[PATCH v2] seccomp: add ptrace options for suspend/resume

[Tycho Andersen <tycho.andersen@canonical.com>]

This patch is the first step in enabling checkpoint/restore of processes
with seccomp enabled.

One of the things CRIU does while dumping tasks is inject code into them
via ptrace to collect information that is only available to the process
itself. However, if we are in a seccomp mode where these processes are
prohibited from making these syscalls, then what CRIU does kills the task.

This patch adds a new ptrace option, PTRACE_O_SUSPEND_SECCOMP, that enables
a task from the init user namespace which has CAP_SYS_ADMIN and no seccomp
filters to disable (and re-enable) seccomp filters for another task so that
they can be successfully dumped (and restored). We restrict the set of
processes that can disable seccomp through ptrace because although today
ptrace can be used to bypass seccomp, there is some discussion of closing
this loophole in the future and we would like this patch to not depend on
that behavior and be future proofed for when it is removed.

Note that seccomp can be suspended before any filters are actually
installed; this behavior is useful on criu restore, so that we can suspend
seccomp, restore the filters, unmap our restore code from the restored
process' address space, and then resume the task by detaching and have the
filters resumed as well.

v2 changes:

* require that the tracer have no seccomp filters installed
* drop TIF_NOTSC manipulation from the patch
* change from ptrace command to a ptrace option and use this ptrace option
  as the flag to check. This means that as soon as the tracer
  detaches/dies, seccomp is re-enabled and as a corrollary that one can not
  disable seccomp across PTRACE_ATTACHs.

Signed-off-by: Tycho Andersen <tycho.andersen@canonical.com>
CC: Kees Cook <keescook@chromium.org>
CC: Andy Lutomirski <luto@amacapital.net>
CC: Will Drewry <wad@chromium.org>
CC: Roland McGrath <roland@hack.frob.com>
CC: Oleg Nesterov <oleg@redhat.com>
CC: Pavel Emelyanov <xemul@parallels.com>
CC: Serge E. Hallyn <serge.hallyn@ubuntu.com>
---
 include/linux/ptrace.h      |  1 +
 include/linux/seccomp.h     |  4 ++++
 include/uapi/linux/ptrace.h |  6 ++++--
 kernel/ptrace.c             |  6 ++++++
 kernel/seccomp.c            | 23 +++++++++++++++++++++++
 5 files changed, 38 insertions(+), 2 deletions(-)

diff --git a/include/linux/ptrace.h b/include/linux/ptrace.h
index 987a73a..061265f 100644
--- a/include/linux/ptrace.h
+++ b/include/linux/ptrace.h
@@ -34,6 +34,7 @@
 #define PT_TRACE_SECCOMP	PT_EVENT_FLAG(PTRACE_EVENT_SECCOMP)
 
 #define PT_EXITKILL		(PTRACE_O_EXITKILL << PT_OPT_FLAG_SHIFT)
+#define PT_SUSPEND_SECCOMP	(PTRACE_O_SUSPEND_SECCOMP << PT_OPT_FLAG_SHIFT)
 
 /* single stepping state bits (used on ARM and PA-RISC) */
 #define PT_SINGLESTEP_BIT	31
diff --git a/include/linux/seccomp.h b/include/linux/seccomp.h
index a19ddac..ae3ec52 100644
--- a/include/linux/seccomp.h
+++ b/include/linux/seccomp.h
@@ -53,6 +53,10 @@ static inline int seccomp_mode(struct seccomp *s)
 	return s->mode;
 }
 
+#ifdef CONFIG_CHECKPOINT_RESTORE
+extern bool may_suspend_seccomp(void);
+#endif
+
 #else /* CONFIG_SECCOMP */
 
 #include <linux/errno.h>
diff --git a/include/uapi/linux/ptrace.h b/include/uapi/linux/ptrace.h
index cf1019e..a7a6979 100644
--- a/include/uapi/linux/ptrace.h
+++ b/include/uapi/linux/ptrace.h
@@ -89,9 +89,11 @@ struct ptrace_peeksiginfo_args {
 #define PTRACE_O_TRACESECCOMP	(1 << PTRACE_EVENT_SECCOMP)
 
 /* eventless options */
-#define PTRACE_O_EXITKILL	(1 << 20)
+#define PTRACE_O_EXITKILL		(1 << 20)
+#define PTRACE_O_SUSPEND_SECCOMP	(1 << 21)
 
-#define PTRACE_O_MASK		(0x000000ff | PTRACE_O_EXITKILL)
+#define PTRACE_O_MASK		(\
+	0x000000ff | PTRACE_O_EXITKILL | PTRACE_O_SUSPEND_SECCOMP)
 
 #include <asm/ptrace.h>
 
diff --git a/kernel/ptrace.c b/kernel/ptrace.c
index c8e0e05..e3e68a2 100644
--- a/kernel/ptrace.c
+++ b/kernel/ptrace.c
@@ -15,6 +15,7 @@
 #include <linux/highmem.h>
 #include <linux/pagemap.h>
 #include <linux/ptrace.h>
+#include <linux/seccomp.h>
 #include <linux/security.h>
 #include <linux/signal.h>
 #include <linux/uio.h>
@@ -556,6 +557,11 @@ static int ptrace_setoptions(struct task_struct *child, unsigned long data)
 	if (data & ~(unsigned long)PTRACE_O_MASK)
 		return -EINVAL;
 
+#ifdef CONFIG_CHECKPOINT_RESTORE
+	if (data & PTRACE_O_SUSPEND_SECCOMP && !may_suspend_seccomp())
+		return -EPERM;
+#endif
+
 	/* Avoid intermediate state when all opts are cleared */
 	flags = child->ptrace;
 	flags &= ~(PTRACE_O_MASK << PT_OPT_FLAG_SHIFT);
diff --git a/kernel/seccomp.c b/kernel/seccomp.c
index 980fd26..2a1bd35 100644
--- a/kernel/seccomp.c
+++ b/kernel/seccomp.c
@@ -590,6 +590,11 @@ void secure_computing_strict(int this_syscall)
 {
 	int mode = current->seccomp.mode;
 
+#ifdef CONFIG_CHECKPOINT_RESTORE
+	if (unlikely(current->ptrace & PT_SUSPEND_SECCOMP))
+		return;
+#endif
+
 	if (mode == 0)
 		return;
 	else if (mode == SECCOMP_MODE_STRICT)
@@ -691,6 +696,11 @@ u32 seccomp_phase1(struct seccomp_data *sd)
 	int this_syscall = sd ? sd->nr :
 		syscall_get_nr(current, task_pt_regs(current));
 
+#ifdef CONFIG_CHECKPOINT_RESTORE
+	if (unlikely(current->ptrace & PT_SUSPEND_SECCOMP))
+		return SECCOMP_PHASE1_OK;
+#endif
+
 	switch (mode) {
 	case SECCOMP_MODE_STRICT:
 		__secure_computing_strict(this_syscall);  /* may call do_exit */
@@ -901,3 +911,16 @@ long prctl_set_seccomp(unsigned long seccomp_mode, char __user *filter)
 	/* prctl interface doesn't have flags, so they are always zero. */
 	return do_seccomp(op, 0, uargs);
 }
+
+#ifdef CONFIG_CHECKPOINT_RESTORE
+bool may_suspend_seccomp(void)
+{
+	if (!capable(CAP_SYS_ADMIN))
+		return false;
+
+	if (current->seccomp.mode != SECCOMP_MODE_DISABLED)
+		return false;
+
+	return true;
+}
+#endif /* CONFIG_CHECKPOINT_RESTORE */
-- 
2.1.4