Re: [PATCH v2] vfs: Tighten up linkat(..., AT_EMPTY_PATH)

[Al Viro]

On Fri, Aug 23, 2013 at 02:07:26AM +0100, Al Viro wrote:
> On Thu, Aug 22, 2013 at 01:54:15PM -0700, Linus Torvalds wrote:
> > On Thu, Aug 22, 2013 at 1:48 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
> > >
> > > Sure.  But aren't they always last?
> > 
> > What do you mean? I'd say that the /proc lookup is always *innermost*.
> > Which means that it certainly cannot bail out, since there are many
> > levels of nesting outside of it.
> > 
> > > With the current code structure, trying to enforce some kind of
> > > security restriction in the middle of lookup seems really unpleasant.
> > 
> > If it's conditional (ie "linkat behaves differently from openat"), it
> > certainly means that we'd have to pass in that info in annoying ways.
> 
> Nope.  All we need to pass is one more LOOKUP_...  Add
> 	if (unlikely(nd->last_type == LAST_BIND)) {
> 		if ((nd->flags & LOOKUP_BLAH) && !may_flink(...)) {
> 			terminate_walk(nd);
> 			return -EINVAL;
> 		}
> 	}
> in the beginning of lookup_last() and pass LOOKUP_BLAH in flags when
> linkat() calls user_path_at().  That will affect *only* the terminal
> symlinks and cost nothing in all normal cases.  The same check can
> bloody well go into path_init() - take
>                 if (*name) {
>                         if (!can_lookup(dentry->d_inode)) {
>                                 fdput(f);
>                                 return -ENOTDIR;
>                         }
>                 }
> in there and slap
> 		else {
> 			if ((flags & LOOKUP_BLAH) && !may_flink(...)) {
>                                 fdput(f);
> 				return -EINVAL;
> 			}
> 		}
> after it.

OK, let me summarize these threads so far:
	* restrictions for flink() are needed and they'd better be
consistent for AT_SYMLINK_FOLLOW + /proc/<pid>/fd/<n> and simply
passing the descriptor as dfd.
	* CAP_DAC_OVERRIDE is sufficient; so should be O_TMPFILE used
to open that sucker.
	* lookup_last() is the natural place for catching the case
of following a trailing procfs symlink - it can be done very cheaply
there.

FWIW, I'm tempted to try the following trick:
	* introduce FMODE_FLINK in file->f_mode; O_TMPFILE would set it,
unless O_EXCL is present.
	* introduce LOOKUP_LINK, to be passed by sys_linkat() when
resolving the target.
	* have path_init() called with empty pathname and LOOKUP_LINK in
flags do checks for FMODE_FLINK or CAP_DAC_OVERRIDE
	* have ->proc_get_link() report whether the target is linkable
(either as bool * or by returning 1 instead of 0).  After the call of
->proc_get_link() check that and set nd->last_type to LAST_BIND_LINKABLE.
Note that *all* places looking at ->last_type treat LAST_BIND as "none
of the above" - we never compare with it, so splitting it in two wouldn't
break anything.
	* have lookup_last() check if LOOKUP_LINK is present and ->last_type
is LAST_BIND; fail unless we have CAP_DAC_OVERRIDE.

AFAICS, it gets more or less sane behaviour; additionally, it makes possible
to introduce explicit "I want that descriptor to be suitable for flink()"
open(2) flag - that would require teaching do_last() about LOOKUP_LINK,
making it check for CAP_DAC_OVERRIDE if it sees LAST_BIND / LOOKUP_LINK,
same as lookup_last() above (we obviously want to avoid the possibility
to take a non-flinkable descriptor and use it to reopen the sucker in
flinkable way).

Alternatively we can revert "fs: Allow unprivileged linkat(..., AT_EMPTY_PATH)
aka flink" for the time being.  flink() is certainly an awful mess and I
seriously regret touching it ;-/

Comments?  Hell, maybe somebody even has printable ones - stranger things
have happened...
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