Re: [PATCH V4] audit: add tty field to LOGIN event

[Peter Hurley <peter@hurleysoftware.com>]

On 04/27/2016 06:31 PM, Richard Guy Briggs wrote:
> On 16/04/22, Peter Hurley wrote:
>> On 04/21/2016 11:14 AM, Richard Guy Briggs wrote:
>>> The tty field was missing from AUDIT_LOGIN events.
>>>
>>> Refactor code to create a new function audit_get_tty(), using it to
>>> replace the call in audit_log_task_info() and to add it to
>>> audit_log_set_loginuid().  Lock and bump the kref to protect it, adding
>>> audit_put_tty() alias to decrement it.
>>>
>>> Signed-off-by: Richard Guy Briggs <rgb@redhat.com>
>>> ---
>>>
>>> V4: Add missing prototype for audit_put_tty() when audit syscall is not
>>>     enabled (MIPS).
>>>
>>> V3: Introduce audit_put_tty() alias to decrement kref.
>>>
>>> V2: Use kref to protect tty signal struct while in use.
>>>
>>> ---
>>>
>>>  include/linux/audit.h |   24 ++++++++++++++++++++++++
>>>  kernel/audit.c        |   18 +++++-------------
>>>  kernel/auditsc.c      |    8 ++++++--
>>>  3 files changed, 35 insertions(+), 15 deletions(-)
>>>
>>> diff --git a/include/linux/audit.h b/include/linux/audit.h
>>> index b40ed5d..32cdafb 100644
>>> --- a/include/linux/audit.h
>>> +++ b/include/linux/audit.h
>>> @@ -26,6 +26,7 @@
>>>  #include <linux/sched.h>
>>>  #include <linux/ptrace.h>
>>>  #include <uapi/linux/audit.h>
>>> +#include <linux/tty.h>
>>>  
>>>  #define AUDIT_INO_UNSET ((unsigned long)-1)
>>>  #define AUDIT_DEV_UNSET ((dev_t)-1)
>>> @@ -343,6 +344,23 @@ static inline unsigned int audit_get_sessionid(struct task_struct *tsk)
>>>  	return tsk->sessionid;
>>>  }
>>>  
>>> +static inline struct tty_struct *audit_get_tty(struct task_struct *tsk)
>>> +{
>>> +	struct tty_struct *tty = NULL;
>>> +	unsigned long flags;
>>> +
>>> +	spin_lock_irqsave(&tsk->sighand->siglock, flags);
>>> +	if (tsk->signal)
>>> +		tty = tty_kref_get(tsk->signal->tty);
>>> +	spin_unlock_irqrestore(&tsk->sighand->siglock, flags);
>>
>>
>> Not that I'm objecting because I get that you're just refactoring
>> existing code, but I thought I'd point out some stuff.
>>
>> 1. There's no need to check if signal_struct is NULL (ie. tsk->signal)
>>    because if it is, this will blow up trying to dereference the
>>    sighand_struct (ie tsk->sighand).
> 
> Ok.  This logic goes back 10 years and one month less two days. (45d9bb0e)
> 
>> 2. The existing usage is always tsk==current
> 
> My understanding is that when it is called via:
> 
> 	copy_process()
> 		audit_free()
> 			__audit_free()
> 				audit_log_exit()
> 					audit_log_task_info()
> 
> then tsk != current.

While it's true that tsk != current here, everything relevant to tty
in task_struct is the same because the nascent task is not even half-done.
So tsk->sighand == current->sighand, tsk->signal == current->signal etc.

If you're uncomfortable with pass-through execution like that, then the
simple solution is:


	struct tty_struct *tty = NULL;

	/* tsk != current when copy_process() failed */
	if (tsk == current)
		tty = get_current_tty();


because tty_kref_put(tty) accepts NULL tty and (obviously) so does
tty_name(tty).


Regards,
Peter Hurley


>  This appears to be the only case which appears to
> force lugging around tsk.  This is noted in that commit referenced
> above.
> 
>> 3. If the idea is to make this invulnerable to tsk being gone, then
>>    the usage is unsafe anyway.
>>
>>
>> So ultimately (but not necessarily for this patch) I'd prefer that either
>> a. audit use existing tty api instead of open-coding, or
>> b. add any tty api functions required.
> 
> This latter option did cross my mind...
> 
>> Peter Hurley
>>
>>> +	return tty;
>>> +}
>>> +
>>> +static inline void audit_put_tty(struct tty_struct *tty)
>>> +{
>>> +	tty_kref_put(tty);
>>> +}
>>> +
>>>  extern void __audit_ipc_obj(struct kern_ipc_perm *ipcp);
>>>  extern void __audit_ipc_set_perm(unsigned long qbytes, uid_t uid, gid_t gid, umode_t mode);
>>>  extern void __audit_bprm(struct linux_binprm *bprm);
>>> @@ -500,6 +518,12 @@ static inline unsigned int audit_get_sessionid(struct task_struct *tsk)
>>>  {
>>>  	return -1;
>>>  }
>>> +static inline struct tty_struct *audit_get_tty(struct task_struct *tsk)
>>> +{
>>> +	return NULL;
>>> +}
>>> +static inline void audit_put_tty(struct tty_struct *tty)
>>> +{ }
>>>  static inline void audit_ipc_obj(struct kern_ipc_perm *ipcp)
>>>  { }
>>>  static inline void audit_ipc_set_perm(unsigned long qbytes, uid_t uid,
>>> diff --git a/kernel/audit.c b/kernel/audit.c
>>> index 3a3e5de..7edd776 100644
>>> --- a/kernel/audit.c
>>> +++ b/kernel/audit.c
>>> @@ -64,7 +64,6 @@
>>>  #include <linux/security.h>
>>>  #endif
>>>  #include <linux/freezer.h>
>>> -#include <linux/tty.h>
>>>  #include <linux/pid_namespace.h>
>>>  #include <net/netns/generic.h>
>>>  
>>> @@ -1873,21 +1872,14 @@ void audit_log_task_info(struct audit_buffer *ab, struct task_struct *tsk)
>>>  {
>>>  	const struct cred *cred;
>>>  	char comm[sizeof(tsk->comm)];
>>> -	char *tty;
>>> +	struct tty_struct *tty;
>>>  
>>>  	if (!ab)
>>>  		return;
>>>  
>>>  	/* tsk == current */
>>>  	cred = current_cred();
>>> -
>>> -	spin_lock_irq(&tsk->sighand->siglock);
>>> -	if (tsk->signal && tsk->signal->tty && tsk->signal->tty->name)
>>> -		tty = tsk->signal->tty->name;
>>> -	else
>>> -		tty = "(none)";
>>> -	spin_unlock_irq(&tsk->sighand->siglock);
>>> -
>>> +	tty = audit_get_tty(tsk);
>>>  	audit_log_format(ab,
>>>  			 " ppid=%d pid=%d auid=%u uid=%u gid=%u"
>>>  			 " euid=%u suid=%u fsuid=%u"
>>> @@ -1903,11 +1895,11 @@ void audit_log_task_info(struct audit_buffer *ab, struct task_struct *tsk)
>>>  			 from_kgid(&init_user_ns, cred->egid),
>>>  			 from_kgid(&init_user_ns, cred->sgid),
>>>  			 from_kgid(&init_user_ns, cred->fsgid),
>>> -			 tty, audit_get_sessionid(tsk));
>>> -
>>> +			 tty ? tty_name(tty) : "(none)",
>>> +			 audit_get_sessionid(tsk));
>>> +	audit_put_tty(tty);
>>>  	audit_log_format(ab, " comm=");
>>>  	audit_log_untrustedstring(ab, get_task_comm(comm, tsk));
>>> -
>>>  	audit_log_d_path_exe(ab, tsk->mm);
>>>  	audit_log_task_context(ab);
>>>  }
>>> diff --git a/kernel/auditsc.c b/kernel/auditsc.c
>>> index 195ffae..71e14d8 100644
>>> --- a/kernel/auditsc.c
>>> +++ b/kernel/auditsc.c
>>> @@ -1980,6 +1980,7 @@ static void audit_log_set_loginuid(kuid_t koldloginuid, kuid_t kloginuid,
>>>  {
>>>  	struct audit_buffer *ab;
>>>  	uid_t uid, oldloginuid, loginuid;
>>> +	struct tty_struct *tty;
>>>  
>>>  	if (!audit_enabled)
>>>  		return;
>>> @@ -1987,14 +1988,17 @@ static void audit_log_set_loginuid(kuid_t koldloginuid, kuid_t kloginuid,
>>>  	uid = from_kuid(&init_user_ns, task_uid(current));
>>>  	oldloginuid = from_kuid(&init_user_ns, koldloginuid);
>>>  	loginuid = from_kuid(&init_user_ns, kloginuid),
>>> +	tty = audit_get_tty(current);
>>>  
>>>  	ab = audit_log_start(NULL, GFP_KERNEL, AUDIT_LOGIN);
>>>  	if (!ab)
>>>  		return;
>>>  	audit_log_format(ab, "pid=%d uid=%u", task_pid_nr(current), uid);
>>>  	audit_log_task_context(ab);
>>> -	audit_log_format(ab, " old-auid=%u auid=%u old-ses=%u ses=%u res=%d",
>>> -			 oldloginuid, loginuid, oldsessionid, sessionid, !rc);
>>> +	audit_log_format(ab, " old-auid=%u auid=%u tty=%s old-ses=%u ses=%u res=%d",
>>> +			 oldloginuid, loginuid, tty ? tty_name(tty) : "(none)",
>>> +			 oldsessionid, sessionid, !rc);
>>> +	audit_put_tty(tty);
>>>  	audit_log_end(ab);
>>>  }
>>>  
>>>
>>
> 
> - RGB
> 
> --
> Richard Guy Briggs <rgb@redhat.com>
> Kernel Security Engineering, Base Operating Systems, Red Hat
> Remote, Ottawa, Canada
> Voice: +1.647.777.2635, Internal: (81) 32635
>