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

[Andy Lutomirski]

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>
---
 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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