Message-ID: <57226535.9000000@hurleysoftware.com>
Date:	Thu, 28 Apr 2016 12:32:05 -0700
From:	Peter Hurley <peter@...leysoftware.com>
To:	Richard Guy Briggs <rgb@...hat.com>
Cc:	linux-audit@...hat.com, pmoore@...hat.com,
	linux-kernel@...r.kernel.org, sgrubb@...hat.com, eparis@...hat.com
Subject: Re: [PATCH V4] audit: add tty field to LOGIN event

On 04/28/2016 12:28 PM, Richard Guy Briggs wrote:
> On 16/04/27, Peter Hurley wrote:
>> 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@...hat.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.
> 
> I agree this is true except in the case of !CLONE_SIGHAND, if it fails
> after copy_sighand() or copy_signal() then it would be null and would
> get freed before audit_free() is called.  By the time tty gets copied
> from current in this case, it is past the point of failure in
> copy_process().

Oh, right.


>> 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).
> 
> Given the circumstances above, this appears reasonable to me at first
> look.

Ok.

I'll spend more analysis time before I actually submit a patch for this.


>>>  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@...hat.com>
>>> Kernel Security Engineering, Base Operating Systems, Red Hat
>>> Remote, Ottawa, Canada
>>> Voice: +1.647.777.2635, Internal: (81) 32635
>>>
>>
> 
> - RGB
> 
> --
> Richard Guy Briggs <rgb@...hat.com>
> Kernel Security Engineering, Base Operating Systems, Red Hat
> Remote, Ottawa, Canada
> Voice: +1.647.777.2635, Internal: (81) 32635
> 

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