linux-kernel - Re: new ...at() flag: AT_NO

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Date:   Mon, 1 May 2017 19:36:52 +0200
From:   Jann Horn <jannh@...gle.com>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     Linux API <linux-api@...r.kernel.org>,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: new ...at() flag: AT_NO_JUMPS

On Sun, Apr 30, 2017 at 12:04 AM, Al Viro <viro@...iv.linux.org.uk> wrote:
> New AT_... flag - AT_NO_JUMPS
>
> Semantics: pathname resolution must not involve
>         * traversals of absolute symlinks
>         * traversals of procfs-style symlinks
>         * traversals of mountpoints (including bindings, referrals, etc.)
>         * traversal of .. in the starting point of pathname resolution.
>
> All of those lead to failure with -ELOOP.  Relative symlinks are fine,
> as long as their resolution does not end up stepping into the conditions
> above.
>
> It guarantees that result of successful pathname resolution will be on the
> same filesystem as its starting point and within the subtree rooted at
> the starting point.
>
> Right now I have it hooked only for fstatat() and friends; it could be
> easily extended to any ...at() syscalls.  Objections?

Oh, nice!

It looks like this is somewhat similar to the old O_BENEATH proposal,
but because the intentions behind the proposals are different
(application sandboxing versus permitting an application to restrict its
own filesystem accesses), the semantics differ: AT_NO_JUMPS
doesn't prevent starting the path with "/", but does prevent mountpoint
traversal. Is that correct?

I think that, as Andy mentioned, it might make sense to split out (or
even remove?) the prevention of mountpoint traversal. A user who
can create visible mountpoints needs to have capabilities over the
mount namespace the file descriptor refers to already.

I suspect that if this lands, it would be pretty straightforward to add
another flag AT_NO_ABSOLUTE or so that, combined with
AT_NO_JUMPS, has the same semantics as O_BENEATH?