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Subject[PATCH v2 1/4] Add PR_{GET,SET}_NO_NEW_PRIVS to prevent execve from granting privs
Date
With this set, a lot of dangerous operations (chroot, unshare, etc)
become a lot less dangerous because there is no possibility of
subverting privileged binaries.

This patch completely breaks apparmor. Someone who understands (and
uses) apparmor should fix it or at least give me a hint.

Signed-off-by: Andy Lutomirski <luto@amacapital.net>
---
fs/exec.c | 10 +++++++++-
include/linux/prctl.h | 15 +++++++++++++++
include/linux/sched.h | 2 ++
include/linux/security.h | 1 +
kernel/fork.c | 2 ++
kernel/sys.c | 10 ++++++++++
security/apparmor/domain.c | 4 ++++
security/commoncap.c | 7 +++++--
security/selinux/hooks.c | 10 +++++++++-
9 files changed, 57 insertions(+), 4 deletions(-)
diff --git a/fs/exec.c b/fs/exec.c
index 25dcbe5..ca6a966 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1242,6 +1242,13 @@ int check_unsafe_exec(struct linux_binprm *bprm)
bprm->unsafe |= LSM_UNSAFE_PTRACE;
}

+ /*
+ * This isn't strictly necessary, but it makes it harder for LSMs to
+ * mess up.
+ */
+ if (current->no_new_privs)
+ bprm->unsafe |= LSM_UNSAFE_NO_NEW_PRIVS;
+
n_fs = 1;
spin_lock(&p->fs->lock);
rcu_read_lock();
@@ -1285,7 +1292,8 @@ int prepare_binprm(struct linux_binprm *bprm)
bprm->cred->euid = current_euid();
bprm->cred->egid = current_egid();

- if (!(bprm->file->f_path.mnt->mnt_flags & MNT_NOSUID)) {
+ if (!(bprm->file->f_path.mnt->mnt_flags & MNT_NOSUID) &&
+ !current->no_new_privs) {
/* Set-uid? */
if (mode & S_ISUID) {
bprm->per_clear |= PER_CLEAR_ON_SETID;
diff --git a/include/linux/prctl.h b/include/linux/prctl.h
index a3baeb2..7bafc98 100644
--- a/include/linux/prctl.h
+++ b/include/linux/prctl.h
@@ -102,4 +102,19 @@

#define PR_MCE_KILL_GET 34

+/*
+ * If no_new_privs is set, then operations that grant new privileges (i.e.
+ * execve) will either fail or not grant them. This affects suid/sgid,
+ * file capabilities, and LSMs.
+ *
+ * Operations that merely manipulate or drop existing privileges (setresuid,
+ * capset, etc.) will still work. Drop those privileges if you want them gone.
+ *
+ * Changing LSM security domain is considered a new privilege. So, for example,
+ * asking selinux for a specific new context (e.g. with runcon) will result
+ * in execve returning -EPERM.
+ */
+#define PR_SET_NO_NEW_PRIVS 35
+#define PR_GET_NO_NEW_PRIVS 36
+
#endif /* _LINUX_PRCTL_H */
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 41d0237..02b342c 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1300,6 +1300,8 @@ struct task_struct {
* execve */
unsigned in_iowait:1;

+ /* task may not gain privileges */
+ unsigned no_new_privs:1;

/* Revert to default priority/policy when forking */
unsigned sched_reset_on_fork:1;
diff --git a/include/linux/security.h b/include/linux/security.h
index ebd2a53..48294e0 100644
--- a/include/linux/security.h
+++ b/include/linux/security.h
@@ -129,6 +129,7 @@ struct request_sock;
#define LSM_UNSAFE_SHARE 1
#define LSM_UNSAFE_PTRACE 2
#define LSM_UNSAFE_PTRACE_CAP 4
+#define LSM_UNSAFE_NO_NEW_PRIVS 8

#ifdef CONFIG_MMU
/*
diff --git a/kernel/fork.c b/kernel/fork.c
index 8e6b6f4..97cd34a 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1117,6 +1117,8 @@ static struct task_struct *copy_process(unsigned long clone_flags,
if (retval < 0)
goto bad_fork_free;

+ p->no_new_privs = current->no_new_privs;
+
/*
* If multiple threads are within copy_process(), then this check
* triggers too late. This doesn't hurt, the check is only there
diff --git a/kernel/sys.c b/kernel/sys.c
index 1dbbe69..219b3dc 100644
--- a/kernel/sys.c
+++ b/kernel/sys.c
@@ -1837,6 +1837,16 @@ SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3,
else
error = PR_MCE_KILL_DEFAULT;
break;
+ case PR_SET_NO_NEW_PRIVS:
+ if (arg2 != 1 || arg3 || arg4 || arg5)
+ return -EINVAL;
+
+ current->no_new_privs = 1;
+ break;
+ case PR_GET_NO_NEW_PRIVS:
+ if (arg2 || arg3 || arg4 || arg5)
+ return -EINVAL;
+ return current->no_new_privs ? 1 : 0;
default:
error = -EINVAL;
break;
diff --git a/security/apparmor/domain.c b/security/apparmor/domain.c
index c1e18ba..a24ee44 100644
--- a/security/apparmor/domain.c
+++ b/security/apparmor/domain.c
@@ -360,6 +360,10 @@ int apparmor_bprm_set_creds(struct linux_binprm *bprm)
if (bprm->cred_prepared)
return 0;

+ /* XXX: no_new_privs is not useable with AppArmor yet */
+ if (bprm->unsafe & LSM_UNSAFE_NO_NEW_PRIVS)
+ return -EPERM;
+
cxt = bprm->cred->security;
BUG_ON(!cxt);

diff --git a/security/commoncap.c b/security/commoncap.c
index a93b3b7..b0e0f7b 100644
--- a/security/commoncap.c
+++ b/security/commoncap.c
@@ -511,14 +511,17 @@ int cap_bprm_set_creds(struct linux_binprm *bprm)
skip:

/* Don't let someone trace a set[ug]id/setpcap binary with the revised
- * credentials unless they have the appropriate permit
+ * credentials unless they have the appropriate permit.
+ *
+ * In addition, if NO_NEW_PRIVS, then ensure we get no new privs.
*/
if ((new->euid != old->uid ||
new->egid != old->gid ||
!cap_issubset(new->cap_permitted, old->cap_permitted)) &&
bprm->unsafe & ~LSM_UNSAFE_PTRACE_CAP) {
/* downgrade; they get no more than they had, and maybe less */
- if (!capable(CAP_SETUID)) {
+ if (!capable(CAP_SETUID) ||
+ (bprm->unsafe & LSM_UNSAFE_NO_NEW_PRIVS)) {
new->euid = new->uid;
new->egid = new->gid;
}
diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
index 266a229..43eea9b 100644
--- a/security/selinux/hooks.c
+++ b/security/selinux/hooks.c
@@ -2003,6 +2003,13 @@ static int selinux_bprm_set_creds(struct linux_binprm *bprm)
new_tsec->sid = old_tsec->exec_sid;
/* Reset exec SID on execve. */
new_tsec->exec_sid = 0;
+
+ /*
+ * Minimize confusion: if no_new_privs and a transition is
+ * explicitly requested, then fail the exec.
+ */
+ if (bprm->unsafe & LSM_UNSAFE_NO_NEW_PRIVS)
+ return -EPERM;
} else {
/* Check for a default transition on this program. */
rc = security_transition_sid(old_tsec->sid, isec->sid,
@@ -2015,7 +2022,8 @@ static int selinux_bprm_set_creds(struct linux_binprm *bprm)
COMMON_AUDIT_DATA_INIT(&ad, PATH);
ad.u.path = bprm->file->f_path;

- if (bprm->file->f_path.mnt->mnt_flags & MNT_NOSUID)
+ if ((bprm->file->f_path.mnt->mnt_flags & MNT_NOSUID) ||
+ (bprm->unsafe & LSM_UNSAFE_NO_NEW_PRIVS))
new_tsec->sid = old_tsec->sid;

if (new_tsec->sid == old_tsec->sid) {
--
1.7.7.5


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