Re: [PATCH 04/11] fs: Don't remove suid for CAP_FSETID for userns root

[Serge E. Hallyn]

On Fri, Dec 22, 2017 at 03:32:28PM +0100, Dongsu Park wrote:
> From: Seth Forshee <seth.forshee@xxxxxxxxxxxxx>
> 
> Expand the check in should_remove_suid() to keep privileges for

I realize this description came from Seth, but reading it now,
'Expand' seems wrong.  Expanding a check brings to my mind making
it stricter, not looser.  How about 'Relax the check' ?

> CAP_FSETID in s_user_ns rather than init_user_ns.
> 
> Patch v4 is available: https://patchwork.kernel.org/patch/8944621/
> 
> --EWB Changed from ns_capable(sb->s_user_ns, ) to capable_wrt_inode_uidgid

Why exactly?

This is wrong, because capable_wrt_inode_uidgid() does a check
against current_user_ns, not the  inode->i_sb->s_user_ns

> 
> Cc: linux-fsdevel@xxxxxxxxxxxxxxx
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> Cc: Alexander Viro <viro@xxxxxxxxxxxxxxxxxx>
> Cc: Serge Hallyn <serge@xxxxxxxxxx>
> Signed-off-by: Seth Forshee <seth.forshee@xxxxxxxxxxxxx>
> Signed-off-by: Dongsu Park <dongsu@xxxxxxxxxx>
> ---
>  fs/inode.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/inode.c b/fs/inode.c
> index fd401028..6459a437 100644
> --- a/fs/inode.c
> +++ b/fs/inode.c
> @@ -1749,7 +1749,8 @@ EXPORT_SYMBOL(touch_atime);
>   */
>  int should_remove_suid(struct dentry *dentry)
>  {
> -	umode_t mode = d_inode(dentry)->i_mode;
> +	struct inode *inode = d_inode(dentry);
> +	umode_t mode = inode->i_mode;
>  	int kill = 0;
>  
>  	/* suid always must be killed */
> @@ -1763,7 +1764,8 @@ int should_remove_suid(struct dentry *dentry)
>  	if (unlikely((mode & S_ISGID) && (mode & S_IXGRP)))
>  		kill |= ATTR_KILL_SGID;
>  
> -	if (unlikely(kill && !capable(CAP_FSETID) && S_ISREG(mode)))
> +	if (unlikely(kill && !capable_wrt_inode_uidgid(inode, CAP_FSETID) &&
> +		     S_ISREG(mode)))
>  		return kill;
>  
>  	return 0;
> -- 
> 2.13.6