[PATCH] xacct_add_tsk: fix pure theoretical ->mm use-after-free

Paranoid fix. The task can free its ->mm after the 'if (p->mm)' check.
Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Cc: Shailabh Nagar <nagar@watson.ibm.com>
Cc: Balbir Singh <balbir@in.ibm.com>
Cc: Jay Lan <jlan@sgi.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

kernel/tsacct.c

@@ -80,13 +80,17 @@ void bacct_add_tsk(struct taskstats *stats, struct task_struct *tsk)
  */
 void xacct_add_tsk(struct taskstats *stats, struct task_struct *p)
 {
+	struct mm_struct *mm;
+
 	/* convert pages-jiffies to Mbyte-usec */
 	stats->coremem = jiffies_to_usecs(p->acct_rss_mem1) * PAGE_SIZE / MB;
 	stats->virtmem = jiffies_to_usecs(p->acct_vm_mem1) * PAGE_SIZE / MB;
-	if (p->mm) {
+	mm = get_task_mm(p);
+	if (mm) {
 		/* adjust to KB unit */
-		stats->hiwater_rss   = p->mm->hiwater_rss * PAGE_SIZE / KB;
-		stats->hiwater_vm    = p->mm->hiwater_vm * PAGE_SIZE / KB;
+		stats->hiwater_rss   = mm->hiwater_rss * PAGE_SIZE / KB;
+		stats->hiwater_vm    = mm->hiwater_vm * PAGE_SIZE / KB;
+		mmput(mm);
 	}
 	stats->read_char	= p->rchar;
 	stats->write_char	= p->wchar;