[PATCH 3.16 100/366] fuse: fix control dir setup and teardown

Ben Hutchings
2018-11-11T19:58:42Z

From: Miklos Szeredi <mszeredi@redhat.com>

commit 6becdb601bae2a043d7fb9762c4d48699528ea6e upstream.

syzbot is reporting NULL pointer dereference at fuse_ctl_remove_conn() [1].
Since fc->ctl_ndents is incremented by fuse_ctl_add_conn() when new_inode()
failed, fuse_ctl_remove_conn() reaches an inode-less dentry and tries to
clear d_inode(dentry)->i_private field.

Fix by only adding the dentry to the array after being fully set up.

When tearing down the control directory, do d_invalidate() on it to get rid
of any mounts that might have been added.

[1] https://syzkaller.appspot.com/bug?id=f396d863067238959c91c0b7cfc10b163638cac6
Reported-by: syzbot <syzbot+32c236387d66c4516827@syzkaller.appspotmail.com>
Fixes: bafa96541b25 ("[PATCH] fuse: add control filesystem")
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
[bwh: Backported to 3.16: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
fs/fuse/control.c | 13 ++++++++++---
1 file changed, 10 insertions(+), 3 deletions(-)

--- a/fs/fuse/control.c
+++ b/fs/fuse/control.c
@@ -211,10 +211,11 @@ static struct dentry *fuse_ctl_add_dentr
if (!dentry)
return NULL;

- fc->ctl_dentry[fc->ctl_ndents++] = dentry;
inode = new_inode(fuse_control_sb);
- if (!inode)
+ if (!inode) {
+ dput(dentry);
return NULL;
+ }

inode->i_ino = get_next_ino();
inode->i_mode = mode;
@@ -228,6 +229,9 @@ static struct dentry *fuse_ctl_add_dentr
set_nlink(inode, nlink);
inode->i_private = fc;
d_add(dentry, inode);
+
+ fc->ctl_dentry[fc->ctl_ndents++] = dentry;
+
return dentry;
}

@@ -284,7 +288,10 @@ void fuse_ctl_remove_conn(struct fuse_co
for (i = fc->ctl_ndents - 1; i >= 0; i--) {
struct dentry *dentry = fc->ctl_dentry[i];
dentry->d_inode->i_private = NULL;
- d_drop(dentry);
+ if (!i) {
+ /* Get rid of submounts: */
+ d_invalidate(dentry);
+ }
dput(dentry);
}
drop_nlink(fuse_control_sb->s_root->d_inode);