Re: KASAN: use-after-free Read in path_lookupat

[Al Viro]

On Sun, Mar 24, 2019 at 06:23:24PM -0700, Linus Torvalds wrote:


> Al, comments? At the very least, if we don't make
> simple_symlink_inode_operations() do that, we should have a *big*
> comment that if it's not part of the inode data, it needs to be
> RCU-delayed.

simple_symlink_inode_operations is red herring here - what matters
is ->i_link being set; those have ->get_link == simple_get_link,
but note that it is *not* called:
        res = inode->i_link;
        if (!res) {
                const char * (*get)(struct dentry *, struct inode *,
                                struct delayed_call *);
                get = inode->i_op->get_link;
                if (nd->flags & LOOKUP_RCU) {
                        res = get(NULL, inode, &last->done);
                        if (res == ERR_PTR(-ECHILD)) {
                                if (unlikely(unlazy_walk(nd)))
                                        return ERR_PTR(-ECHILD);
                                res = get(dentry, inode, &last->done);
                        }
                } else {
                        res = get(dentry, inode, &last->done);
                }
                if (IS_ERR_OR_NULL(res))
                        return res;
	}
for traversal and similar for readlink(2).  And we certainly don't want
to allocate copies in those cases - it would fuck RCU traversals for
all fast symlinks (i.e. for the majority of symlinks out there).

Actual situation:

* shmem, erofs: OK, kfree() from the thing ->destroy_inode() is calling via
call_rcu().
* befs, ext2, ext4, freevxfs, jfs, orangefs, ufs: OK, coallocated with inode
* debugfs: broken
* jffs2: broken, freeing of f->target should be moved to jffs2_i_callback().
* ubifs: broken, ought to move kfree(ui->data); from ubifs_destroy_inode() to
ubifs_i_callback()
* ceph: broken, needs to move kfree(ci->symlink) from ceph_destroy_inode()
to ceph_i_callback().
* bpf: broken

So we have 5 broken cases, all with the same kind of fix: move freeing
into the RCU-delayed part of ->destroy_inode(); for debugfs and bpf
that requires adding ->alloc_inode()/->destroy_inode(), rather than
relying upon the defaults from fs/inode.c

> Or maybe we could add a final inode callback function for "rcu_free"
> that is called as the RCU-delayed freeing of the inode itself happens?
> And then people could hook into that for freeing the inode->i_link
> data.

You mean, split ->destroy_inode() into immediate and RCU-delayed parts?
There are filesystems where both parts are non-empty - we can't just
switch all ->destroy_inode() work to call_rcu().

> So many choices.. But the current situation seems unnecessarily
> complex for the filesystem, and isn't really documented.
> 
> Our documentation currently says for get_link(): "If the body won't go
> away until the inode is gone, nothing else is needed", which is wrong
> (or at least very misleading, since the last "inode is gone" callback
> we have is that evict() function).

s/inode is gone/struct inode is freed/, but it's obviously not clear
enough.