[PATCH 1/5] exec: binfmt_misc: don't nullify Node->dentry in kill_node()

From: Oleg Nesterov
Date: Fri Sep 22 2017 - 10:36:53 EST


kill_node() nullifies/checks Node->dentry to avoid double free. This
complicates the next changes and this is very confusing:

- we do not need to check dentry != NULL under entries_lock, kill_node()
is always called under inode_lock(d_inode(root)) and we rely on this
inode_lock() anyway, without this lock the MISC_FMT_OPEN_FILE cleanup
could race with itself.

- if kill_inode() was already called and ->dentry == NULL we should not
even try to close e->interp_file.

We can change bm_entry_write() to simply check !list_empty(list) before
kill_node. Again, we rely on inode_lock(), in particular it saves us from
the race with bm_status_write(), another caller of kill_node().

Signed-off-by: Oleg Nesterov <oleg@xxxxxxxxxx>
---
fs/binfmt_misc.c | 22 +++++++++-------------
1 file changed, 9 insertions(+), 13 deletions(-)

diff --git a/fs/binfmt_misc.c b/fs/binfmt_misc.c
index f471809..f4de5ae 100644
--- a/fs/binfmt_misc.c
+++ b/fs/binfmt_misc.c
@@ -600,11 +600,7 @@ static void kill_node(Node *e)
struct dentry *dentry;

write_lock(&entries_lock);
- dentry = e->dentry;
- if (dentry) {
- list_del_init(&e->list);
- e->dentry = NULL;
- }
+ list_del_init(&e->list);
write_unlock(&entries_lock);

if ((e->flags & MISC_FMT_OPEN_FILE) && e->interp_file) {
@@ -612,12 +608,11 @@ static void kill_node(Node *e)
e->interp_file = NULL;
}

- if (dentry) {
- drop_nlink(d_inode(dentry));
- d_drop(dentry);
- dput(dentry);
- simple_release_fs(&bm_mnt, &entry_count);
- }
+ dentry = e->dentry;
+ drop_nlink(d_inode(dentry));
+ d_drop(dentry);
+ dput(dentry);
+ simple_release_fs(&bm_mnt, &entry_count);
}

/* /<entry> */
@@ -662,7 +657,8 @@ static ssize_t bm_entry_write(struct file *file, const char __user *buffer,
root = file_inode(file)->i_sb->s_root;
inode_lock(d_inode(root));

- kill_node(e);
+ if (!list_empty(&e->list))
+ kill_node(e);

inode_unlock(d_inode(root));
break;
@@ -791,7 +787,7 @@ static ssize_t bm_status_write(struct file *file, const char __user *buffer,
inode_lock(d_inode(root));

while (!list_empty(&entries))
- kill_node(list_entry(entries.next, Node, list));
+ kill_node(list_first_entry(&entries, Node, list));

inode_unlock(d_inode(root));
break;
--
2.5.0