[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>

1 files changed, 3 insertions, 6 deletions

diff --git a/kernel/acct.c b/kernel/acct.c
index dc12db8600e7..ca5619039367 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);