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From: "Jürg Billeter" <>
To: Aleksa Sarai <>,
	Al Viro <>,
	Jeff Layton <>,
	"J. Bruce Fields" <>,
	Arnd Bergmann <>,
	David Howells <>
Cc: Eric Biederman <>,
	Christian Brauner <>,, Andy Lutomirski <>,
	Jann Horn <>,
	David Drysdale <>,
	Aleksa Sarai <>,,,,
Subject: Re: [PATCH v4 2/4] namei: O_BENEATH-style path resolution flags
Date: Fri, 23 Nov 2018 13:10:17 +0100
Message-ID: <> (raw)
In-Reply-To: <>

Hi Aleksa,

On Tue, 2018-11-13 at 01:26 +1100, Aleksa Sarai wrote:
> * O_BENEATH: Disallow "escapes" from the starting point of the
>   filesystem tree during resolution (you must stay "beneath" the
>   starting point at all times). Currently this is done by disallowing
>   ".." and absolute paths (either in the given path or found during
>   symlink resolution) entirely, as well as all "magic link" jumping.

With open_tree(2) and OPEN_TREE_CLONE, will O_BENEATH still be
necessary? As I understand it, O_BENEATH could be replaced by a much
simpler flag that only disallows absolute paths (incl. absolute
symlinks). And it would have the benefit that you can actually pass the
tree/directory fd to another process and escaping would not be possible
even if that other process doesn't use O_BENEATH (after calling
mount_setattr(2) to make sure it's locked down).

This approach would also make it easy to restrict writes via a cloned
tree/directory fd by marking it read-only via mount_setattr(2) (and
locking down the read-only flag). This would again be especially useful
when passing tree/directory fds across processes, or for voluntary
self-lockdown within a process for robustness against security bugs.

This wouldn't affect any of the other flags in this patch. And for full
equivalence to O_BENEATH you'd have to use O_NOMAGICLINKS in addition
to O_NOABSOLUTE, or whatever that new flag would be called.

Or is OPEN_TREE_CLONE too expensive for this use case? Or is there
anything else I'm missing?


  reply index

Thread overview: 10+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-11-12 14:26 [PATCH v4 0/4] namei: O_* flags to restrict path resolution Aleksa Sarai
2018-11-12 14:26 ` [PATCH v4 1/4] namei: split out nd->dfd handling to dirfd_path_init Aleksa Sarai
2018-11-12 14:26 ` [PATCH v4 2/4] namei: O_BENEATH-style path resolution flags Aleksa Sarai
2018-11-23 12:10   ` Jürg Billeter [this message]
2018-11-23 16:07     ` Andy Lutomirski
2018-11-23 16:48       ` Aleksa Sarai
2018-11-23 16:52         ` Aleksa Sarai
2018-11-23 16:58     ` Aleksa Sarai
2018-11-12 14:26 ` [PATCH v4 3/4] namei: O_THISROOT: chroot-like path resolution Aleksa Sarai
2018-11-12 14:26 ` [PATCH v4 4/4] namei: aggressively check for nd->root escape on ".." resolution Aleksa Sarai

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