On 12/01/2017 04:48 AM, Al Viro wrote:
> On Fri, Dec 01, 2017 at 01:33:04AM +0000, Al Viro wrote:
>
>> Use of file descriptors should be limited to "got a number from userland,
>> convert to struct file *" on the way in and "install struct file * into
>> descriptor table and return the descriptor to userland" on the way out.
>> And the latter - *ONLY* after the last possible point of failure. Once
>> a file reference is inserted into descriptor table, that's it - you
>> can't undo that.
>>
>> The only way to use bpf_obj_get_user() is to pass its return value to
>> userland. As return value of syscall - not even put_user() (for that
>> you'd need to reserve the descriptor, copy it to userland and only
>> then attach struct file * to it).
>>
>> The whole approach stinks - what it needs is something that would
>> take struct filename * and return struct bpf_prog * or struct file *
>> reference. With bpf_obj_get_user() and this thing implemented
>> via that.
Agree, the "fix" is completely buggy due to fd being exposed to user
space during that period of time ...
>> I'm looking into that thing...
>
> What it tries to pull off is something not far from
>
> static struct bpf_prog *__get_prog(struct inode *inode, enum bpf_prog_type
> type)
> {
> struct bpf_prog *prog;
> int err = inode_permission(inode, FMODE_READ | FMODE_WRITE);
> if (err)
> return ERR_PTR(err);
>
> if (inode->i_op == &bpf_map_iops)
> return ERR_PTR(-EINVAL);
>
> if (inode->i_op != &bpf_prog_iops)
> return ERR_PTR(-EACCES);
>
> prog = inode->i_private;
> err = security_bpf_prog(prog);
> if (err < 0)
> return ERR_PTR(err);
>
> if (!bpf_prog_get_ok(prog, &type, false))
> return ERR_PTR(-EINVAL);
>
> return bpf_prog_inc(prog);
> }
>
> struct bpf_prog *get_prog_path_type(const char *name, enum bpf_prog_type type)
> {
> struct path path;
> struct bpf_prog *prog;
> int err = kern_path(name, LOOKUP_FOLLOW, &path);
> if (err)
> return ERR_PTR(err);
> prog = __get_prog(d_backing_inode(path.dentry), type);
> if (!IS_ERR(prog))
> touch_atime(&path);
> path_put(&path);
> return prog;
> }
>
> static int __bpf_mt_check_path(const char *path, struct bpf_prog **ret)
> {
> *ret = get_prog_path_type(path, BPF_PROG_TYPE_SOCKET_FILTER);
> return PTR_ERR_OR_ZERO(*ret);
> }
>
> That skips all tracepoint random shite (pardon the triple redundance) and
> makes
> a somewhat arbitrary change for touch_atime() logics. And, of course, it is
> not even compile-tested.
>
> Something similar to get_prog_path_type() above might make for a usable
> primitive, IMO...
The above looks good to me!
