linux-ext4 - Re: WARNING in ext4

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Date:   Tue, 16 Oct 2018 11:53:41 -0400
From:   "Theodore Y. Ts'o" <tytso@....edu>
To:     Dmitry Vyukov <dvyukov@...gle.com>
Cc:     syzbot <syzbot+85da7ac734f7ba432ee4@...kaller.appspotmail.com>,
        Andreas Dilger <adilger.kernel@...ger.ca>,
        linux-ext4@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>
Subject: Re: WARNING in ext4_invalidatepage

On Tue, Oct 16, 2018 at 04:02:07PM +0200, Dmitry Vyukov wrote:
> I am not sure how exactly this should be classified. To significant
> degree these "$FOO" discriminations are informational and only really
> affect the data format passed in.

The problem is that putting something which is simply and plainly
*wrong* is going to be actively misleading.  You *have* to know what
ioctl you are trying to be trying to execute because some of them
require input data structures that you have to fill in, or that it
requires a file descriptor as opposed to an integer or a pointer to a
struct.  I assume you're not just picking ioctl numbers purely at
random, right?

So I assume you had a table that said, this ioctl, 0x6611 takes a file
descriptor, and has thus-and-so-a-name.  If that name is wrong, I'd
say it's pretty clearly a bug, right?

(I'll again gently point out that a tiny amount of work in making
syzkaller a tiny bit more developer toil would make it much more
effective, since all of this extra developer toil --- now I know I
can't even trust the $FOO discriminators to be right --- makes
syzkaller less scalable by pursuading developers that it's not
worthwhile to pay attention...)

> > The patch I referenced in my previous e-mail protects against
> > additional scenarios where someone might be trying to punch a whole
> > into a file that is being swapped into the bootloader ioctl.  This
> > particular ioctl isn't yet being used by anyone, so it had some other
> > issues as well, such as not interacting well with inline_data-enabled
> > file systems --- not that any bootloader would be small enough that it
> > would fit in an inline_data inode, but we're basically proofing the
> > code against a malicious (or buggy) root-privileged program... such as
> > syzbot.  :-)
> 
> ... or paving the way to opening all of this to non-root users. Why
> not if not bugs? ;)

The intent behind this particular ioctl is used to install a boot
loader.  It will *never* be opened to non-root users.  It doesn't even
make sense to make it available to pseudo-containers-root users.  :-)

							- Ted