Re: [PATCH 3/4] capability: Create a new capability CAP_SIGNED

Casey Schaufler
2013-03-15T21:12:53Z

On 3/15/2013 1:35 PM, Vivek Goyal wrote:
> Create a new capability CAP_SIGNED which can be given to signed executables.

This would drive anyone who is trying to use
capabilities as the privilege mechanism it is
intended to be absolutely crazy.

Capabilities aren't just random attribute bits. They
indicate that a task has permission to violate a
system policy (e.g. change the mode bits of a file
the user doesn't own). Think about how this will
interact with programs using file based capabilities.

>
> Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
> ---
> include/uapi/linux/capability.h | 12 +++++++++++-
> kernel/cred.c | 7 +++++++
> security/commoncap.c | 2 ++
> 3 files changed, 20 insertions(+), 1 deletions(-)
>
> diff --git a/include/uapi/linux/capability.h b/include/uapi/linux/capability.h
> index ba478fa..1bbe671 100644
> --- a/include/uapi/linux/capability.h
> +++ b/include/uapi/linux/capability.h
> @@ -343,7 +343,17 @@ struct vfs_cap_data {
>
> #define CAP_BLOCK_SUSPEND 36
>
> -#define CAP_LAST_CAP CAP_BLOCK_SUSPEND
> +/*
> + * Allow certain kernel services with secureboot enabled. One of such
> + * service is sys_kexec() which can be invoked by process only if it
> + * has CAP_SIGNED capability (with secureboot enabled).
> + *
> + * This capability is given by kernel automatically if executable
> + * file is validly signed.
> + */
> +#define CAP_SIGNED 37
> +
> +#define CAP_LAST_CAP CAP_SIGNED
>
> #define cap_valid(x) ((x) >= 0 && (x) <= CAP_LAST_CAP)
>
> diff --git a/kernel/cred.c b/kernel/cred.c
> index e0573a4..f554d1b 100644
> --- a/kernel/cred.c
> +++ b/kernel/cred.c
> @@ -555,6 +555,12 @@ void revert_creds(const struct cred *old)
> }
> EXPORT_SYMBOL(revert_creds);
>
> +static void remove_cap_signed_init_cred(void)
> +{
> + cap_lower((&init_cred)->cap_bset, CAP_SIGNED);
> + cap_lower((&init_cred)->cap_permitted, CAP_SIGNED);
> +}
> +
> /*
> * initialise the credentials stuff
> */
> @@ -563,6 +569,7 @@ void __init cred_init(void)
> /* allocate a slab in which we can store credentials */
> cred_jar = kmem_cache_create("cred_jar", sizeof(struct cred),
> 0, SLAB_HWCACHE_ALIGN|SLAB_PANIC, NULL);
> + remove_cap_signed_init_cred();
> }
>
> /**
> diff --git a/security/commoncap.c b/security/commoncap.c
> index c44b6fe..4190eb9 100644
> --- a/security/commoncap.c
> +++ b/security/commoncap.c
> @@ -272,6 +272,8 @@ int cap_capset(struct cred *new,
> new->cap_effective = *effective;
> new->cap_inheritable = *inheritable;
> new->cap_permitted = *permitted;
> + if (cap_raised(old->cap_effective, CAP_SIGNED))
> + cap_raise(new->cap_effective, CAP_SIGNED);
> return 0;
> }
>