[PATCH v2] kernel: Conditionally support non-root users, groups and capabilities

From: Iulia Manda
Date: Thu Jan 29 2015 - 13:43:43 EST

There are a lot of embedded systems that run most or all of their functionality
in init, running as root:root. For these systems, supporting multiple users is
not necessary.

This patch adds a new symbol, CONFIG_NON_ROOT, that makes support for non-root
users, non-root groups, and capabilities optional.

When this symbol is not defined, UID and GID are zero in any possible case
and processes always have all capabilities.

The following syscalls are compiled out: setuid, setregid, setgid,
setreuid, setresuid, getresuid, setresgid, getresgid, setgroups, getgroups,
setfsuid, setfsgid, capget, capset.

Also, groups.c is compiled out completely.

This change saves about 25 KB on a defconfig build.

The kernel was booted in Qemu. All the common functionalities work. Adding
users/groups is not possible, failing with -ENOSYS.

Bloat-o-meter output:
add/remove: 7/87 grow/shrink: 19/397 up/down: 1675/-26325 (-24650)

Signed-off-by: Iulia Manda <iulia.manda21@xxxxxxxxx>
Reviewed-by: Josh Triplett <josh@xxxxxxxxxxxxxxxx>
---
Changes since v1:
- refactor code;
- compile out groups.c;
- if groups_alloc is called, enable NON_ROOT;

arch/s390/Kconfig | 1 +
drivers/staging/lustre/lustre/Kconfig | 1 +
fs/nfsd/Kconfig | 1 +
include/linux/capability.h | 29 +++++++++++++++++++++++++++
include/linux/cred.h | 23 ++++++++++++++++++----
include/linux/uidgid.h | 12 +++++++++++
init/Kconfig | 19 +++++++++++++++++-
kernel/Makefile | 4 +++-
kernel/capability.c | 35 ++++++++++++++++++---------------
kernel/cred.c | 3 +++
kernel/groups.c | 3 ---
kernel/sys.c | 2 ++
kernel/sys_ni.c | 14 +++++++++++++
net/sunrpc/Kconfig | 2 ++
14 files changed, 124 insertions(+), 25 deletions(-)

diff --git a/arch/s390/Kconfig b/arch/s390/Kconfig
index 68b68d7..b2d2116 100644
--- a/arch/s390/Kconfig
+++ b/arch/s390/Kconfig
@@ -324,6 +324,7 @@ config COMPAT
select COMPAT_BINFMT_ELF if BINFMT_ELF
select ARCH_WANT_OLD_COMPAT_IPC
select COMPAT_OLD_SIGACTION
+ select NON_ROOT
help
Select this option if you want to enable your system kernel to
handle system-calls from ELF binaries for 31 bit ESA. This option
diff --git a/drivers/staging/lustre/lustre/Kconfig b/drivers/staging/lustre/lustre/Kconfig
index 6725467..b975f62 100644
--- a/drivers/staging/lustre/lustre/Kconfig
+++ b/drivers/staging/lustre/lustre/Kconfig
@@ -10,6 +10,7 @@ config LUSTRE_FS
select CRYPTO_SHA1
select CRYPTO_SHA256
select CRYPTO_SHA512
+ select NON_ROOT
help
This option enables Lustre file system client support. Choose Y
here if you want to access a Lustre file system cluster. To compile
diff --git a/fs/nfsd/Kconfig b/fs/nfsd/Kconfig
index 7339515..1a8d6d9 100644
--- a/fs/nfsd/Kconfig
+++ b/fs/nfsd/Kconfig
@@ -6,6 +6,7 @@ config NFSD
select SUNRPC
select EXPORTFS
select NFS_ACL_SUPPORT if NFSD_V2_ACL
+ select NON_ROOT
help
Choose Y here if you want to allow other computers to access
files residing on this system using Sun's Network File System
diff --git a/include/linux/capability.h b/include/linux/capability.h
index aa93e5e..601c5de 100644
--- a/include/linux/capability.h
+++ b/include/linux/capability.h
@@ -205,6 +205,7 @@ static inline kernel_cap_t cap_raise_nfsd_set(const kernel_cap_t a,
cap_intersect(permitted, __cap_nfsd_set));
}

+#ifdef CONFIG_NON_ROOT
extern bool has_capability(struct task_struct *t, int cap);
extern bool has_ns_capability(struct task_struct *t,
struct user_namespace *ns, int cap);
@@ -213,6 +214,34 @@ extern bool has_ns_capability_noaudit(struct task_struct *t,
struct user_namespace *ns, int cap);
extern bool capable(int cap);
extern bool ns_capable(struct user_namespace *ns, int cap);
+#else
+static inline bool has_capability(struct task_struct *t, int cap)
+{
+ return true;
+}
+static inline bool has_ns_capability(struct task_struct *t,
+ struct user_namespace *ns, int cap)
+{
+ return true;
+}
+static inline bool has_capability_noaudit(struct task_struct *t, int cap)
+{
+ return true;
+}
+static inline bool has_ns_capability_noaudit(struct task_struct *t,
+ struct user_namespace *ns, int cap)
+{
+ return true;
+}
+static inline bool capable(int cap)
+{
+ return true;
+}
+static inline bool ns_capable(struct user_namespace *ns, int cap)
+{
+ return true;
+}
+#endif /* CONFIG_NON_ROOT */
extern bool capable_wrt_inode_uidgid(const struct inode *inode, int cap);
extern bool file_ns_capable(const struct file *file, struct user_namespace *ns, int cap);

diff --git a/include/linux/cred.h b/include/linux/cred.h
index 2fb2ca2..08ea5c6 100644
--- a/include/linux/cred.h
+++ b/include/linux/cred.h
@@ -62,9 +62,27 @@ do { \
groups_free(group_info); \
} while (0)

-extern struct group_info *groups_alloc(int);
extern struct group_info init_groups;
+#ifdef CONFIG_NON_ROOT
+extern struct group_info *groups_alloc(int);
extern void groups_free(struct group_info *);
+
+extern int in_group_p(kgid_t);
+extern int in_egroup_p(kgid_t);
+#else
+static inline void groups_free(struct group_info *group_info)
+{
+}
+
+static inline int in_group_p(kgid_t grp)
+{
+ return 1;
+}
+static inline int in_egroup_p(kgid_t grp)
+{
+ return 1;
+}
+#endif
extern int set_current_groups(struct group_info *);
extern void set_groups(struct cred *, struct group_info *);
extern int groups_search(const struct group_info *, kgid_t);
@@ -74,9 +92,6 @@ extern bool may_setgroups(void);
#define GROUP_AT(gi, i) \
((gi)->blocks[(i) / NGROUPS_PER_BLOCK][(i) % NGROUPS_PER_BLOCK])

-extern int in_group_p(kgid_t);
-extern int in_egroup_p(kgid_t);
-
/*
* The security context of a task
*
diff --git a/include/linux/uidgid.h b/include/linux/uidgid.h
index 2d1f9b6..22bd1fa 100644
--- a/include/linux/uidgid.h
+++ b/include/linux/uidgid.h
@@ -29,6 +29,7 @@ typedef struct {
#define KUIDT_INIT(value) (kuid_t){ value }
#define KGIDT_INIT(value) (kgid_t){ value }

+#ifdef CONFIG_NON_ROOT
static inline uid_t __kuid_val(kuid_t uid)
{
return uid.val;
@@ -38,6 +39,17 @@ static inline gid_t __kgid_val(kgid_t gid)
{
return gid.val;
}
+#else
+static inline uid_t __kuid_val(kuid_t uid)
+{
+ return 0;
+}
+
+static inline gid_t __kgid_val(kgid_t gid)
+{
+ return 0;
+}
+#endif

#define GLOBAL_ROOT_UID KUIDT_INIT(0)
#define GLOBAL_ROOT_GID KGIDT_INIT(0)
diff --git a/init/Kconfig b/init/Kconfig
index 9afb971..dc5bfd4 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -394,6 +394,7 @@ endchoice

config BSD_PROCESS_ACCT
bool "BSD Process Accounting"
+ select NON_ROOT
help
If you say Y here, a user level program will be able to instruct the
kernel (via a special system call) to write process accounting
@@ -420,6 +421,7 @@ config BSD_PROCESS_ACCT_V3
config TASKSTATS
bool "Export task/process statistics through netlink"
depends on NET
+ select NON_ROOT
default n
help
Export selected statistics for tasks/processes through the
@@ -1140,6 +1142,7 @@ config CHECKPOINT_RESTORE

menuconfig NAMESPACES
bool "Namespaces support" if EXPERT
+ depends on NON_ROOT
default !EXPERT
help
Provides the way to make tasks work with different objects using
@@ -1352,11 +1355,25 @@ menuconfig EXPERT

config UID16
bool "Enable 16-bit UID system calls" if EXPERT
- depends on HAVE_UID16
+ depends on HAVE_UID16 && NON_ROOT
default y
help
This enables the legacy 16-bit UID syscall wrappers.

+config NON_ROOT
+ bool "Multiple users, groups and capabilities support" if EXPERT
+ default y
+ help
+ This option enables support for non-root users, groups and
+ capabilities.
+
+ If you say N here, all processes will run with UID 0, GID 0, and all
+ possible capabilities. Saying N here also compiles out support for
+ system calls related to UIDs, GIDs, and capabilities, such as setuid,
+ setgid, and capset.
+
+ If unsure, say Y here.
+
config SGETMASK_SYSCALL
bool "sgetmask/ssetmask syscalls support" if EXPERT
def_bool PARISC || MN10300 || BLACKFIN || M68K || PPC || MIPS || X86 || SPARC || CRIS || MICROBLAZE || SUPERH
diff --git a/kernel/Makefile b/kernel/Makefile
index a59481a..d5ca6b8 100644
--- a/kernel/Makefile
+++ b/kernel/Makefile
@@ -9,7 +9,9 @@ obj-y = fork.o exec_domain.o panic.o \
extable.o params.o \
kthread.o sys_ni.o nsproxy.o \
notifier.o ksysfs.o cred.o reboot.o \
- async.o range.o groups.o smpboot.o
+ async.o range.o smpboot.o
+
+obj-$(CONFIG_NON_ROOT) += groups.o

ifdef CONFIG_FUNCTION_TRACER
# Do not trace debug files and internal ftrace files
diff --git a/kernel/capability.c b/kernel/capability.c
index 989f5bf..2638412 100644
--- a/kernel/capability.c
+++ b/kernel/capability.c
@@ -35,6 +35,7 @@ static int __init file_caps_disable(char *str)
}
__setup("no_file_caps", file_caps_disable);

+#ifdef CONFIG_NON_ROOT
/*
* More recent versions of libcap are available from:
*
@@ -386,6 +387,24 @@ bool ns_capable(struct user_namespace *ns, int cap)
}
EXPORT_SYMBOL(ns_capable);

+
+/**
+ * capable - Determine if the current task has a superior capability in effect
+ * @cap: The capability to be tested for
+ *
+ * Return true if the current task has the given superior capability currently
+ * available for use, false if not.
+ *
+ * This sets PF_SUPERPRIV on the task if the capability is available on the
+ * assumption that it's about to be used.
+ */
+bool capable(int cap)
+{
+ return ns_capable(&init_user_ns, cap);
+}
+EXPORT_SYMBOL(capable);
+#endif /* CONFIG_NON_ROOT */
+
/**
* file_ns_capable - Determine if the file's opener had a capability in effect
* @file: The file we want to check
@@ -412,22 +431,6 @@ bool file_ns_capable(const struct file *file, struct user_namespace *ns,
EXPORT_SYMBOL(file_ns_capable);

/**
- * capable - Determine if the current task has a superior capability in effect
- * @cap: The capability to be tested for
- *
- * Return true if the current task has the given superior capability currently
- * available for use, false if not.
- *
- * This sets PF_SUPERPRIV on the task if the capability is available on the
- * assumption that it's about to be used.
- */
-bool capable(int cap)
-{
- return ns_capable(&init_user_ns, cap);
-}
-EXPORT_SYMBOL(capable);
-
-/**
* capable_wrt_inode_uidgid - Check nsown_capable and uid and gid mapped
* @inode: The inode in question
* @cap: The capability in question
diff --git a/kernel/cred.c b/kernel/cred.c
index e0573a4..ec1c076 100644
--- a/kernel/cred.c
+++ b/kernel/cred.c
@@ -29,6 +29,9 @@

static struct kmem_cache *cred_jar;

+/* init to 2 - one for init_task, one to ensure it is never freed */
+struct group_info init_groups = { .usage = ATOMIC_INIT(2) };
+
/*
* The initial credentials for the initial task
*/
diff --git a/kernel/groups.c b/kernel/groups.c
index 664411f..74d431d 100644
--- a/kernel/groups.c
+++ b/kernel/groups.c
@@ -9,9 +9,6 @@
#include <linux/user_namespace.h>
#include <asm/uaccess.h>

-/* init to 2 - one for init_task, one to ensure it is never freed */
-struct group_info init_groups = { .usage = ATOMIC_INIT(2) };
-
struct group_info *groups_alloc(int gidsetsize)
{
struct group_info *group_info;
diff --git a/kernel/sys.c b/kernel/sys.c
index a8c9f5a..bfe532b 100644
--- a/kernel/sys.c
+++ b/kernel/sys.c
@@ -319,6 +319,7 @@ out_unlock:
* SMP: There are not races, the GIDs are checked only by filesystem
* operations (as far as semantic preservation is concerned).
*/
+#ifdef CONFIG_NON_ROOT
SYSCALL_DEFINE2(setregid, gid_t, rgid, gid_t, egid)
{
struct user_namespace *ns = current_user_ns();
@@ -809,6 +810,7 @@ change_okay:
commit_creds(new);
return old_fsgid;
}
+#endif /* CONFIG_NON_ROOT */

/**
* sys_getpid - return the thread group id of the current process
diff --git a/kernel/sys_ni.c b/kernel/sys_ni.c
index 5adcb0a..7995ef5 100644
--- a/kernel/sys_ni.c
+++ b/kernel/sys_ni.c
@@ -159,6 +159,20 @@ cond_syscall(sys_uselib);
cond_syscall(sys_fadvise64);
cond_syscall(sys_fadvise64_64);
cond_syscall(sys_madvise);
+cond_syscall(sys_setuid);
+cond_syscall(sys_setregid);
+cond_syscall(sys_setgid);
+cond_syscall(sys_setreuid);
+cond_syscall(sys_setresuid);
+cond_syscall(sys_getresuid);
+cond_syscall(sys_setresgid);
+cond_syscall(sys_getresgid);
+cond_syscall(sys_setgroups);
+cond_syscall(sys_getgroups);
+cond_syscall(sys_setfsuid);
+cond_syscall(sys_setfsgid);
+cond_syscall(sys_capget);
+cond_syscall(sys_capset);

/* arch-specific weak syscall entries */
cond_syscall(sys_pciconfig_read);
diff --git a/net/sunrpc/Kconfig b/net/sunrpc/Kconfig
index fb78117..2b2c471 100644
--- a/net/sunrpc/Kconfig
+++ b/net/sunrpc/Kconfig
@@ -1,9 +1,11 @@
config SUNRPC
tristate
+ select NON_ROOT

config SUNRPC_GSS
tristate
select OID_REGISTRY
+ select NON_ROOT

config SUNRPC_BACKCHANNEL
bool
--
1.7.10.4

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