[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>

2 files changed, 9 insertions, 6 deletions

diff --git a/fs/dquot.c b/fs/dquot.c
index f9cd5e23ebdf..89066b19124d 100644
--- a/fs/dquot.c
+++ b/fs/dquot.c
@@ -828,6 +828,7 @@ static inline int need_print_warning(struct dquot *dquot)
 static void print_warning(struct dquot *dquot, const char warntype)
 {
         char *msg = NULL;
+        struct tty_struct *tty;
         int flag = (warntype == BHARDWARN || warntype == BSOFTLONGWARN) ? DQ_BLKS_B :
           ((warntype == IHARDWARN || warntype == ISOFTLONGWARN) ? DQ_INODES_B : 0);
 
@@ -835,14 +836,15 @@ static void print_warning(struct dquot *dquot, const char warntype)
                 return;
 
         mutex_lock(&tty_mutex);
-        if (!current->signal->tty)
+        tty = get_current_tty();
+        if (!tty)
                 goto out_lock;
-        tty_write_message(current->signal->tty, dquot->dq_sb->s_id);
+        tty_write_message(tty, dquot->dq_sb->s_id);
         if (warntype == ISOFTWARN || warntype == BSOFTWARN)
-                tty_write_message(current->signal->tty, ": warning, ");
+                tty_write_message(tty, ": warning, ");
         else
-                tty_write_message(current->signal->tty, ": write failed, ");
-        tty_write_message(current->signal->tty, quotatypes[dquot->dq_type]);
+                tty_write_message(tty, ": write failed, ");
+        tty_write_message(tty, quotatypes[dquot->dq_type]);
         switch (warntype) {
                 case IHARDWARN:
                         msg = " file limit reached.\r\n";
@@ -863,7 +865,7 @@ static void print_warning(struct dquot *dquot, const char warntype)
                         msg = " block quota exceeded.\r\n";
                         break;
         }
-        tty_write_message(current->signal->tty, msg);
+        tty_write_message(tty, msg);
 out_lock:
         mutex_unlock(&tty_mutex);
 }
diff --git a/fs/open.c b/fs/open.c
index 89e0c237a636..3b56192816ca 100644
--- a/fs/open.c
+++ b/fs/open.c
@@ -1087,6 +1087,7 @@ EXPORT_SYMBOL(sys_close);
 asmlinkage long sys_vhangup(void)
 {
         if (capable(CAP_SYS_TTY_CONFIG)) {
+                /* XXX: this needs locking */
                 tty_vhangup(current->signal->tty);
                 return 0;
         }