linux-kernel - Re: [RFC 2/2] fs,proc: Respect FMODE

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Message-ID: <CANq1E4T1P-g+wA6bZQe6=79-+QLmXReu4namcPifw-SZViqW-Q@mail.gmail.com>
Date:	Tue, 22 Apr 2014 14:44:50 +0200
From:	David Herrmann <dh.herrmann@...il.com>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Pavel Machek <pavel@....cz>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	"Theodore Ts'o" <tytso@....edu>
Subject: Re: [RFC 2/2] fs,proc: Respect FMODE_WRITE when opening /proc/pid/fd/N

Hi

On Mon, Apr 21, 2014 at 6:22 PM, Andy Lutomirski <luto@...capital.net> wrote:
> This patch does this:

I can see _what_ the patch does, but your patch lacks any discussion
_why_ it is needed. Can you provide at least one real example where
this fixes a security issue?

> This may break userspace.  If so, I would guess that anything broken
> by it is either an actual exploit or is so broken that it doesn't
> deserve to continue working.  If it breaks something important, then
> maybe it will need a sysctl.

This patch breaks the following use-case:

fd = open("/run", O_RDWR | O_TMPFILE);
sprintf(path, "/proc/self/fd/%d", fd);
fd2 = open(buf, O_RDONLY);
sprintf(path, "/proc/self/fd/%d", fd2);
linkat(AT_FDCWD, path, AT_FDCWD, "/run/some_lock_file", AT_FOLLOW_SYMLINK);

I mean I explicitly create the object as _writable_ but then keep a
read-only descriptor for debugging purposes (to make sure that the
program no longer writes to the file). This is no security feature,
but only a safety feature in case something goes wrong. But I still
want to be able to create hard-links (I _do_ have write-permissions on
the object/inode).

Thanks
David
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