[patch 3/5] fs: fix bdi writeback use after free 1

[npiggin]

By the time bdi_work_on_stack gets evaluated again in bdi_work_free, it
cna already have been deallocated and used for something else in the
!on stack case, giving a false positive in this test and causing corruption.

Signed-off-by: Nick Piggin <npiggin@xxxxxxx>
---
 fs/fs-writeback.c |    5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

Index: linux-2.6/fs/fs-writeback.c
===================================================================
--- linux-2.6.orig/fs/fs-writeback.c
+++ linux-2.6/fs/fs-writeback.c
@@ -114,6 +114,7 @@ static void bdi_work_free(struct rcu_hea
 static void wb_work_complete(struct bdi_work *work)
 {
 	const enum writeback_sync_modes sync_mode = work->sync_mode;
+	int onstack = bdi_work_on_stack(work);
 
 	/*
 	 * For allocated work, we can clear the done/seen bit right here.
@@ -121,9 +122,9 @@ static void wb_work_complete(struct bdi_
 	 * to after the RCU grace period, since the stack could be invalidated
 	 * as soon as bdi_work_clear() has done the wakeup.
 	 */
-	if (!bdi_work_on_stack(work))
+	if (!onstack)
 		bdi_work_clear(work);
-	if (sync_mode == WB_SYNC_NONE || bdi_work_on_stack(work))
+	if (sync_mode == WB_SYNC_NONE || onstack)
 		call_rcu(&work->rcu_head, bdi_work_free);
 }
 


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