Re: [PATCH v2 2/2] capabilities: Add a securebit to disable PR_CAP_AMBIENT_RAISE

[Serge E. Hallyn]

On Thu, May 14, 2015 at 11:39:49PM -0700, Andy Lutomirski wrote:
> Per Andrew Morgan's request, add a securebit to allow admins to
> disable PR_CAP_AMBIENT_RAISE.  This securebit will prevent processes
> from adding capabilities to their ambient set.
> 
> For simplicity, this disables PR_CAP_AMBIENT_RAISE entirely rather
> than just disabling setting previously cleared bits.
> 
> Requested-by: Andrew G. Morgan <morgan@xxxxxxxxxx>
> Cc: Kees Cook <keescook@xxxxxxxxxxxx>
> Cc: Christoph Lameter <cl@xxxxxxxxx>
> Cc: Serge Hallyn <serge.hallyn@xxxxxxxxxxxxx>
> Cc: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
> Cc: Jonathan Corbet <corbet@xxxxxxx>
> Cc: Aaron Jones <aaronmdjones@xxxxxxxxx>
> CC: Ted Ts'o <tytso@xxxxxxx>
> Cc: linux-security-module@xxxxxxxxxxxxxxx
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> Cc: linux-api@xxxxxxxxxxxxxxx
> Cc: akpm@xxxxxxxxxxxxxxxxxxx
> Cc: Andrew G. Morgan <morgan@xxxxxxxxxx>
> Cc: Mimi Zohar <zohar@xxxxxxxxxxxxxxxxxx>
> Cc: Austin S Hemmelgarn <ahferroin7@xxxxxxxxx>
> Cc: Markku Savela <msa@xxxxxxxxxxx>
> Cc: Jarkko Sakkinen <jarkko.sakkinen@xxxxxxxxxxxxxxx>
> Cc: Michael Kerrisk <mtk.manpages@xxxxxxxxx>
> Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxx>

Interesting - at first I thought this wouldn't please Andrew as he'd
want the inverse, but I guess this way keeps the pure-capabilities
mode (with SECURE_ALL_BITS set) ambient-free which I suppose is
really the important thing.

Acked-by: Serge Hallyn <serge.hallyn@xxxxxxxxxxxxx>

> ---
>  include/uapi/linux/securebits.h | 11 ++++++++++-
>  security/commoncap.c            |  3 ++-
>  2 files changed, 12 insertions(+), 2 deletions(-)
> 
> diff --git a/include/uapi/linux/securebits.h b/include/uapi/linux/securebits.h
> index 985aac9e6bf8..35ac35cef217 100644
> --- a/include/uapi/linux/securebits.h
> +++ b/include/uapi/linux/securebits.h
> @@ -43,9 +43,18 @@
>  #define SECBIT_KEEP_CAPS	(issecure_mask(SECURE_KEEP_CAPS))
>  #define SECBIT_KEEP_CAPS_LOCKED (issecure_mask(SECURE_KEEP_CAPS_LOCKED))
>  
> +/* When set, a process cannot add new capabilities to its ambient set. */
> +#define SECURE_NO_CAP_AMBIENT_RAISE		6
> +#define SECURE_NO_CAP_AMBIENT_RAISE_LOCKED	7  /* make bit-6 immutable */
> +
> +#define SECBIT_NO_CAP_AMBIENT_RAISE (issecure_mask(SECURE_NO_CAP_AMBIENT_RAISE))
> +#define SECBIT_NO_CAP_AMBIENT_RAISE_LOCKED \
> +			(issecure_mask(SECURE_NO_CAP_AMBIENT_RAISE_LOCKED))
> +
>  #define SECURE_ALL_BITS		(issecure_mask(SECURE_NOROOT) | \
>  				 issecure_mask(SECURE_NO_SETUID_FIXUP) | \
> -				 issecure_mask(SECURE_KEEP_CAPS))
> +				 issecure_mask(SECURE_KEEP_CAPS) | \
> +				 issecure_mask(SECURE_NO_CAP_AMBIENT_RAISE))
>  #define SECURE_ALL_LOCKS	(SECURE_ALL_BITS << 1)
>  
>  #endif /* _UAPI_LINUX_SECUREBITS_H */
> diff --git a/security/commoncap.c b/security/commoncap.c
> index 09541a6a85a0..98aa1b129c12 100644
> --- a/security/commoncap.c
> +++ b/security/commoncap.c
> @@ -983,7 +983,8 @@ int cap_task_prctl(int option, unsigned long arg2, unsigned long arg3,
>  			if (arg2 == PR_CAP_AMBIENT_RAISE &&
>  			    (!cap_raised(current_cred()->cap_permitted, arg3) ||
>  			     !cap_raised(current_cred()->cap_inheritable,
> -					 arg3)))
> +					 arg3) ||
> +			     issecure(SECURE_NO_CAP_AMBIENT_RAISE)))
>  				return -EPERM;
>  
>  			new = prepare_creds();
> -- 
> 2.1.0
> 
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