[PATCH v2 2/4] capabilities: Add securebit to restrict userns caps

[Serge E. Hallyn]

On Sun, Jun 09, 2024 at 03:43:35AM -0700, Jonathan Calmels wrote:
> This patch adds a new capability security bit designed to constrain a
> task’s userns capability set to its bounding set. The reason for this is
> twofold:
> 
> - This serves as a quick and easy way to lock down a set of capabilities
>   for a task, thus ensuring that any namespace it creates will never be
>   more privileged than itself is.
> - This helps userspace transition to more secure defaults by not requiring
>   specific logic for the userns capability set, or libcap support.
> 
> Example:
> 
>     # capsh --secbits=$((1 << 8)) --drop=cap_sys_rawio -- \
>             -c 'unshare -r grep Cap /proc/self/status'
>     CapInh: 0000000000000000
>     CapPrm: 000001fffffdffff
>     CapEff: 000001fffffdffff
>     CapBnd: 000001fffffdffff
>     CapAmb: 0000000000000000
>     CapUNs: 000001fffffdffff

But you are not (that I can see, in this or the previous patch)
keeping SECURE_USERNS_STRICT_CAPS in securebits on the next
level unshare.  Though I think it's ok, because by then both
cap_userns and cap_bset are reduced and cap_userns can't be
expanded.  (Sorry, just thinking aloud here)

> Signed-off-by: Jonathan Calmels <jcalmels at 3xx0.net>
> ---
>  include/linux/securebits.h      |  1 +
>  include/uapi/linux/securebits.h | 11 ++++++++++-
>  kernel/user_namespace.c         |  5 +++++
>  3 files changed, 16 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/securebits.h b/include/linux/securebits.h
> index 656528673983..5f9d85cd69c3 100644
> --- a/include/linux/securebits.h
> +++ b/include/linux/securebits.h
> @@ -5,4 +5,5 @@
>  #include <uapi/linux/securebits.h>
>  
>  #define issecure(X)		(issecure_mask(X) & current_cred_xxx(securebits))
> +#define iscredsecure(cred, X)	(issecure_mask(X) & cred->securebits)
>  #endif /* !_LINUX_SECUREBITS_H */
> diff --git a/include/uapi/linux/securebits.h b/include/uapi/linux/securebits.h
> index d6d98877ff1a..2da3f4be4531 100644
> --- a/include/uapi/linux/securebits.h
> +++ b/include/uapi/linux/securebits.h
> @@ -52,10 +52,19 @@
>  #define SECBIT_NO_CAP_AMBIENT_RAISE_LOCKED \
>  			(issecure_mask(SECURE_NO_CAP_AMBIENT_RAISE_LOCKED))
>  
> +/* When set, user namespace capabilities are restricted to their parent's bounding set. */
> +#define SECURE_USERNS_STRICT_CAPS			8
> +#define SECURE_USERNS_STRICT_CAPS_LOCKED		9  /* make bit-8 immutable */
> +
> +#define SECBIT_USERNS_STRICT_CAPS (issecure_mask(SECURE_USERNS_STRICT_CAPS))
> +#define SECBIT_USERNS_STRICT_CAPS_LOCKED \
> +			(issecure_mask(SECURE_USERNS_STRICT_CAPS_LOCKED))
> +
>  #define SECURE_ALL_BITS		(issecure_mask(SECURE_NOROOT) | \
>  				 issecure_mask(SECURE_NO_SETUID_FIXUP) | \
>  				 issecure_mask(SECURE_KEEP_CAPS) | \
> -				 issecure_mask(SECURE_NO_CAP_AMBIENT_RAISE))
> +				 issecure_mask(SECURE_NO_CAP_AMBIENT_RAISE) | \
> +				 issecure_mask(SECURE_USERNS_STRICT_CAPS))
>  #define SECURE_ALL_LOCKS	(SECURE_ALL_BITS << 1)
>  
>  #endif /* _UAPI_LINUX_SECUREBITS_H */
> diff --git a/kernel/user_namespace.c b/kernel/user_namespace.c
> index 7e624607330b..53848e2b68cd 100644
> --- a/kernel/user_namespace.c
> +++ b/kernel/user_namespace.c
> @@ -10,6 +10,7 @@
>  #include <linux/cred.h>
>  #include <linux/securebits.h>
>  #include <linux/security.h>
> +#include <linux/capability.h>
>  #include <linux/keyctl.h>
>  #include <linux/key-type.h>
>  #include <keys/user-type.h>
> @@ -42,6 +43,10 @@ static void dec_user_namespaces(struct ucounts *ucounts)
>  
>  static void set_cred_user_ns(struct cred *cred, struct user_namespace *user_ns)
>  {
> +	/* Limit userns capabilities to our parent's bounding set. */

In the case of userns_install(), it will be the target user namespace
creator's bounding set, right?  Not "our parent's"?

> +	if (iscredsecure(cred, SECURE_USERNS_STRICT_CAPS))
> +		cred->cap_userns = cap_intersect(cred->cap_userns, cred->cap_bset);
> +
>  	/* Start with the capabilities defined in the userns set. */
>  	cred->cap_bset = cred->cap_userns;
>  	cred->cap_permitted = cred->cap_userns;
> -- 
> 2.45.2