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From:   David Howells <dhowells@redhat.com>
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To:     Jann Horn <jannh@google.com>
Cc:     dhowells@redhat.com, Al Viro <viro@zeniv.linux.org.uk>,
        Linux API <linux-api@vger.kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        linux-fsdevel@vger.kernel.org,
        kernel list <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 29/38] vfs: syscall: Add fsconfig() for configuring and managing a context [ver #10]
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Date:   Sun, 29 Jul 2018 09:50:00 +0100
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Jann Horn <jannh@google.com> wrote:

> [...]
> > +       case fsconfig_set_binary:
> > +               param.type = fs_value_is_blob;
> > +               param.size = aux;
> > +               param.blob = memdup_user_nul(_value, aux);
> > +               if (IS_ERR(param.blob)) {
> > +                       ret = PTR_ERR(param.blob);
> > +                       goto out_key;
> > +               }
> > +               break;
> 
> This means that a namespace admin (iow, an unprivileged user) can
> allocate 1MB of unswappable kmalloc memory per userspace task, right?
> Using userfaultfd or FUSE, you can then stall the task as long as you
> want while it has that allocation. Is that problematic, or is that
> normal?

That's not exactly the case.  A userspace task can make a temporary
allocation, but unless the filesystem grabs it, it's released again on exit
from the system call.

Note that I should probably use vmalloc() rather than kmalloc(), but that
doesn't really affect your point.  I could also pass the user pointer through
to the filesystem instead - I wanted to avoid that for this interface, but it
make sense in this instance.

David