dlm: fix plock use-after-free

Fix a regression from the original addition of nfs lock support
586759f03e2e9031ac5589912a51a909ed53c30a.  When a synchronous
(non-nfs) plock completes, the waiting thread will wake up and
free the op struct.  This races with the user thread in
dev_write() which goes on to read the op's callback field to
check if the lock is async and needs a callback.  This check
can happen on the freed op.  The fix is to note the callback
value before the op can be freed.

Signed-off-by: David Teigland <teigland@redhat.com>

1 files changed, 10 insertions, 7 deletions

diff --git a/fs/dlm/plock.c b/fs/dlm/plock.c
index 894a32d438d5..16f682e26c07 100644
--- a/fs/dlm/plock.c
+++ b/fs/dlm/plock.c
@@ -353,7 +353,7 @@ static ssize_t dev_write(struct file *file, const char __user *u, size_t count,
 {
         struct dlm_plock_info info;
         struct plock_op *op;
-        int found = 0;
+        int found = 0, do_callback = 0;
 
         if (count != sizeof(info))
                 return -EINVAL;
@@ -366,21 +366,24 @@ static ssize_t dev_write(struct file *file, const char __user *u, size_t count,
 
         spin_lock(&ops_lock);
         list_for_each_entry(op, &recv_list, list) {
-                if (op->info.fsid == info.fsid && op->info.number == info.number &&
+                if (op->info.fsid == info.fsid &&
+                    op->info.number == info.number &&
                     op->info.owner == info.owner) {
+                        struct plock_xop *xop = (struct plock_xop *)op;
                         list_del_init(&op->list);
-                        found = 1;
-                        op->done = 1;
                         memcpy(&op->info, &info, sizeof(info));
+                        if (xop->callback)
+                                do_callback = 1;
+                        else
+                                op->done = 1;
+                        found = 1;
                         break;
                 }
         }
         spin_unlock(&ops_lock);
 
         if (found) {
-                struct plock_xop *xop;
-                xop = (struct plock_xop *)op;
-                if (xop->callback)
+                if (do_callback)
                         dlm_plock_callback(op);
                 else
                         wake_up(&recv_wq);