linux-kernel - Re: [PATCH RFC 0/1] mount: universally disallow mounting over symlinks

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Date:   Thu, 9 Jan 2020 16:08:16 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     Aleksa Sarai <cyphar@...har.com>,
        David Howells <dhowells@...hat.com>,
        Eric Biederman <ebiederm@...ssion.com>,
        stable <stable@...r.kernel.org>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Serge Hallyn <serge@...lyn.com>, dev@...ncontainers.org,
        Linux Containers <containers@...ts.linux-foundation.org>,
        Linux API <linux-api@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Ian Kent <raven@...maw.net>
Subject: Re: [PATCH RFC 0/1] mount: universally disallow mounting over symlinks

On Wed, Jan 8, 2020 at 1:34 PM Al Viro <viro@...iv.linux.org.uk> wrote:
>
> The point is, we'd never followed mounts on /proc/self/cwd et.al.
> I hadn't checked 2.0, but 2.1.100 ('97, before any changes from me)
> is that way.

Hmm. If that's the case, maybe they should be marked implicitly as
O_PATH when opened?

> Actually, scratch that - 2.0 behaves the same way
> (mountpoint crossing is done in iget() there; is that Minix influence
> or straight from the Lions' book?)

I don't think I ever had access to Lions' - I've _seen_ a printout of
it later, and obviously maybe others did,

More likely it's from Maurice Bach: the Design of the Unix Operating
System. I'm pretty sure that's where a lot of the FS layer stuff came
from.  Certainly the bad old buffer head interfaces, and quite likely
the iget() stuff too.

> 0.10: forward traversal in iget(), back traversal in fs/namei.c:find_entry()

Whee, you _really_ went back in time.

So I did too.

And looking at that code in iget(), I doubt it came from anywhere.
Christ. It's just looping over a fixed-size array, both when finding
the inode, and finding the superblock.

Cute, but unbelievably stupid. It was a more innocent time.

In other words, I think you can chalk it up to just me, because
blaming anybody else for that garbage would be very very unfair indeed
;)

> How would your proposal deal with access("/proc/self/fd/42/foo", MAY_READ)
> vs. faccessat(42, "foo", MAY_READ)?

I think that in a perfect world, the O_PATH'ness of '42' would be the
deciding factor. Wouldn't those be the best and most consistent
semantics?

And then 'cwd'/'root' always have the O_PATH behavior.

> The latter would trigger automount,
> the former would not...  Or would you extend that to "traverse mounts
> upon following procfs links, if the file in question had been opened with
> O_PATH"?

Exactly.

But you know what? I do not believe this is all that important, and I
doubt it will matter to anybody.

So what matters most is what makes the most sense to the VFS layer,
and what makes the most sense to _you_.

Because my reaction from this thread is that not only have you thought
about this issue and followed the history a whole lot more than I
would ever have done, it's also that I trust you to DTRT.

I think it would be good to have some self-consistency, but at the
same time clearly we already don't really, and our behavior here has
subtly changed over the years (and not so subtly - if you go back
sufficiently far, /proc behavior wrt file descriptors has had both
"dup()" behavior and "make a new file descriptor with the same inode"
behavior, afaik).

               Linus