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

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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@redhat.com>
---
 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

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[PATCH 0/5] exec: binfmt_misc: fix use-after-free, kill  iname[BINPRM_BUF_SIZE] Oleg Nesterov <oleg@redhat.com> - 2017-09-22 16:40 +0200
  [PATCH 1/5] exec: binfmt_misc: don't nullify Node->dentry in  kill_node() Oleg Nesterov <oleg@redhat.com> - 2017-09-22 16:40 +0200
  [PATCH 4/5] exec: binfmt_misc: fix race between load_misc_binary()  and kill_node() Oleg Nesterov <oleg@redhat.com> - 2017-09-22 16:40 +0200
  Re: [PATCH 0/5] exec: binfmt_misc: fix use-after-free, kill iname[BINPRM_BUF_SIZE] Kees Cook <keescook@chromium.org> - 2017-09-22 17:30 +0200

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