linux-kernel - Re: [GIT PULL] proc fixes v2 for v5.8-rc1

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Date:   Fri, 12 Jun 2020 15:02:16 -0500
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Alexey Dobriyan <adobriyan@...il.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Alexey Gladkov <gladkov.alexey@...il.com>
Subject: Re: [GIT PULL] proc fixes v2 for v5.8-rc1

Linus Torvalds <torvalds@...ux-foundation.org> writes:

> On Fri, Jun 12, 2020 at 12:34 PM Eric W. Biederman
> <ebiederm@...ssion.com> wrote:
>>
>> ebiederm@...ssion.com (Eric W. Biederman) writes:
>
> What happened to that first version of the email? I never got it..

A little distracted I think.  I forgot to edit the above line out,
and v2 because it is my second pull request for a proc fix into
v5.8-rc1.

>> Looking at the code the fsnotify watch should have been removed by
>> fsnotify_sb_delete in generic_shutdown_super.
>
> Hmm. Correct. The new_inode_pseudo() is for things that don't have
> quotas, fsnotify or writeback.
>
> That was used somewhat intentionally on /proc, though. /proc certainly
> doesn't have quotas or writeback.

It also means we break our debugging by not putting inodes on the s_inodes list.

AKA this line in generic_shutdown_super is also depent on that behavior.
	if (!list_empty(&sb->s_inodes)) {
		printk("VFS: Busy inodes after unmount of %s. "
		   "Self-destruct in 5 seconds.  Have a nice day...\n",
		   sb->s_id);
	}

> And fsnotify on /proc seems a bit questionably too. Do people actually
> _do_ this and depend on it, or is this just about syzbot doing
> something odd and thus showing the problem?
>
> Anyway, I have pulled your fix, because I think it's reasonable and
> safe, but I do wonder if we should have kept the new_inode_pseudo(),
> and instead just make fsnotify say "you can't notify on an inode that
> isn't on the superblock list". Hmm?
>
> Is fsnotify on /proc really sensible? Do we actually generate any
> useful notifications?

I don't know of any proc code that does anything to make
fsnotify/inotify work.  Since changes to proc are not initialiated
through the vfs that probably means fsnotify is pretty much useless.

I have a sense that a use after free that anyone can trigger could be a
bit dangerous, and despite not being the only virtual filesystem in the
kernel proc is the only virtual filesystem that called new_inode_pseudo.

Eric