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Subject[RFC][PATCH v3] Unprivileged: Disable raising of privileges
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Date

If we can know that a process will never raise
it's priveleges we can enable a lot of features
without privilege (such as unsharing namespaces
and unprivileged mounts) 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 <ebiederm@xmission.com>
---
arch/x86/include/asm/thread_info.h | 2 ++
fs/exec.c | 6 ++++--
include/linux/binfmts.h | 1 +
include/linux/prctl.h | 3 +++
kernel/ptrace.c | 4 ++++
kernel/sys.c | 21 +++++++++++++++++++++
security/commoncap.c | 3 +--
security/selinux/hooks.c | 2 +-
8 files changed, 37 insertions(+), 5 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..8adc517 100644
--- a/include/linux/prctl.h
+++ b/include/linux/prctl.h
@@ -102,4 +102,7 @@

#define PR_MCE_KILL_GET 34

+#define PR_SET_NOSUID 35
+#define PR_GET_NOSUID 36
+
#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..8731f2a 100644
--- a/kernel/sys.c
+++ b/kernel/sys.c
@@ -1578,6 +1578,27 @@ 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;
+ /* Don't support cases that could be unsafe */
+ 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) ||
+ !cap_isclear(cred->cap_permitted) ||
+ (atomic_read(&current->signal->count) != 1))
+ break;
+ error = 0;
+ set_tsk_thread_flag(current, TIF_NOSUID);
+ break;
+ }
+ case PR_GET_NOSUID:
+ error = !!test_tsk_thread_flag(current, TIF_NOSUID);
+ break;
default:
error = -EINVAL;
break;
diff --git a/security/commoncap.c b/security/commoncap.c
index f800fdb..34500e3 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);
@@ -868,7 +868,6 @@ int cap_task_prctl(int option, unsigned long arg2, unsigned long arg3,
else
new->securebits &= ~issecure_mask(SECURE_KEEP_CAPS);
goto changed;
-
default:
/* No functionality available - continue with default */
error = -ENOSYS;
diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
index 7a374c2..bd77a2b 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->nosuid)
new_tsec->sid = old_tsec->sid;

if (new_tsec->sid == old_tsec->sid) {
--
1.6.5.2.143.g8cc62


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