From: Andy Lutomirski <luto@amacapital.net>
Date: Fri, 2 Aug 2013 04:44:31 +0000 (-0700)
Subject: fs: Allow unprivileged linkat(..., AT_EMPTY_PATH) aka flink
X-Git-Tag: v3.11-rc5~29^2~1
X-Git-Url: http://git.openpandora.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=bb2314b47996491bbc5add73633905c3120b6268;p=pandora-kernel.git

fs: Allow unprivileged linkat(..., AT_EMPTY_PATH) aka flink

Every now and then someone proposes a new flink syscall, and this spawns
a long discussion of whether it would be a security problem.  I think
that this is missing the point: flink is *already* allowed without
privilege as long as /proc is mounted -- it's called AT_SYMLINK_FOLLOW.

Now that O_TMPFILE is here, the ability to create a file with O_TMPFILE,
write it, and link it in is very convenient.  The only problem is that
it requires that /proc be mounted so that you can do:

linkat(AT_FDCWD, "/proc/self/fd/<tmpfd>", dfd, path, AT_SYMLINK_NOFOLLOW)

This sucks -- it's much nicer to do:

linkat(tmpfd, "", dfd, path, AT_EMPTY_PATH)

Let's allow it.

If this turns out to be excessively scary, it we could instead require
that the inode in question be I_LINKABLE, but this seems pointless given
the /proc situation

Signed-off-by: Andy Lutomirski <luto@amacapital.net>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
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