Re: [CFT][PATCH 4/7] userns: Check euid no fsuid when establishing an unprivileged uid mapping

[Andy Lutomirski]

On Mon, Dec 8, 2014 at 2:08 PM, Eric W. Biederman <ebiederm@xxxxxxxxxxxx> wrote:
>
> setresuid allows the euid to be set to any of uid, euid, suid, and
> fsuid.  Therefor it is safe to allow an unprivileged user to map their
> euid and use CAP_SETUID privileged with exactly that uid, as no new
> credentials can be obtained.
>
> I can not find a combination of existing system calls that allows
> setting uid, euid, suid, and fsuid from the fsuid making the previous
> use of fsuid for allowing unprivileged mappings a bug.

Right.

>
> This is part of a fix for CVE-2014-8989.

Reviewed-by: Andy Lutomirski <luto@xxxxxxxxxxxxxx>

>
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
> ---
>  kernel/user_namespace.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/kernel/user_namespace.c b/kernel/user_namespace.c
> index 8e7c87162171..da1eeb927b21 100644
> --- a/kernel/user_namespace.c
> +++ b/kernel/user_namespace.c
> @@ -819,7 +819,7 @@ static bool new_idmap_permitted(const struct file *file,
>                 u32 id = new_map->extent[0].lower_first;
>                 if (cap_setid == CAP_SETUID) {
>                         kuid_t uid = make_kuid(ns->parent, id);
> -                       if (uid_eq(uid, file->f_cred->fsuid))
> +                       if (uid_eq(uid, file->f_cred->euid))
>                                 return true;
>                 }
>         }
> --
> 1.9.1
>



-- 
Andy Lutomirski
AMA Capital Management, LLC
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