Re: [RFC 2/2] fs,proc: Respect FMODE_WRITE when opening /proc/pid/fd/N

[Pavel Machek <pavel@ucw.cz>]

Hi!

> On Mon, Apr 21, 2014 at 6:22 PM, Andy Lutomirski <luto@amacapital.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?

Such as here?

http://www.securityfocus.com/archive/1/507386

> > 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);

You meant open(path, ) here?

> 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).

Does some real code do it? I believe this deserves to be broken -- you
explicitely opened that read-only...

     	  							Pavel
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