul Moore
2021-02-21 12:57   ` John Johansen
2021-02-21 22:12     ` Paul Moore
2021-02-20  1:49 ` [RFC PATCH 0/4] Split security_task_getsecid() into subj and obj variants Casey Schaufler
2021-02-20 14:41   ` Paul Moore
2021-02-22 23:58     ` Casey Schaufler
2021-02-23 14:14       ` Mimi Zohar
2021-02-24  0:03         ` Paul Moore
2021-03-04  0:46       ` Paul Moore
2021-03-04  2:21         ` Casey Schaufler
2021-03-04 23:41           ` Paul Moore

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