linux-kernel - Re: [PATCH] Introduce v3 namespaced file capabilities

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Date:   Thu, 27 Apr 2017 11:52:45 -0500
From:   "Serge E. Hallyn" <serge@...lyn.com>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     "Serge E. Hallyn" <serge@...lyn.com>,
        Seth Forshee <seth.forshee@...onical.com>,
        lkml <linux-kernel@...r.kernel.org>, linux-api@...r.kernel.org,
        linux-security-module@...r.kernel.org,
        Kees Cook <keescook@...omium.org>,
        Andreas Gruenbacher <agruenba@...hat.com>,
        Andy Lutomirski <luto@...nel.org>,
        "Andrew G. Morgan" <morgan@...nel.org>
Subject: Re: [PATCH] Introduce v3 namespaced file capabilities

Quoting Eric W. Biederman (ebiederm@...ssion.com):
> ebiederm@...ssion.com (Eric W. Biederman) writes:
> 
> > "Serge E. Hallyn" <serge@...lyn.com> writes:
> >
> >> Quoting Eric W. Biederman (ebiederm@...ssion.com):
> >>> 
> >>> "Serge E. Hallyn" <serge@...lyn.com> writes:
> >>> 
> >>> > diff --git a/fs/xattr.c b/fs/xattr.c
> >>> > index 7e3317c..75cc65a 100644
> >>> > --- a/fs/xattr.c
> >>> > +++ b/fs/xattr.c
> >>> > @@ -170,12 +170,29 @@ int __vfs_setxattr_noperm(struct dentry *dentry, const char *name,
> >>> >  		const void *value, size_t size, int flags)
> >>> >  {
> >>> >  	struct inode *inode = dentry->d_inode;
> >>> > -	int error = -EAGAIN;
> >>> > +	int error;
> >>> > +	void *wvalue = NULL;
> >>> > +	size_t wsize = 0;
> >>> >  	int issec = !strncmp(name, XATTR_SECURITY_PREFIX,
> >>> >  				   XATTR_SECURITY_PREFIX_LEN);
> >>> >  
> >>> > -	if (issec)
> >>> > +	if (issec) {
> >>> >  		inode->i_flags &= ~S_NOSEC;
> >>> > +
> >>> > +		if (!strcmp(name, "security.capability")) {
> >>> > +			error = cap_setxattr_convert_nscap(dentry, value, size,
> >>> > +					&wvalue, &wsize);
> >>> > +			if (error < 0)
> >>> > +				return error;
> >>> > +			if (wvalue) {
> >>> > +				value = wvalue;
> >>> > +				size = wsize;
> >>> > +			}
> >>> > +		}
> >>> > +	}
> >>> > +
> >>> > +	error = -EAGAIN;
> >>> > +
> >>> 
> >>> Why is the conversion in __vfs_setxattr_noperm and not in setattr as
> >>> was done for posix_acl_fix_xattr_from_user?
> >>
> >> I think I was thinking I wanted to catch all the vfs_setxattr operations,
> >> but I don't think that's right.  Moving to setxattr seems right.  I'll
> >> look around a bit more.
> >
> > Thanks.  This is one of these little details that we want a good answer
> > to why there.  If you can document that in your patch description when
> > you resend I would appreciate it.
> 
> Ok. Grrr.
> 
> Looking at this a little more getting it correct where we call the
> conversion operation is critical. 
> 
> I believe the current placement of cap_setxattr_convert_nscap is
> actively wrong.  In particular unless I am misleading something this
> will trigger multiple conversions when setting one of these attributes
> on overlayfs.
> 
> The stragey I adopted for for posix acls is:
> 
>    On a write from userspace convert from current_user_ns() to &init_user_ns.
>    On a write to the filesystem convert from &init_user_ns to fs_user_ns.
>    
>    On a read from the filesystem convert from fs_user_ns to &init_user_ns
>    On a read from the kernel to userspace convert from &init_user_ns
>       to current_user_ns().
> 
> Overall a good strategy but no one we can trivially adopt for the
> capability xattr as the second write to filesystem method does not
> appear to actually exist for anything except for posix acls.
> 
> I need to think a little more about how we want to accomplish this for
> the capability xattr.  My apoligies for leading you down a path that has
> all of these bumps and then being sufficiently distracted not to help
> you through this maze.
> 
> The only easy solution I can see is to just always keep things in
> &init_user_ns inside the kernel.   That works until we bring fuse or
> other unprivileged mounts onboard that have storage outside of the
> kernel.
> 
> Seth and I will have to rework that for fuse support but that sounds
> better than not letting such an issue prevent us from merging the code.

Ok, in the meantime I've made a few updates in my tree which I think
make the code a lot nicer (and do move the conversion to setxattr()),
but there's a bug in that which I'm still trying to nail down.  I'll
send a new version when I get that figured, and we can see how close
to ok that is.

Note that upstream cap_inode_removexattr and cap_inode_setxattr()
upstream still don't respect the fs_user_ns properly either (the
proper code is in the Ubuntu kernel, maybe it's in your -next
tree, I don't know how you and Seth are coordinating that)

-serge