Subject: [RFC][PATCH v2] Unprivileged: Disable raising of privileges

From: [email-masked] (Eric W. Biederman)
Date: Wed, 30 Dec 2009 04:49:43 -0800

If we can know that a process will never raise
it's priveleges we can enable a lot of features
that otherwise would be unsafe, because they
could break assumptions of existing suid executables.

To allow this to be used as a sand boxing feature
also disable ptracing other executables without
this new restriction.

For the moment I have used a per thread flag because
we are out of per process flags.

To ensure all descendants get this flag I rely on
the default copying of procss structures.

Added bprm->nosuid to make remove the need to add
duplicate error prone checks.  This ensures that
the disabling of suid executables is exactly the
same as MNT_NOSUID.

Signed-off-by: Eric W. Biederman <[email-masked]>
---
 arch/x86/include/asm/thread_info.h |    2 ++
 fs/exec.c                          |    6 ++++--
 include/linux/binfmts.h            |    1 +
 include/linux/prctl.h              |    2 ++
 kernel/ptrace.c                    |    4 ++++
 kernel/sys.c                       |   16 ++++++++++++++++
 security/commoncap.c               |   14 +++++++++++++-
 security/selinux/hooks.c           |    2 +-
 8 files changed, 43 insertions(+), 4 deletions(-)

diff --git a/arch/x86/include/asm/thread_info.h b/arch/x86/include/asm/thread_info.h
index 375c917..e716203 100644
--- a/arch/x86/include/asm/thread_info.h
+++ b/arch/x86/include/asm/thread_info.h
@@ -82,6 +82,7 @@ struct thread_info {
 #define TIF_SYSCALL_EMU		6	/* syscall emulation active */
 #define TIF_SYSCALL_AUDIT	7	/* syscall auditing active */
 #define TIF_SECCOMP		8	/* secure computing */
+#define TIF_NOSUID		9	/* suid exec permanently disabled */
 #define TIF_MCE_NOTIFY		10	/* notify userspace of an MCE */
 #define TIF_USER_RETURN_NOTIFY	11	/* notify kernel of userspace return */
 #define TIF_NOTSC		16	/* TSC is not accessible in userland */
@@ -107,6 +108,7 @@ struct thread_info {
 #define _TIF_SYSCALL_EMU	(1 << TIF_SYSCALL_EMU)
 #define _TIF_SYSCALL_AUDIT	(1 << TIF_SYSCALL_AUDIT)
 #define _TIF_SECCOMP		(1 << TIF_SECCOMP)
+#define _TIF_NOSUID		(1 << TIF_NOSUID)
 #define _TIF_MCE_NOTIFY		(1 << TIF_MCE_NOTIFY)
 #define _TIF_USER_RETURN_NOTIFY	(1 << TIF_USER_RETURN_NOTIFY)
 #define _TIF_NOTSC		(1 << TIF_NOTSC)
diff --git a/fs/exec.c b/fs/exec.c
index 632b02e..5cba5ac 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1131,8 +1131,10 @@ int prepare_binprm(struct linux_binprm *bprm)
 	/* clear any previous set[ug]id data from a previous binary */
 	bprm->cred->euid = current_euid();
 	bprm->cred->egid = current_egid();
-
-	if (!(bprm->file->f_path.mnt->mnt_flags & MNT_NOSUID)) {
+	bprm->nosuid =
+		(bprm->file->f_path.mnt->mnt_flags & MNT_NOSUID) ||
+		test_tsk_thread_flag(current, TIF_NOSUID);
+	if (bprm->nosuid) {
 		/* Set-uid? */
 		if (mode & S_ISUID) {
 			bprm->per_clear |= PER_CLEAR_ON_SETID;
diff --git a/include/linux/binfmts.h b/include/linux/binfmts.h
index cd4349b..c3b5a30 100644
--- a/include/linux/binfmts.h
+++ b/include/linux/binfmts.h
@@ -44,6 +44,7 @@ struct linux_binprm{
 #ifdef __alpha__
 	unsigned int taso:1;
 #endif
+	unsigned int nosuid:1;	/* True if suid bits are ignored */
 	unsigned int recursion_depth;
 	struct file * file;
 	struct cred *cred;	/* new credentials */
diff --git a/include/linux/prctl.h b/include/linux/prctl.h
index a3baeb2..acb3516 100644
--- a/include/linux/prctl.h
+++ b/include/linux/prctl.h
@@ -102,4 +102,6 @@
 
 #define PR_MCE_KILL_GET 34
 
+#define PR_SET_NOSUID	35
+
 #endif /* _LINUX_PRCTL_H */
diff --git a/kernel/ptrace.c b/kernel/ptrace.c
index 23bd09c..b91040c 100644
--- a/kernel/ptrace.c
+++ b/kernel/ptrace.c
@@ -152,6 +152,10 @@ int __ptrace_may_access(struct task_struct *task, unsigned int mode)
 	if (!dumpable && !capable(CAP_SYS_PTRACE))
 		return -EPERM;
 
+	if (test_tsk_thread_flag(current, TIF_NOSUID) &&
+	    !test_tsk_thread_flag(task, TIF_NOSUID))
+		return -EPERM;
+
 	return security_ptrace_access_check(task, mode);
 }
 
diff --git a/kernel/sys.c b/kernel/sys.c
index 26a6b73..1d1902a 100644
--- a/kernel/sys.c
+++ b/kernel/sys.c
@@ -1578,6 +1578,22 @@ SYSCALL_DEFINE5(prctl, int, option, unsigned long, arg2, unsigned long, arg3,
 			else
 				error = PR_MCE_KILL_DEFAULT;
 			break;
+		case PR_SET_NOSUID:
+		{
+			const struct cred *cred = current->cred;
+			error = -EINVAL;
+			if (	(cred->uid != cred->suid) ||
+				(cred->uid != cred->euid) ||
+				(cred->uid != cred->fsuid) ||
+				(cred->gid != cred->sgid) ||
+				(cred->gid != cred->egid) ||
+				(cred->gid != cred->fsgid) ||
+				(atomic_read(&current->signal->count) != 1))
+				break;
+			error = 0;
+			set_tsk_thread_flag(current, TIF_NOSUID);
+			break;
+		}
 		default:
 			error = -EINVAL;
 			break;
diff --git a/security/commoncap.c b/security/commoncap.c
index f800fdb..28ab286 100644
--- a/security/commoncap.c
+++ b/security/commoncap.c
@@ -389,7 +389,7 @@ static int get_file_caps(struct linux_binprm *bprm, bool *effective)
 	if (!file_caps_enabled)
 		return 0;
 
-	if (bprm->file->f_vfsmnt->mnt_flags & MNT_NOSUID)
+	if (bprm->nosuid)
 		return 0;
 
 	dentry = dget(bprm->file->f_dentry);
@@ -869,6 +869,18 @@ int cap_task_prctl(int option, unsigned long arg2, unsigned long arg3,
 			new->securebits &= ~issecure_mask(SECURE_KEEP_CAPS);
 		goto changed;
 
+	case PR_SET_NOSUID:
+	{
+		const struct cred *cred = current->cred;
+		error = -EINVAL;
+		/* Perform the capabilities checks */
+		if (!cap_isclear(cred->cap_permitted) ||
+		    !cap_isclear(cred->cap_effective))
+			goto error;
+		/* Have the default perform the rest of the work. */
+		error = -ENOSYS;
+		goto error;
+	}
 	default:
 		/* No functionality available - continue with default */
 		error = -ENOSYS;
diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
index 7a374c2..d14cd24 100644
--- a/security/selinux/hooks.c
+++ b/security/selinux/hooks.c
@@ -2147,7 +2147,7 @@ static int selinux_bprm_set_creds(struct linux_binprm *bprm)
 	COMMON_AUDIT_DATA_INIT(&ad, FS);
 	ad.u.fs.path = bprm->file->f_path;
 
-	if (bprm->file->f_path.mnt->mnt_flags & MNT_NOSUID)
+	if (bprm->nosid)
 		new_tsec->sid = old_tsec->sid;
 
 	if (new_tsec->sid == old_tsec->sid) {
-- 
1.6.5.2.143.g8cc62