[PATCH 1/2] file capabilities: add no_file_caps switch (v2)

[Serge Hallyn]

Add a no_file_caps boot option when file capabilities are
compiled into the kernel (CONFIG_SECURITY_FILE_CAPABILITIES=y).

This allows distributions to ship a kernel with file capabilities
compiled in, without forcing users to use (and understand and
trust) them.

When no_file_caps is specified at boot, then when a process executes
a file, any file capabilities stored with that file will not be
used in the calculation of the process' new capability sets.

This means that booting with the no_file_caps boot option will
not be the same as booting a kernel with file capabilities
compiled out - in particular a task with  CAP_SETPCAP will not
have any chance of passing capabilities to another task (which
isn't "really" possible anyway, and which may soon by killed
altogether by David Howells in any case), and it will instead
be able to put new capabilities in its pI.  However since fI
will always be empty and pI is masked with fI, it gains the
task nothing.

We also support the extra prctl options, setting securebits and
dropping capabilities from the per-process bounding set.

The other remaining difference is that killpriv, task_setscheduler,
setioprio, and setnice will continue to be hooked.  That will
be noticable in the case where a root task changed its uid
while keeping some caps, and another task owned by the new uid
tries to change settings for the more privileged task.

Signed-off-by: Serge Hallyn <serue@xxxxxxxxxx>
---
 include/linux/capability.h |    4 ++++
 kernel/capability.c        |   11 +++++++++++
 security/commoncap.c       |    9 +++++++++
 3 files changed, 24 insertions(+), 0 deletions(-)

diff --git a/include/linux/capability.h b/include/linux/capability.h
index 9d1fe30..c96c455 100644
--- a/include/linux/capability.h
+++ b/include/linux/capability.h
@@ -359,6 +359,10 @@ typedef struct kernel_cap_struct {
 
 #ifdef __KERNEL__
 
+#ifdef CONFIG_SECURITY_FILE_CAPABILITIES
+extern int file_caps_enabled;
+#endif
+
 /*
  * Internal kernel functions only
  */
diff --git a/kernel/capability.c b/kernel/capability.c
index 33e51e7..e13a685 100644
--- a/kernel/capability.c
+++ b/kernel/capability.c
@@ -33,6 +33,17 @@ EXPORT_SYMBOL(__cap_empty_set);
 EXPORT_SYMBOL(__cap_full_set);
 EXPORT_SYMBOL(__cap_init_eff_set);
 
+#ifdef CONFIG_SECURITY_FILE_CAPABILITIES
+int file_caps_enabled = 1;
+
+static int __init file_caps_disable(char *str)
+{
+	file_caps_enabled = 0;
+	return 1;
+}
+__setup("no_file_caps", file_caps_disable);
+#endif
+
 /*
  * More recent versions of libcap are available from:
  *
diff --git a/security/commoncap.c b/security/commoncap.c
index e4c4b3f..e33f632 100644
--- a/security/commoncap.c
+++ b/security/commoncap.c
@@ -27,6 +27,10 @@
 #include <linux/prctl.h>
 #include <linux/securebits.h>
 
+#ifndef CONFIG_SECURITY_FILE_CAPABILITIES
+static const int file_caps_enabled;
+#endif
+
 int cap_netlink_send(struct sock *sk, struct sk_buff *skb)
 {
 	NETLINK_CB(skb).eff_cap = current->cap_effective;
@@ -279,6 +283,11 @@ static int get_file_caps(struct linux_binprm *bprm)
 	struct vfs_cap_data vcaps;
 	struct inode *inode;
 
+	if (!file_caps_enabled) {
+		bprm_clear_caps(bprm);
+		return 0;
+	}
+
 	if (bprm->file->f_vfsmnt->mnt_flags & MNT_NOSUID) {
 		bprm_clear_caps(bprm);
 		return 0;
-- 
1.5.4.3

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