linux-kernel - flink (AT_EMPTY_PATH / AT_SYMLINK

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Message-ID: <CALCETrWOmLF6DoAAdjccx3UHwuciAnu=FjGzdv5D+MKO2UutwA@mail.gmail.com>
Date:	Tue, 13 Aug 2013 10:21:36 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	linux-kernel@...r.kernel.org,
	Linux FS Devel <linux-fsdevel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	spender@...ecurity.net, Al Viro <viro@...iv.linux.org.uk>
Subject: flink (AT_EMPTY_PATH / AT_SYMLINK_FOLLOW) security considerations

Linux 3.10 (and many earlier kernels) allow flink using an incantation
like linkat(AT_FDCWD, "/proc/self/fd/N", destdirfd, newname,
AT_SYMLINK_FOLLOW);  It's possible to do much the same thing using
linkat(oldfd, "", destdirfd, newname, AT_EMPTY_PATH) if you're
privileged on 3.10, and the requirement for privilege is dropped in
3.11-rc5.

The immediate motivation for dropping the privilege requirement is the
O_TMPFILE changes: you can create a temporary file with O_TMPFILE,
write to it, and then give it a name with linkat(..., AT_EMPTY_PATH).
You can prevent this behavior by using O_TMPFILE | O_EXCL.

Apparently there's some kind of new security issue here [1], but I
don't know what it is.  So I'd like to get other people's thoughts.

Some notes:

All linkat variations do this:

	/* Make sure we don't allow creating hardlink to an unlinked file */
	if (inode->i_nlink == 0 && !(inode->i_state & I_LINKABLE))
		error =  -ENOENT;

That means that deleted files (except for O_TMPFILE, which sets
I_LINKABLE) can't be flinked.

Both flink variants work on O_PATH fds.

I've attached some test code if you want to play with this stuff.

Possible changes include inspecting f_cred before flink, requiring
I_ILINKABLE if unprivileged, and reverting the 3.11 change.

[1] https://lwn.net/Articles/562488/ -- see the comments

View attachment "tmpfile_flink.c" of type "text/x-csrc" (811 bytes)

View attachment "o_path_flink.c" of type "text/x-csrc" (937 bytes)