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

[Richard Guy Briggs]

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@xxxxxxxxxx>
> > ---
> > 
> > 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.  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@xxxxxxxxxx>
Kernel Security Engineering, Base Operating Systems, Red Hat
Remote, Ottawa, Canada
Voice: +1.647.777.2635, Internal: (81) 32635