[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 dc12db8600e..ca561903936 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 40722e26de9..b6cb802fbcd 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 4e3f919edc4..fa235779b6a 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 a0c1a29a507..1ac2d1c5d84 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;
 }