[PATCH 2/3] capabilities: add securebit for strict userns caps

[Jonathan Calmels <jcalmels@xxxxxxxx>]

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@xxxxxxxx>
---
 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;
-- 
2.45.0