From 614a4e3773148a31f58dc174bbf578ceb63510c2 Mon Sep 17 00:00:00 2001 From: Tejun Heo Date: Fri, 18 Mar 2016 13:50:03 -0400 Subject: [PATCH] writeback, cgroup: fix premature wb_put() in locked_inode_to_wb_and_lock_list() locked_inode_to_wb_and_lock_list() wb_get()'s the wb associated with the target inode, unlocks inode, locks the wb's list_lock and verifies that the inode is still associated with the wb. To prevent the wb going away between dropping inode lock and acquiring list_lock, the wb is pinned while inode lock is held. The wb reference is put right after acquiring list_lock citing that the wb won't be dereferenced anymore. This isn't true. If the inode is still associated with the wb, the inode has reference and it's safe to return the wb; however, if inode has been switched, the wb still needs to be unlocked which is a dereference and can lead to use-after-free if it it races with wb destruction. Fix it by putting the reference after releasing list_lock. Signed-off-by: Tejun Heo Fixes: 87e1d789bf55 ("writeback: implement [locked_]inode_to_wb_and_lock_list()") Cc: stable@vger.kernel.org # v4.2+ Tested-by: Tahsin Erdogan Signed-off-by: Jens Axboe --- fs/fs-writeback.c | 8 +++++--- 1 file changed, 5 insertions(+), 3 deletions(-) diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c index 5c46ed9f3e14..7b9582ed26f2 100644 --- a/fs/fs-writeback.c +++ b/fs/fs-writeback.c @@ -281,13 +281,15 @@ locked_inode_to_wb_and_lock_list(struct inode *inode) wb_get(wb); spin_unlock(&inode->i_lock); spin_lock(&wb->list_lock); - wb_put(wb); /* not gonna deref it anymore */ /* i_wb may have changed inbetween, can't use inode_to_wb() */ - if (likely(wb == inode->i_wb)) - return wb; /* @inode already has ref */ + if (likely(wb == inode->i_wb)) { + wb_put(wb); /* @inode already has ref */ + return wb; + } spin_unlock(&wb->list_lock); + wb_put(wb); cpu_relax(); spin_lock(&inode->i_lock); } -- 2.20.1