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Subject[PATCHv2 1/2] capability: introduce sysctl for controlled user-ns capability whitelist
Date
From: Mahesh Bandewar <maheshb@google.com>

Add a sysctl variable kernel.controlled_userns_caps_whitelist. This
takes input as capability mask expressed as two comma separated hex
u32 words. The mask, however, is stored in kernel as kernel_cap_t type.

Any capabilities that are not part of this mask will be controlled and
will not be allowed to processes in controlled user-ns.

Signed-off-by: Mahesh Bandewar <maheshb@google.com>
---
v2:
Rebase
v1:
Initial submission

Documentation/sysctl/kernel.txt | 21 ++++++++++++++++++
include/linux/capability.h | 3 +++
kernel/capability.c | 47 +++++++++++++++++++++++++++++++++++++++++
kernel/sysctl.c | 5 +++++
4 files changed, 76 insertions(+)

diff --git a/Documentation/sysctl/kernel.txt b/Documentation/sysctl/kernel.txt
index 694968c7523c..a1d39dbae847 100644
--- a/Documentation/sysctl/kernel.txt
+++ b/Documentation/sysctl/kernel.txt
@@ -25,6 +25,7 @@ show up in /proc/sys/kernel:
- bootloader_version [ X86 only ]
- callhome [ S390 only ]
- cap_last_cap
+- controlled_userns_caps_whitelist
- core_pattern
- core_pipe_limit
- core_uses_pid
@@ -187,6 +188,26 @@ CAP_LAST_CAP from the kernel.

==============================================================

+controlled_userns_caps_whitelist
+
+Capability mask that is whitelisted for "controlled" user namespaces.
+Any capability that is missing from this mask will not be allowed to
+any process that is attached to a controlled-userns. e.g. if CAP_NET_RAW
+is not part of this mask, then processes running inside any controlled
+userns's will not be allowed to perform action that needs CAP_NET_RAW
+capability. However, processes that are attached to a parent user-ns
+hierarchy that is *not* controlled and has CAP_NET_RAW can continue
+performing those actions. User-namespaces are marked "controlled" at
+the time of their creation based on the capabilities of the creator.
+A process that does not have CAP_SYS_ADMIN will create user-namespaces
+that are controlled.
+
+The value is expressed as two comma separated hex words (u32). This
+sysctl is avaialble in init-ns and users with CAP_SYS_ADMIN in init-ns
+are allowed to make changes.
+
+==============================================================
+
core_pattern:

core_pattern is used to specify a core dumpfile pattern name.
diff --git a/include/linux/capability.h b/include/linux/capability.h
index f640dcbc880c..7d79a4689625 100644
--- a/include/linux/capability.h
+++ b/include/linux/capability.h
@@ -14,6 +14,7 @@
#define _LINUX_CAPABILITY_H

#include <uapi/linux/capability.h>
+#include <linux/sysctl.h>


#define _KERNEL_CAPABILITY_VERSION _LINUX_CAPABILITY_VERSION_3
@@ -248,6 +249,8 @@ extern bool ptracer_capable(struct task_struct *tsk, struct user_namespace *ns);

/* audit system wants to get cap info from files as well */
extern int get_vfs_caps_from_disk(const struct dentry *dentry, struct cpu_vfs_cap_data *cpu_caps);
+int proc_douserns_caps_whitelist(struct ctl_table *table, int write,
+ void __user *buff, size_t *lenp, loff_t *ppos);

extern int cap_convert_nscap(struct dentry *dentry, void **ivalue, size_t size);

diff --git a/kernel/capability.c b/kernel/capability.c
index 1e1c0236f55b..4a859b7d4902 100644
--- a/kernel/capability.c
+++ b/kernel/capability.c
@@ -29,6 +29,8 @@ EXPORT_SYMBOL(__cap_empty_set);

int file_caps_enabled = 1;

+kernel_cap_t controlled_userns_caps_whitelist = CAP_FULL_SET;
+
static int __init file_caps_disable(char *str)
{
file_caps_enabled = 0;
@@ -507,3 +509,48 @@ bool ptracer_capable(struct task_struct *tsk, struct user_namespace *ns)
rcu_read_unlock();
return (ret == 0);
}
+
+/* Controlled-userns capabilities routines */
+#ifdef CONFIG_SYSCTL
+int proc_douserns_caps_whitelist(struct ctl_table *table, int write,
+ void __user *buff, size_t *lenp, loff_t *ppos)
+{
+ DECLARE_BITMAP(caps_bitmap, CAP_LAST_CAP);
+ struct ctl_table caps_table;
+ char tbuf[NAME_MAX];
+ int ret;
+
+ ret = bitmap_from_u32array(caps_bitmap, CAP_LAST_CAP,
+ controlled_userns_caps_whitelist.cap,
+ _KERNEL_CAPABILITY_U32S);
+ if (ret != CAP_LAST_CAP)
+ return -1;
+
+ scnprintf(tbuf, NAME_MAX, "%*pb", CAP_LAST_CAP, caps_bitmap);
+
+ caps_table.data = tbuf;
+ caps_table.maxlen = NAME_MAX;
+ caps_table.mode = table->mode;
+ ret = proc_dostring(&caps_table, write, buff, lenp, ppos);
+ if (ret)
+ return ret;
+ if (write) {
+ kernel_cap_t tmp;
+
+ if (!capable(CAP_SYS_ADMIN))
+ return -EPERM;
+
+ ret = bitmap_parse_user(buff, *lenp, caps_bitmap, CAP_LAST_CAP);
+ if (ret)
+ return ret;
+
+ ret = bitmap_to_u32array(tmp.cap, _KERNEL_CAPABILITY_U32S,
+ caps_bitmap, CAP_LAST_CAP);
+ if (ret != CAP_LAST_CAP)
+ return -1;
+
+ controlled_userns_caps_whitelist = tmp;
+ }
+ return 0;
+}
+#endif /* CONFIG_SYSCTL */
diff --git a/kernel/sysctl.c b/kernel/sysctl.c
index d9c31bc2eaea..25c3f7b76ece 100644
--- a/kernel/sysctl.c
+++ b/kernel/sysctl.c
@@ -1226,6 +1226,11 @@ static struct ctl_table kern_table[] = {
.extra2 = &one,
},
#endif
+ {
+ .procname = "controlled_userns_caps_whitelist",
+ .mode = 0644,
+ .proc_handler = proc_douserns_caps_whitelist,
+ },
{ }
};

--
2.15.0.448.gf294e3d99a-goog
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