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

Al Viro
2019-07-12T04:14:54Z

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?

> static __always_inline
> -const char *get_link(struct nameidata *nd)
> +const char *get_link(struct nameidata *nd, bool trailing)
> {
> struct saved *last = nd->stack + nd->depth - 1;
> struct dentry *dentry = last->link.dentry;
> @@ -1081,6 +1134,44 @@ const char *get_link(struct nameidata *nd)
> } else {
> res = get(dentry, inode, &last->done);
> }
> + /* If we just jumped it was because of a magic-link. */
> + if (unlikely(nd->flags & LOOKUP_JUMPED)) {

That's not quite guaranteed (it is possible to bind a symlink on top
of a regular file, and you will get LOOKUP_JUMPED on the entry into
trailing_symlink() when looking the result up). Moreover, why bother
with LOOKUP_JUMPED here? See that
nd->last_type = LAST_BIND;
several lines prior? That's precisely to be able to recognize those
suckers.

And _that_ would've avoided another piece of ugliness - your LOOKUP_JUMPED
kludge forces you to handle that cra^Wsclero^Wvaluable security hardening
in get_link(), instead of trailing_symlink() where you apparently want
it to be. Simply because nd_jump_root() done later in get_link() will set
LOOKUP_JUMPED for absolute symlinks, confusing your test.

Moreover, I'm not sure that trailing_symlink() is the right place for
that either - I would be rather tempted to fold do_o_path() into
path_openat(), inline path_lookupat() there (as in
s = path_init(nd, flags);

while (!(error = link_path_walk(s, nd))
&& ((error = lookup_last(nd)) > 0)) {
s = trailing_symlink(nd);
}
if (!error)
error = complete_walk(nd);
if (!error && nd->flags & LOOKUP_DIRECTORY)
if (!d_can_lookup(nd->path.dentry))
error = -ENOTDIR;
if (!error) {
audit_inode(nd->name, nd->path.dentry, 0);
error = vfs_open(&nd->path, file);
}
terminate_walk(nd);
- we don't need LOOKUP_DOWN there) and then we only care about the
two callers of trailing_symlink() that are in path_openat(). Which
is where you have your ->acc_mode and ->opath_mask without the need
to dump them into nameidata. Or to bring that mess into the
things like stat(2) et.al. - it simply doesn't belong there.

In any case, this "bool trailing" is completely wrong; whether that
check belongs in trailing_symlink() or (some of) its callers, putting
it into get_link() is a mistake, forced by kludgy check for procfs-style
symlinks.