Re: [RFC PATCH] vfs: don't check may_create_in_sticky if the file is already open/created

[Jeff Layton]

On Wed, 2022-07-27 at 13:34 +0200, Christian Brauner wrote:
> On Tue, Jul 26, 2022 at 04:27:56PM -0400, Jeff Layton wrote:
> > On Tue, 2022-07-26 at 16:23 -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. Only call may_create_in_sticky
> > > if the file hasn't already been opened or created.
> > > 
> > > Cc: Christian Brauner <brauner@xxxxxxxxxx>
> > > Link: https://bugzilla.redhat.com/show_bug.cgi?id=1976829
> > > Reported-by: Yongchen Yang <yoyang@xxxxxxxxxx>
> > > Signed-off-by: Jeff Layton <jlayton@xxxxxxxxxx>
> > > ---
> > >  fs/namei.c | 13 +++++++++----
> > >  1 file changed, 9 insertions(+), 4 deletions(-)
> > > 
> > > diff --git a/fs/namei.c b/fs/namei.c
> > > index 1f28d3f463c3..7480b6dc8d27 100644
> > > --- a/fs/namei.c
> > > +++ b/fs/namei.c
> > > @@ -3495,10 +3495,15 @@ static int do_open(struct nameidata *nd,
> > >  			return -EEXIST;
> > >  		if (d_is_dir(nd->path.dentry))
> > >  			return -EISDIR;
> > > -		error = may_create_in_sticky(mnt_userns, nd,
> > > -					     d_backing_inode(nd->path.dentry));
> > > -		if (unlikely(error))
> > > -			return error;
> > > +		if (!(file->f_mode & (FMODE_OPENED | FMODE_CREATED))) {
> > > +			error = may_create_in_sticky(mnt_userns, nd,
> > > +						d_backing_inode(nd->path.dentry));
> > > +			if (unlikely(error)) {
> > > +				printk("%s: f_mode=0x%x oflag=0x%x\n",
> > > +					__func__, file->f_mode, open_flag);
> > > +				return error;
> > > +			}
> > > +		}
> > >  	}
> > >  	if ((nd->flags & LOOKUP_DIRECTORY) && !d_can_lookup(nd->path.dentry))
> > >  		return -ENOTDIR;
> > 
> > I'm pretty sure this patch is the wrong approach, actually, since it
> > doesn't fix the regular (non-atomic) open codepath. Any thoughts on what
> 
> Hey Jeff,
> 
> I haven't quite understood why that won't work for the regular open
> codepaths. I'm probably missing something obvious.
> 

In the normal open codepaths, FMODE_OPENED | FMODE_CREATED are still
clear. If we're not doing an atomic_open (i.e. the dentry doesn't exist
yet or is negative), then nothing really happens until you get to the
vfs_open call.

> > the right fix might be?
> 
> When an actual creation has taken place - and not just a lookup - then
> may_create_in_sticky() assumes that the owner of the inode matches
> current_fsuid(). That'd would also be problematic on fat or in fact on
> any fs where the actual inode->i_{g,u}id are based on e.g. uid/gid mount
> options and not on current_fsuid(), I think?
> 
> So in order to improve this we would need to work around that assumption
> in some way. Either by skipping may_create_in_sticky() if the file got
> created or by adapting may_create_in_sticky().
> 
> I only wonder whether skipping may_create_in_sticky() altogether might
> be a bit too lax. One possibility that came to my mind might be to relax
> this assumption when the file has been created and the creator has
> CAP_FOWNER.
> 

That may be the best option. I'll tinker around with that and see if I
can get it to work. Thanks for the suggestion.

> So (not compile tested or in any way) sm like:
> 
> diff --git a/fs/namei.c b/fs/namei.c
> index 1f28d3f463c3..239e9f423346 100644
> --- a/fs/namei.c
> +++ b/fs/namei.c
> @@ -1221,7 +1221,8 @@ int may_linkat(struct user_namespace *mnt_userns, struct path *link)
>   * Returns 0 if the open is allowed, -ve on error.
>   */
>  static int may_create_in_sticky(struct user_namespace *mnt_userns,
> -                               struct nameidata *nd, struct inode *const inode)
> +                               struct nameidata *nd, struct inode *const inode,
> +                               bool created)
>  {
>         umode_t dir_mode = nd->dir_mode;
>         kuid_t dir_uid = nd->dir_uid;
> @@ -1230,7 +1231,9 @@ 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)) ||
> +           (created &&
> +            capable_wrt_inode_uidgid(mnt_userns, inode, CAP_FOWNER)))
>                 return 0;
> 
>         if (likely(dir_mode & 0002) ||
> @@ -3496,7 +3499,8 @@ static int do_open(struct nameidata *nd,
>                 if (d_is_dir(nd->path.dentry))
>                         return -EISDIR;
>                 error = may_create_in_sticky(mnt_userns, nd,
> -                                            d_backing_inode(nd->path.dentry));
> +                                            d_backing_inode(nd->path.dentry),
> +                                            (file->f_mode & FMODE_CREATED));
>                 if (unlikely(error))
>                         return error;
>         }

I think that still won't fix it in the normal open codepath.
FMODE_CREATED won't be set, so this will just end up failing again.

-- 
Jeff Layton <jlayton@xxxxxxxxxx>