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

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Message-ID: <20160428192803.GG18994@madcap2.tricolour.ca>
Date:	Thu, 28 Apr 2016 15:28:03 -0400
From:	Richard Guy Briggs <rgb@...hat.com>
To:	Peter Hurley <peter@...leysoftware.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 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().

> 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.

> 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@...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