linux-kernel - Re: [PATCH 29/38] vfs: syscall: Add fsconfig() for configuring and managing a context [ver #10]

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Date:   Mon, 30 Jul 2018 13:32:35 +0100
From:   David Howells <dhowells@...hat.com>
To:     Jann Horn <jannh@...gle.com>
Cc:     dhowells@...hat.com, Al Viro <viro@...iv.linux.org.uk>,
        Linux API <linux-api@...r.kernel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-fsdevel@...r.kernel.org,
        kernel list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 29/38] vfs: syscall: Add fsconfig() for configuring and managing a context [ver #10]

Jann Horn <jannh@...gle.com> wrote:

> > > 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.
> 
> That's what I said.

Sorry, I wasn't clear what you meant.  I assumed you were thinking it was then
automatically attached to the context, say:

	fd = fsopen("fuse", 0);
	fsconfig(fd, fsconfig_set_binary, "foo", buffer, size);

> Each userspace task can make a 1MB allocation by calling this syscall, and
> this temporary allocation stays allocated until the end of the syscall. But
> the runtime of the syscall is unbounded - even just the memdup_user_nul()
> can stall forever if the copy_from_user() call inside it faults on e.g. a
> userfault region or a memory-mapped file from a FUSE filesystem.

Okay, I see what you're getting at.  Note that this affects other syscalls
too, keyctl, module loading and read() with readahead for example.  Not sure
what the answer should be.

David