Re: [PATCH] ovl: fix uid/gid when creating over whiteout

[Vivek Goyal <vgoyal@redhat.com>]

On Wed, Jun 15, 2016 at 03:30:02PM +0200, Miklos Szeredi wrote:
> Hi Vivek,
> 
> I've tested this to fix the regresion that Stephen reported.  I think this
> also is a good base for the selinux fix.
> 
> Pushed to overlayfs-linus and overlayfs-next branches of
> 
>   git://git.kernel.org/pub/scm/linux/kernel/git/mszeredi/vfs.git
> 
> Please let me know if you see any problem with this.
> 
> Thanks,
> Miklos
> 
> ---
> From: Miklos Szeredi <mszeredi@redhat.com>
> Subject: ovl: fix uid/gid when creating over whiteout
> 
> Fix a regression when creating a file over a whiteout.  The new
> file/directory needs to use the current fsuid/fsgid, not the ones from the
> mounter's credentials.
> 
> The refcounting is a bit tricky: prepare_creds() sets an original refcount,
> override_creds() gets one more, which revert_cred() drops.  So
> 
>   1) we need to expicitly put the mounter's credentials when overriding
>      with the updated one
> 
>   2) we need to put the original ref to the updated creds (and this can
>      safely be done before revert_creds(), since we'll still have the ref
>      from override_creds()).
> 
> Reported-by: Stephen Smalley <sds@tycho.nsa.gov>
> Fixes: 3fe6e52f0626 ("ovl: override creds with the ones from the superblock mounter")
> Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
> ---
>  fs/overlayfs/dir.c |   13 +++++++++++--
>  1 file changed, 11 insertions(+), 2 deletions(-)
> 
> --- a/fs/overlayfs/dir.c
> +++ b/fs/overlayfs/dir.c
> @@ -405,12 +405,21 @@ static int ovl_create_or_link(struct den
>  		err = ovl_create_upper(dentry, inode, &stat, link, hardlink);
>  	} else {
>  		const struct cred *old_cred;
> +		struct cred *override_cred;
>  
>  		old_cred = ovl_override_creds(dentry->d_sb);
>  
> -		err = ovl_create_over_whiteout(dentry, inode, &stat, link,
> -					       hardlink);
> +		err = -ENOMEM;
> +		override_cred = prepare_creds();
> +		if (override_cred) {
> +			override_cred->fsuid = old_cred->fsuid;
> +			override_cred->fsgid = old_cred->fsgid;

Hi Miklos,

I am wondering if we are switching to tasks's ->fsuid and ->fsgid too
early. ovl_create_over_whiteout() calls ovl_lookup_temp(workdir) and
IIUC, task might not have permission to do lookup in workdir.

Should we switch to this override_cred, just before ovl_create_real()
so that task ->fsuid and ->fsgid are used only for creation purposes
only.

Thanks
Vivek

> +			put_cred(override_creds(override_cred));
> +			put_cred(override_cred);
>  
> +			err = ovl_create_over_whiteout(dentry, inode, &stat,
> +						       link, hardlink);
> +		}
>  		revert_creds(old_cred);
>  	}
>