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
Signed-off-by: Jonathan Calmels jcalmels@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. */ + 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;