[PATCH] tty: ->signal->tty locking

Fix the locking of signal->tty.

Use ->sighand->siglock to protect ->signal->tty; this lock is already used
by most other members of ->signal/->sighand.  And unless we are 'current'
or the tasklist_lock is held we need ->siglock to access ->signal anyway.

(NOTE: sys_unshare() is broken wrt ->sighand locking rules)

Note that tty_mutex is held over tty destruction, so while holding
tty_mutex any tty pointer remains valid.  Otherwise the lifetime of ttys
are governed by their open file handles.  This leaves some holes for tty
access from signal->tty (or any other non file related tty access).

It solves the tty SLAB scribbles we were seeing.

(NOTE: the change from group_send_sig_info to __group_send_sig_info needs to
       be examined by someone familiar with the security framework, I think
       it is safe given the SEND_SIG_PRIV from other __group_send_sig_info
       invocations)

[schwidefsky@de.ibm.com: 3270 fix]
[akpm@osdl.org: various post-viro fixes]
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Acked-by: Alan Cox <alan@redhat.com>
Cc: Oleg Nesterov <oleg@tv-sign.ru>
Cc: Prarit Bhargava <prarit@redhat.com>
Cc: Chris Wright <chrisw@sous-sol.org>
Cc: Roland McGrath <roland@redhat.com>
Cc: Stephen Smalley <sds@tycho.nsa.gov>
Cc: James Morris <jmorris@namei.org>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Jeff Dike <jdike@addtoit.com>
Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
Cc: Jan Kara <jack@ucw.cz>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

4 files changed, 10 insertions, 11 deletions

diff --git a/kernel/acct.c b/kernel/acct.c
index dc12db8..ca56190 100644
--- a/kernel/acct.c
+++ b/kernel/acct.c
@@ -428,6 +428,7 @@ static void do_acct_process(struct file *file)
 	u64 elapsed;
 	u64 run_time;
 	struct timespec uptime;
+	struct tty_struct *tty;
 
 	/*
 	 * First check to see if there is enough free_space to continue
@@ -485,12 +486,8 @@ static void do_acct_process(struct file *file)
 #endif
 
 	mutex_lock(&tty_mutex);
-	/* FIXME: Whoever is responsible for current->signal locking needs
-	   to use the same locking all over the kernel and document it */
-	read_lock(&tasklist_lock);
-	ac.ac_tty = current->signal->tty ?
-		old_encode_dev(tty_devnum(current->signal->tty)) : 0;
-	read_unlock(&tasklist_lock);
+	tty = get_current_tty();
+	ac.ac_tty = tty ? old_encode_dev(tty_devnum(tty)) : 0;
 	mutex_unlock(&tty_mutex);
 
 	spin_lock_irq(&current->sighand->siglock);
diff --git a/kernel/auditsc.c b/kernel/auditsc.c
index 40722e2..b6cb802 100644
--- a/kernel/auditsc.c
+++ b/kernel/auditsc.c
@@ -826,10 +826,12 @@ static void audit_log_exit(struct audit_context *context, struct task_struct *ts
 				 context->return_code);
 
 	mutex_lock(&tty_mutex);
+	read_lock(&tasklist_lock);
 	if (tsk->signal && tsk->signal->tty && tsk->signal->tty->name)
 		tty = tsk->signal->tty->name;
 	else
 		tty = "(none)";
+	read_unlock(&tasklist_lock);
 	audit_log_format(ab,
 		  " a0=%lx a1=%lx a2=%lx a3=%lx items=%d"
 		  " ppid=%d pid=%d auid=%u uid=%u gid=%u"
diff --git a/kernel/exit.c b/kernel/exit.c
index 4e3f919..fa23577 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -384,9 +384,7 @@ void daemonize(const char *name, ...)
 	exit_mm(current);
 
 	set_special_pids(1, 1);
-	mutex_lock(&tty_mutex);
-	current->signal->tty = NULL;
-	mutex_unlock(&tty_mutex);
+	proc_clear_tty(current);
 
 	/* Block and flush all signals */
 	sigfillset(&blocked);
diff --git a/kernel/sys.c b/kernel/sys.c
index a0c1a29..1ac2d1c5 100644
--- a/kernel/sys.c
+++ b/kernel/sys.c
@@ -1484,7 +1484,6 @@ asmlinkage long sys_setsid(void)
 	pid_t session;
 	int err = -EPERM;
 
-	mutex_lock(&tty_mutex);
 	write_lock_irq(&tasklist_lock);
 
 	/* Fail if I am already a session leader */
@@ -1504,12 +1503,15 @@ asmlinkage long sys_setsid(void)
 
 	group_leader->signal->leader = 1;
 	__set_special_pids(session, session);
+
+	spin_lock(&group_leader->sighand->siglock);
 	group_leader->signal->tty = NULL;
 	group_leader->signal->tty_old_pgrp = 0;
+	spin_unlock(&group_leader->sighand->siglock);
+
 	err = process_group(group_leader);
 out:
 	write_unlock_irq(&tasklist_lock);
-	mutex_unlock(&tty_mutex);
 	return err;
 }