Re: [PATCH v9 01/10] namei: obey trailing magic-link DAC permissions

[Aleksa Sarai <cyphar@cyphar.com>]

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On 2019-07-12, Al Viro <viro@zeniv.linux.org.uk> wrote:
> On Fri, Jul 12, 2019 at 10:20:17PM +1000, Aleksa Sarai wrote:
> > On 2019-07-12, Al Viro <viro@zeniv.linux.org.uk> wrote:
> > > On Sun, Jul 07, 2019 at 12:57:28AM +1000, Aleksa Sarai wrote:
> > > > @@ -514,7 +516,14 @@ static void set_nameidata(struct nameidata *p, int dfd, struct filename *name)
> > > >  	p->stack = p->internal;
> > > >  	p->dfd = dfd;
> > > >  	p->name = name;
> > > > -	p->total_link_count = old ? old->total_link_count : 0;
> > > > +	p->total_link_count = 0;
> > > > +	p->acc_mode = 0;
> > > > +	p->opath_mask = FMODE_PATH_READ | FMODE_PATH_WRITE;
> > > > +	if (old) {
> > > > +		p->total_link_count = old->total_link_count;
> > > > +		p->acc_mode = old->acc_mode;
> > > > +		p->opath_mask = old->opath_mask;
> > > > +	}
> > > 
> > > Huh?  Could somebody explain why traversals of NFS4 referrals should inherit
> > > ->acc_mode and ->opath_mask?
> > 
> > I'll be honest -- I don't understand what set_nameidata() did so I just
> > did what I thought would be an obvious change (to just copy the
> > contents). I thought it was related to some aspect of the symlink stack
> > handling.
> 
> No.  It's handling of (very rare) nested pathwalk.  The only case I can think
> of is handling of NFS4 referrals - they are triggered by ->d_automount()
> and include NFS4 mount.  Which does internal pathwalk of its own, to get
> to the root of subtree being automounted.
> 
> NFS has its own recursion protection on that path (no deeper nesting than
> one level of referral traversals), but there some nesting is inevitable;
> we do get another nameidata instance on stack.  And for nd_jump_link() we
> need to keep track of the innermost one.
> 
> For symlinks nothing of that sort happens - they are dealt with on the same
> struct nameidata.  ->total_link_count copying is there for one reason only -
> we want the total amount of symlinks traversed during the pathwalk (including
> the referral processing, etc.) to count towards MAXSYMLINKS check.  It could've
> been moved from nameidata to task_struct, but it's cheaper to handle it that
> way.
> 
> Again, nesting is *rare*.

Thanks for the explanation, much appreciated. I will drop the old->...
copying hunk.

> > In that case, should they both be set to 0 on set_nameidata()? This will
> > mean that fd re-opening (or magic-link opening) through a
> > set_nameidata() would always fail.
> 
> Huh?  set_nameidata() is done for *all* instances - it's pretty much the
> constructor of that object (and restore_nameidata() - a destructor).
> Everything goes through it.

Sorry, I meant to drop the copy-from-old logic -- not set it to zero
explicitly in set_nameidata().

> And again, I'm not sure we want these fields in nameidata - IMO they belong
> in open_flags.  Things like e.g. stat() don't need them at all.

Yup, I'll work up a version that does the consolidation you mentioned
in your other mail.

> Incidentally, O_PATH opening of symlinks combined with subsequent procfs
> symlink traversals is worth testing - that's where the things get subtle
> and that's where it's easy to get in trouble on modifications.

I have some self-tests of a symlink-to-a-magic-link in the last patch of
the series. Did you mean something even more chained like a symlink to a
/proc/self/fd/$n of an O_NOFOLLOW|O_PATH of a symlink?

-- 
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>

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