Re: [PATCH] vfs: bypass may_create_in_sticky check if task has CAP_FOWNER

[Christian Brauner]

On Wed, Jul 27, 2022 at 08:55:35AM -0400, Jeff Layton wrote:
> On Wed, 2022-07-27 at 14:37 +0200, Christian Brauner wrote:
> > On Wed, Jul 27, 2022 at 08:30:48AM -0400, Jeff Layton wrote:
> > > NFS server is exporting a sticky directory (mode 01777) with root
> > > squashing enabled. Client has protect_regular enabled and then tries to
> > > open a file as root in that directory. File is created (with ownership
> > > set to nobody:nobody) but the open syscall returns an error.
> > > 
> > > The problem is may_create_in_sticky, which rejects the open even though
> > > the file has already been created/opened. Bypass the checks in
> > > may_create_in_sticky if the task has CAP_FOWNER in the given namespace.
> > > 
> > > Link: https://bugzilla.redhat.com/show_bug.cgi?id=1976829
> > > Reported-by: Yongchen Yang <yoyang@xxxxxxxxxx>
> > > Suggested-by: Christian Brauner <brauner@xxxxxxxxxx>
> > > Signed-off-by: Jeff Layton <jlayton@xxxxxxxxxx>
> > > ---
> > >  fs/namei.c | 3 ++-
> > >  1 file changed, 2 insertions(+), 1 deletion(-)
> > > 
> > > diff --git a/fs/namei.c b/fs/namei.c
> > > index 1f28d3f463c3..170c2396ba29 100644
> > > --- a/fs/namei.c
> > > +++ b/fs/namei.c
> > > @@ -1230,7 +1230,8 @@ static int may_create_in_sticky(struct user_namespace *mnt_userns,
> > >  	    (!sysctl_protected_regular && S_ISREG(inode->i_mode)) ||
> > >  	    likely(!(dir_mode & S_ISVTX)) ||
> > >  	    uid_eq(i_uid_into_mnt(mnt_userns, inode), dir_uid) ||
> > > -	    uid_eq(current_fsuid(), i_uid_into_mnt(mnt_userns, inode)))
> > > +	    uid_eq(current_fsuid(), i_uid_into_mnt(mnt_userns, inode)) ||
> > > +	    ns_capable(mnt_userns, CAP_FOWNER))
> > >  		return 0;
> > 
> > Hm, no. You really want inode_owner_or_capable() here..
> > You need to verify that you have a mapping for the inode->i_{g,u}id in
> > question and that you're having CAP_FOWNER in the caller's userns.
> > 
> 
> Ok, I should be able to make that change and test it out.
> 
> > I'm pretty sure we should also restrict this to the case were the caller
> > actually created the file otherwise we introduce a potential issue where
> > the caller is susceptible to data spoofing. For example, the file was
> > created by another user racing the caller's O_CREAT.
> 
> That won't be sufficient to fix the testcase, I think. If a file already
> exists in the sticky dir and is owned by nobody:nobody, do we really
> want to prevent root from opening it? I wouldn't think so.

Afaict, the whole stick behind the protected_regular thing in
may_create_in_sticky() thing is that you prevent scenarios where you can
be tricked into opening a file that you didn't intend to with O_CREAT.

That's specifically also a protection for root. So say root specifies
O_CREAT but someone beats root to it and creates the file dumping
malicious data in there. The uid_eq() requirement is supposed to prevent
such attacks and it's a sysctl that userspace opted into.

We'd be relaxing that restriction quite a bit if we not just allow newly
created but also pre-existing file to be opened even with the CAP_FOWNER
requirement.

So the dd call should really fail if O_CREAT is passed but the file is
pre-existing, imho. It's a different story if dd created that file and
has CAP_FOWNER imho.