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SubjectRe: [PATCH bpf-next v9 05/10] bpf,landlock: Add a new map type: inode
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On 26/06/2019 00:52, Al Viro wrote:
> On Tue, Jun 25, 2019 at 11:52:34PM +0200, Mickaël Salaün wrote:
>> +/* must call iput(inode) after this call */
>> +static struct inode *inode_from_fd(int ufd, bool check_access)
>> +{
>> + struct inode *ret;
>> + struct fd f;
>> + int deny;
>> +
>> + f = fdget(ufd);
>> + if (unlikely(!f.file || !file_inode(f.file))) {
>> + ret = ERR_PTR(-EBADF);
>> + goto put_fd;
>> + }
>
> Just when does one get a NULL file_inode()? The reason I'm asking is
> that arseloads of code would break if one managed to create such
> a beast...

I didn't find any API documentation about this guarantee, so I followed
a defensive programming approach. I'll remove the file_inode() check.

>
> Incidentally, that should be return ERR_PTR(-EBADF); fdput() is wrong there.

Right, I'll fix that.

>
>> + }
>> + /* check if the FD is tied to a mount point */
>> + /* TODO: add this check when called from an eBPF program too */
>> + if (unlikely(!f.file->f_path.mnt
>
> Again, the same question - when the hell can that happen?

Defensive programming again, I'll remove it.

> If you are
> sitting on an exploitable roothole, do share it...
>
> || f.file->f_path.mnt->mnt_flags &
>> + MNT_INTERNAL)) {
>> + ret = ERR_PTR(-EINVAL);
>> + goto put_fd;
>
> What does it have to do with mountpoints, anyway?

I want to only manage inodes tied to a userspace-visible file system
(this check may not be enough though). It doesn't make sense to be able
to add inodes which are not mounted, to this kind of map.

>
>> +/* called from syscall */
>> +static int sys_inode_map_delete_elem(struct bpf_map *map, struct inode *key)
>> +{
>> + struct inode_array *array = container_of(map, struct inode_array, map);
>> + struct inode *inode;
>> + int i;
>> +
>> + WARN_ON_ONCE(!rcu_read_lock_held());
>> + for (i = 0; i < array->map.max_entries; i++) {
>> + if (array->elems[i].inode == key) {
>> + inode = xchg(&array->elems[i].inode, NULL);
>> + array->nb_entries--;
>
> Umm... Is that intended to be atomic in any sense?

nb_entries is not used as a bound check but to avoid walking uselessly
through the (pre-allocated) array when adding a new element, but I'll
use an atomic to avoid inconsistencies anyway.

>
>> + iput(inode);
>> + return 0;
>> + }
>> + }
>> + return -ENOENT;
>> +}
>> +
>> +/* called from syscall */
>> +int bpf_inode_map_delete_elem(struct bpf_map *map, int *key)
>> +{
>> + struct inode *inode;
>> + int err;
>> +
>> + inode = inode_from_fd(*key, false);
>> + if (IS_ERR(inode))
>> + return PTR_ERR(inode);
>> + err = sys_inode_map_delete_elem(map, inode);
>> + iput(inode);
>> + return err;
>> +}
>
> Wait a sec... So we have those beasties that can have long-term
> references to arbitrary inodes stuck in them? What will happen
> if you get umount(2) called while such a thing exists?

I though an umount would be denied but no, we get a self-destructed busy
inode and a bug!
What about wrapping the inode's superblock->s_op->destroy_inode() to
first remove the element from the map and then call the real
destroy_inode(), if any?
Or I could update fs/inode.c:destroy_inode() to call inode->free_inode()
if it is set, and set it when such inode is referenced by a map?
Or maybe I could hold the referencing file in the map and then wrap its
f_op?


--
Mickaël Salaün
ANSSI/SDE/ST/LAM

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