Re: Kernel panic at Ubuntu: IMA + Apparmor

Eric W. Biederman
2014-04-25T21:44:21Z

Al Viro <viro@ZenIV.linux.org.uk> writes:

> On Fri, Apr 25, 2014 at 01:45:17PM -0700, Eric W. Biederman wrote:
>
>> IMA-appraisal is fundamentally broken because I can take a mandatory
>> file lock and prevent IMA-apprasial.
>>
>> Using kernel_read is what allows this.
>>
>> > Isn't it a clear motivating case???
>>
>> kernel_read is not appropriate for IMA use. The rest of this is just
>> the messenger.
>>
>> IMA needs to use a cousin of kernel_read that operates at a lower level
>> than vfs_read. A function that all of the permission checks and the
>> fsnotify work.
>
> It's worse than that, actually ;-/ IMA hooks in __fput() have interesting
> interplay with revoke-related stuff as well. Another very messy thing in
> the same area is that it actually does ->read() from under ->i_mutex, leading
> to all kinds of interesting locking issues...
>
> I doubt that your "let's open-code vfs_read() guts" would be a good idea;
> if nothing else, it might make more sense to make rw_verify_area() skip
> the mandlock and security theatre when called in such situation.
>
> What a mess... ;-/

Agreed.

All I really meant is that vfs_read does too much, so it probably needs
to be refactored for this case. But fsnotify_read, add_rchar, and
inc_syscr all seem inappropriate.

So I think we might be able to get away with something like this:

ssize_t __vfs_read(struct file *file, char __user *buf, size_t count, loff_t *pos)
{
ssize_t ret;

if (!(file->f_mode & FMODE_READ))
return -EBADF;
if (!file->f_op->read && !file->f_op->aio_read)
return -EINVAL;
if (unlikely(!access_ok(VERIFY_WRITE, buf, count)))
return -EFAULT;

if (ret >= 0) {
count = ret;
if (file->f_op->read)
ret = file->f_op->read(file, buf, count, pos);
else
ret = do_sync_read(file, buf, count, pos);
}

return ret;
}

How much of the rest we do really would seem to depend on how valuable
the sanity checks are.

This area of code keeps evolving enough that I don't see how we could
possibly avoid going through helper functions to figure out which file
ops we want to use this week.

Eric