Re: [PATCH 01/14] fs_parse: add uid & gid option option parsing helpers

[Jan Kara <jack@suse.cz>]

On Fri 28-06-24 14:23:35, Christian Brauner wrote:
> On Fri, Jun 28, 2024 at 11:45:17AM GMT, Jan Kara wrote:
> > On Thu 27-06-24 19:26:24, Eric Sandeen wrote:
> > > Multiple filesystems take uid and gid as options, and the code to
> > > create the ID from an integer and validate it is standard boilerplate
> > > that can be moved into common helper functions, so do that for
> > > consistency and less cut&paste.
> > > 
> > > This also helps avoid the buggy pattern noted by Seth Jenkins at
> > > https://lore.kernel.org/lkml/CALxfFW4BXhEwxR0Q5LSkg-8Vb4r2MONKCcUCVioehXQKr35eHg@mail.gmail.com/
> > > because uid/gid parsing will fail before any assignment in most
> > > filesystems.
> > > 
> > > Signed-off-by: Eric Sandeen <sandeen@sandeen.net>
> > 
> > I like the idea since this seems like a nobrainer but is actually
> > surprisingly subtle...
> > 
> > > diff --git a/fs/fs_parser.c b/fs/fs_parser.c
> > > index a4d6ca0b8971..24727ec34e5a 100644
> > > --- a/fs/fs_parser.c
> > > +++ b/fs/fs_parser.c
> > > @@ -308,6 +308,40 @@ int fs_param_is_fd(struct p_log *log, const struct fs_parameter_spec *p,
> > >  }
> > >  EXPORT_SYMBOL(fs_param_is_fd);
> > >  
> > > +int fs_param_is_uid(struct p_log *log, const struct fs_parameter_spec *p,
> > > +		    struct fs_parameter *param, struct fs_parse_result *result)
> > > +{
> > > +	kuid_t uid;
> > > +
> > > +	if (fs_param_is_u32(log, p, param, result) != 0)
> > > +		return fs_param_bad_value(log, param);
> > > +
> > > +	uid = make_kuid(current_user_ns(), result->uint_32);
> > 
> > But here is the problem: Filesystems mountable in user namespaces need to use
> > fc->user_ns for resolving uids / gids (e.g. like fuse_parse_param()).
> > Having helpers that work for some filesystems and are subtly broken for
> > others is worse than no helpers... Or am I missing something?
> > 
> > And the problem with fc->user_ns is that currently __fs_parse() does not
> > get fs_context as an argument... So that will need some larger work.
> 
> Not really. If someone does an fsopen() in a namespace but the process
> that actually sets mount options is in another namespace then it's
> completely intransparent what uid/gid this will resolve to if it's
> resovled according to fsopen().
> 
> It's also a bit strange if someone ends up handing off a tmpfs fscontext
> that was created in the initial namespace to some random namespace and
> they now can set uid/gid options that aren't mapped according to their
> namespace but instead are 1:1 resolved according to the intial
> namespace. So this would hinder delegation.
> 
> The expectation is that uid/gid options are resolved in the caller's
> namespace and that shouldn't be any different for fscontexts for
> namespace mountable filesystems. The crucial point is to ensure that the
> resulting kuid/kgid can be resolved in the namespace the filesystem is
> mounted in at the end. That's what was lacking in e.g., tmpfs in commit
> 0200679fc795 ("tmpfs: verify {g,u}id mount options correctly")
> 
> The fuse conversion is the only inconsistency in that regard.

OK, thanks for explanation!

								Honza
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR