[Lucid][CVE-2015-3339] fs: take i_mutex during prepare_binprm for set[ug]id executables

[Seth Forshee]

On Mon, Apr 27, 2015 at 01:43:58PM +0100, Luis Henriques wrote:
> From: Jann Horn <jann at thejh.net>
> 
> This prevents a race between chown() and execve(), where chowning a
> setuid-user binary to root would momentarily make the binary setuid
> root.
> 
> This patch was mostly written by Linus Torvalds.
> 
> Signed-off-by: Jann Horn <jann at thejh.net>
> Signed-off-by: Linus Torvalds <torvalds at linux-foundation.org>
> (backported from commit 8b01fc86b9f425899f8a3a8fc1c47d73c2c20543)
> [ luis: backport to Lucid:
>   - replaced kuid_t/kgid_t by uid_t/gid_t
>   - replaced READ_ONCE() by ACCESS_ONCE()
>   - replaced task_no_new_privs() by current->no_new_privs

<snip>

> +static void bprm_fill_uid(struct linux_binprm *bprm)
> +{
> +	struct inode *inode;
> +	unsigned int mode;
> +	uid_t uid;
> +	gid_t gid;
> +
> +	/* clear any previous set[ug]id data from a previous binary */
> +	bprm->cred->euid = current_euid();
> +	bprm->cred->egid = current_egid();
> +
> +	if (bprm->file->f_path.mnt->mnt_flags & MNT_NOSUID)
> +		return;

The current->no_new_privs bit is missing here, not sure if it's missing
or if the commit log is wrong and that part isn't relevant for lucid.

Otherwise the backport looks correct to me.