[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>

1 files changed, 7 insertions, 3 deletions

diff --git a/kernel/tsacct.c b/kernel/tsacct.c
index 65a5036a3d95..96f77013d3f0 100644
--- a/kernel/tsacct.c
+++ b/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;