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

[Jarkko Sakkinen]

On Sun, 2024-06-09 at 03:43 -0700, Jonathan Calmels wrote:
> This patch adds a new capability security bit designed to constrain a


nit: if you think of it "This patch adds" could be just "add", right? :-)

Also name the exact thing/symbol/whatever here. This is not a HBO series.

> 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
> 
> 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_L
> OCKED))
>  
> +/* 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_LOC
> KED))
> +
>  #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) | \
> +				

spurious new lines in the diff

please as first priority aim for absolute minimal diff or at least
do grow diff proactively like this.

If we really think after that, that we need some "extras" to the
patch set, then we decide that. These only take energy away from
reviewers.


> 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.
> */
> +	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;

Going for 4 week holiday starting for next week so focus in on nits
but since this is something to do access control:

1. Please go surgical with the diff's because this type of patches
also require a surgical review. Now reviewing this like riding on 
a bumpy road with a car of which suspension mechanics is broken
;-)

Hope you grab my argument here. I only want to look at the problem
and solution for that not random stuff..

BR, Jarkko