[PATCH v2 0/3] seccomp: continue syscall from notifier

[PATCH v2 0/3] seccomp: continue syscall from notifier

[Christian Brauner]

Hey everyone,

/* v2 */
This is the patchset coming out of the KSummit session Kees and I gave
in Lisbon last week (cf. [3] which also contains slides with more
details on related things such as deep argument inspection).
The simple idea is to extend the seccomp notifier to allow for the
continuation of a syscall. The rationale for this can be found in the
commit message to [1]. For the curious there is more detail in [2].
This patchset would unblock supervising an extended set of syscalls such
as mount() where a privileged process is supervising the syscalls of a
lesser privileged process and emulates the syscall for the latter in
userspace.
For more comments on security see [1] and the comments in
include/uapi/linux/seccomp.h added by this patchset.

Kees, if you prefer a pr the series can be pulled from:
git@gitolite.kernel.org:pub/scm/linux/kernel/git/brauner/linux tags/seccomp-notify-syscall-continue-v5.5

For anyone who wants to play with this it's sitting in:
https://git.kernel.org/pub/scm/linux/kernel/git/brauner/linux.git/log/?h=seccomp_syscall_continue

/* v1 */
Link: https://lore.kernel.org/r/20190919095903.19370-1-christian.brauner@ubuntu.com
- Kees Cook <keescook@chromium.org>:
  - dropped patch because it is already present in linux-next
    [PATCH 2/4] seccomp: add two missing ptrace ifdefines
    Link: https://lore.kernel.org/r/20190918084833.9369-3-christian.brauner@ubuntu.com

/* v0 */
Link: https://lore.kernel.org/r/20190918084833.9369-1-christian.brauner@ubuntu.com

Thanks!
Christian

*** BLURB HERE ***

Christian Brauner (3):
  seccomp: add SECCOMP_USER_NOTIF_FLAG_CONTINUE
  seccomp: avoid overflow in implicit constant conversion
  seccomp: test SECCOMP_USER_NOTIF_FLAG_CONTINUE

 include/uapi/linux/seccomp.h                  |  28 +++++
 kernel/seccomp.c                              |  28 ++++-
 tools/testing/selftests/seccomp/seccomp_bpf.c | 110 +++++++++++++++++-
 3 files changed, 159 insertions(+), 7 deletions(-)

-- 
2.23.0

[PATCH v2 1/3] seccomp: add SECCOMP_USER_NOTIF_FLAG_CONTINUE

[Christian Brauner]

This allows the seccomp notifier to continue a syscall. A positive
discussion about this feature was triggered by a post to the
ksummit-discuss mailing list (cf. [3]) and took place during KSummit
(cf. [1]) and again at the containers/checkpoint-restore
micro-conference at Linux Plumbers.

Recently we landed seccomp support for SECCOMP_RET_USER_NOTIF (cf. [4])
which enables a process (watchee) to retrieve an fd for its seccomp
filter. This fd can then be handed to another (usually more privileged)
process (watcher). The watcher will then be able to receive seccomp
messages about the syscalls having been performed by the watchee.

This feature is heavily used in some userspace workloads. For example,
it is currently used to intercept mknod() syscalls in user namespaces
aka in containers.
The mknod() syscall can be easily filtered based on dev_t. This allows
us to only intercept a very specific subset of mknod() syscalls.
Furthermore, mknod() is not possible in user namespaces toto coelo and
so intercepting and denying syscalls that are not in the whitelist on
accident is not a big deal. The watchee won't notice a difference.

In contrast to mknod(), a lot of other syscall we intercept (e.g.
setxattr()) cannot be easily filtered like mknod() because they have
pointer arguments. Additionally, some of them might actually succeed in
user namespaces (e.g. setxattr() for all "user.*" xattrs). Since we
currently cannot tell seccomp to continue from a user notifier we are
stuck with performing all of the syscalls in lieu of the container. This
is a huge security liability since it is extremely difficult to
correctly assume all of the necessary privileges of the calling task
such that the syscall can be successfully emulated without escaping
other additional security restrictions (think missing CAP_MKNOD for
mknod(), or MS_NODEV on a filesystem etc.). This can be solved by
telling seccomp to resume the syscall.

One thing that came up in the discussion was the problem that another
thread could change the memory after userspace has decided to let the
syscall continue which is a well known TOCTOU with seccomp which is
present in other ways already.
The discussion showed that this feature is already very useful for any
syscall without pointer arguments. For any accidentally intercepted
non-pointer syscall it is safe to continue.
For syscalls with pointer arguments there is a race but for any cautious
userspace and the main usec cases the race doesn't matter. The notifier
is intended to be used in a scenario where a more privileged watcher
supervises the syscalls of lesser privileged watchee to allow it to get
around kernel-enforced limitations by performing the syscall for it
whenever deemed save by the watcher. Hence, if a user tricks the watcher
into allowing a syscall they will either get a deny based on
kernel-enforced restrictions later or they will have changed the
arguments in such a way that they manage to perform a syscall with
arguments that they would've been allowed to do anyway.
In general, it is good to point out again, that the notifier fd was not
intended to allow userspace to implement a security policy but rather to
work around kernel security mechanisms in cases where the watcher knows
that a given action is safe to perform.

/* References */
[1]: https://linuxplumbersconf.org/event/4/contributions/560
[2]: https://linuxplumbersconf.org/event/4/contributions/477
[3]: https://lore.kernel.org/r/20190719093538.dhyopljyr5ns33qx@brauner.io
[4]: commit 6a21cc50f0c7 ("seccomp: add a return code to trap to userspace")

Co-developed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
Reviewed-by: Tycho Andersen <tycho@tycho.ws>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Will Drewry <wad@chromium.org>
CC: Tyler Hicks <tyhicks@canonical.com>
---
/* v2 */
- Jann Horn <jannh@google.com>:
  - mention that SECCOMP_USER_NOTIF_FLAG_CONTINUE can be used to override lower
    SECCOMP_RET_USER_NOTIF and SECCOMP_RET_TRACE filters

/* v1 */
Link: https://lore.kernel.org/r/20190919095903.19370-2-christian.brauner@ubuntu.com
- Kees Cook <keescook@chromium.org>, Tycho Andersen <tycho@tycho.ws>:
  - s/SECCOMP_RET_USER_NOTIF_ALLOW/SECCOMP_USER_NOTIF_FLAG_CONTINUE/g
- Kees Cook <keescook@chromium.org>:
  - put giant warning about the dangers, and correct usage of the
    SECCOMP_USER_NOTIF_FLAG_CONTINUE flag
- Kees Cook <keescook@chromium.org>:
  - change return type for seccomp_do_user_notification() to int to align with
    similar functions

/* v0 */
Link: https://lore.kernel.org/r/20190918084833.9369-2-christian.brauner@ubuntu.com
---
 include/uapi/linux/seccomp.h | 28 ++++++++++++++++++++++++++++
 kernel/seccomp.c             | 28 ++++++++++++++++++++++------
 2 files changed, 50 insertions(+), 6 deletions(-)

diff --git a/include/uapi/linux/seccomp.h b/include/uapi/linux/seccomp.h
index 90734aa5aa36..61fbbb7c1ee9 100644
--- a/include/uapi/linux/seccomp.h
+++ b/include/uapi/linux/seccomp.h
@@ -76,6 +76,34 @@ struct seccomp_notif {
 	struct seccomp_data data;
 };
 
+/*
+ * Valid flags for struct seccomp_notif_resp
+ *
+ * Note, the SECCOMP_USER_NOTIF_FLAG_CONTINUE flag must be used with caution!
+ * If set by the process supervising the syscalls of another process the
+ * syscall will continue. This is problematic because of an inherent TOCTOU.
+ * An attacker can exploit the time while the supervised process is waiting on
+ * a response from the supervising process to rewrite syscall arguments which
+ * are passed as pointers of the intercepted syscall.
+ * It should be absolutely clear that this means that the seccomp notifier
+ * _cannot_ be used to implement a security policy! It should only ever be used
+ * in scenarios where a more privileged process supervises the syscalls of a
+ * lesser privileged process to get around kernel-enforced security
+ * restrictions when the privileged process deems this safe. In other words,
+ * in order to continue a syscall the supervising process should be sure that
+ * another security mechanism or the kernel itself will sufficiently block
+ * syscalls if arguments are rewritten to something unsafe.
+ *
+ * Similar precautions should be applied when stacking SECCOMP_RET_USER_NOTIF.
+ * For SECCOMP_RET_USER_NOTIF filters acting on the same syscall the uppermost
+ * filter takes precedence. This means that the uppermost
+ * SECCOMP_RET_USER_NOTIF filter can override any SECCOMP_IOCTL_NOTIF_SEND from
+ * lower filters essentially allowing all syscalls to pass by using
+ * SECCOMP_USER_NOTIF_FLAG_CONTINUE. Note that SECCOMP_RET_USER_NOTIF can
+ * equally be overriden by SECCOMP_USER_NOTIF_FLAG_CONTINUE.
+ */
+#define SECCOMP_USER_NOTIF_FLAG_CONTINUE BIT(0)
+
 struct seccomp_notif_resp {
 	__u64 id;
 	__s64 val;
diff --git a/kernel/seccomp.c b/kernel/seccomp.c
index dba52a7db5e8..12d2227e5786 100644
--- a/kernel/seccomp.c
+++ b/kernel/seccomp.c
@@ -75,6 +75,7 @@ struct seccomp_knotif {
 	/* The return values, only valid when in SECCOMP_NOTIFY_REPLIED */
 	int error;
 	long val;
+	u32 flags;
 
 	/* Signals when this has entered SECCOMP_NOTIFY_REPLIED */
 	struct completion ready;
@@ -732,11 +733,12 @@ static u64 seccomp_next_notify_id(struct seccomp_filter *filter)
 	return filter->notif->next_id++;
 }
 
-static void seccomp_do_user_notification(int this_syscall,
-					 struct seccomp_filter *match,
-					 const struct seccomp_data *sd)
+static int seccomp_do_user_notification(int this_syscall,
+					struct seccomp_filter *match,
+					const struct seccomp_data *sd)
 {
 	int err;
+	u32 flags = 0;
 	long ret = 0;
 	struct seccomp_knotif n = {};
 
@@ -764,6 +766,7 @@ static void seccomp_do_user_notification(int this_syscall,
 	if (err == 0) {
 		ret = n.val;
 		err = n.error;
+		flags = n.flags;
 	}
 
 	/*
@@ -780,8 +783,14 @@ static void seccomp_do_user_notification(int this_syscall,
 		list_del(&n.list);
 out:
 	mutex_unlock(&match->notify_lock);
+
+	/* Userspace requests to continue the syscall. */
+	if (flags & SECCOMP_USER_NOTIF_FLAG_CONTINUE)
+		return 0;
+
 	syscall_set_return_value(current, task_pt_regs(current),
 				 err, ret);
+	return -1;
 }
 
 static int __seccomp_filter(int this_syscall, const struct seccomp_data *sd,
@@ -867,8 +876,10 @@ static int __seccomp_filter(int this_syscall, const struct seccomp_data *sd,
 		return 0;
 
 	case SECCOMP_RET_USER_NOTIF:
-		seccomp_do_user_notification(this_syscall, match, sd);
-		goto skip;
+		if (seccomp_do_user_notification(this_syscall, match, sd))
+			goto skip;
+
+		return 0;
 
 	case SECCOMP_RET_LOG:
 		seccomp_log(this_syscall, 0, action, true);
@@ -1087,7 +1098,11 @@ static long seccomp_notify_send(struct seccomp_filter *filter,
 	if (copy_from_user(&resp, buf, sizeof(resp)))
 		return -EFAULT;
 
-	if (resp.flags)
+	if (resp.flags & ~SECCOMP_USER_NOTIF_FLAG_CONTINUE)
+		return -EINVAL;
+
+	if ((resp.flags & SECCOMP_USER_NOTIF_FLAG_CONTINUE) &&
+	    (resp.error || resp.val))
 		return -EINVAL;
 
 	ret = mutex_lock_interruptible(&filter->notify_lock);
@@ -1116,6 +1131,7 @@ static long seccomp_notify_send(struct seccomp_filter *filter,
 	knotif->state = SECCOMP_NOTIFY_REPLIED;
 	knotif->error = resp.error;
 	knotif->val = resp.val;
+	knotif->flags = resp.flags;
 	complete(&knotif->ready);
 out:
 	mutex_unlock(&filter->notify_lock);
-- 
2.23.0

[PATCH v2 2/3] seccomp: avoid overflow in implicit constant conversion

[Christian Brauner]

USER_NOTIF_MAGIC is assigned to int variables in this test so set it to INT_MAX
to avoid warnings:

seccomp_bpf.c: In function ‘user_notification_continue’:
seccomp_bpf.c:3088:26: warning: overflow in implicit constant conversion [-Woverflow]
 #define USER_NOTIF_MAGIC 116983961184613L
                          ^
seccomp_bpf.c:3572:15: note: in expansion of macro ‘USER_NOTIF_MAGIC’
  resp.error = USER_NOTIF_MAGIC;
               ^~~~~~~~~~~~~~~~

Fixes: 6a21cc50f0c7 ("seccomp: add a return code to trap to userspace")
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
Reviewed-by: Tyler Hicks <tyhicks@canonical.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Will Drewry <wad@chromium.org>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Cc: Martin KaFai Lau <kafai@fb.com>
Cc: Song Liu <songliubraving@fb.com>
Cc: Yonghong Song <yhs@fb.com>
Cc: Tycho Andersen <tycho@tycho.ws>
Cc: stable@vger.kernel.org
Cc: linux-kselftest@vger.kernel.org
Cc: netdev@vger.kernel.org
Cc: bpf@vger.kernel.org
---
/* v2 */
unchanged

/* v1 */
Link: https://lore.kernel.org/r/20190919095903.19370-3-christian.brauner@ubuntu.com
unchanged

/* v0 */
Link: https://lore.kernel.org/r/20190918084833.9369-4-christian.brauner@ubuntu.com
---
 tools/testing/selftests/seccomp/seccomp_bpf.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/tools/testing/selftests/seccomp/seccomp_bpf.c b/tools/testing/selftests/seccomp/seccomp_bpf.c
index 6ef7f16c4cf5..e996d7b7fd6e 100644
--- a/tools/testing/selftests/seccomp/seccomp_bpf.c
+++ b/tools/testing/selftests/seccomp/seccomp_bpf.c
@@ -35,6 +35,7 @@
 #include <stdbool.h>
 #include <string.h>
 #include <time.h>
+#include <limits.h>
 #include <linux/elf.h>
 #include <sys/uio.h>
 #include <sys/utsname.h>
@@ -3072,7 +3073,7 @@ static int user_trap_syscall(int nr, unsigned int flags)
 	return seccomp(SECCOMP_SET_MODE_FILTER, flags, &prog);
 }
 
-#define USER_NOTIF_MAGIC 116983961184613L
+#define USER_NOTIF_MAGIC INT_MAX
 TEST(user_notification_basic)
 {
 	pid_t pid;
-- 
2.23.0

[PATCH v2 3/3] seccomp: test SECCOMP_USER_NOTIF_FLAG_CONTINUE

[Christian Brauner]

Test whether a syscall can be performed after having been intercepted by
the seccomp notifier. The test uses dup() and kcmp() since it allows us to
nicely test whether the dup() syscall actually succeeded by comparing whether
the fds refer to the same underlying struct file.

Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Will Drewry <wad@chromium.org>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Cc: Martin KaFai Lau <kafai@fb.com>
Cc: Song Liu <songliubraving@fb.com>
Cc: Yonghong Song <yhs@fb.com>
Cc: Tycho Andersen <tycho@tycho.ws>
CC: Tyler Hicks <tyhicks@canonical.com>
Cc: stable@vger.kernel.org
Cc: linux-kselftest@vger.kernel.org
Cc: netdev@vger.kernel.org
Cc: bpf@vger.kernel.org
---
/* v2 */
- Shuah Khan <shuah@kernel.org>:
  - skip test on missing precondition

/* v1 */
Link: https://lore.kernel.org/r/20190919095903.19370-4-christian.brauner@ubuntu.com
- Christian Brauner <christian.brauner@ubuntu.com>:
  - adapt to new flag name SECCOMP_USER_NOTIF_FLAG_CONTINUE

/* v0 */
Link: https://lore.kernel.org/r/20190918084833.9369-5-christian.brauner@ubuntu.com
---
 tools/testing/selftests/seccomp/seccomp_bpf.c | 107 ++++++++++++++++++
 1 file changed, 107 insertions(+)

diff --git a/tools/testing/selftests/seccomp/seccomp_bpf.c b/tools/testing/selftests/seccomp/seccomp_bpf.c
index e996d7b7fd6e..2519377ebda3 100644
--- a/tools/testing/selftests/seccomp/seccomp_bpf.c
+++ b/tools/testing/selftests/seccomp/seccomp_bpf.c
@@ -44,6 +44,7 @@
 #include <sys/times.h>
 #include <sys/socket.h>
 #include <sys/ioctl.h>
+#include <linux/kcmp.h>
 
 #include <unistd.h>
 #include <sys/syscall.h>
@@ -167,6 +168,10 @@ struct seccomp_metadata {
 
 #define SECCOMP_RET_USER_NOTIF 0x7fc00000U
 
+#ifndef SECCOMP_USER_NOTIF_FLAG_CONTINUE
+#define SECCOMP_USER_NOTIF_FLAG_CONTINUE 0x00000001
+#endif
+
 #define SECCOMP_IOC_MAGIC		'!'
 #define SECCOMP_IO(nr)			_IO(SECCOMP_IOC_MAGIC, nr)
 #define SECCOMP_IOR(nr, type)		_IOR(SECCOMP_IOC_MAGIC, nr, type)
@@ -3481,6 +3486,108 @@ TEST(seccomp_get_notif_sizes)
 	EXPECT_EQ(sizes.seccomp_notif_resp, sizeof(struct seccomp_notif_resp));
 }
 
+static int filecmp(pid_t pid1, pid_t pid2, int fd1, int fd2)
+{
+#ifdef __NR_kcmp
+	return syscall(__NR_kcmp, pid1, pid2, KCMP_FILE, fd1, fd2);
+#else
+	errno = ENOSYS;
+	return -1;
+#endif
+}
+
+TEST(user_notification_continue)
+{
+	pid_t pid;
+	long ret;
+	int status, listener;
+	struct seccomp_notif req = {};
+	struct seccomp_notif_resp resp = {};
+	struct pollfd pollfd;
+
+	ret = prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0);
+	ASSERT_EQ(0, ret) {
+		TH_LOG("Kernel does not support PR_SET_NO_NEW_PRIVS!");
+	}
+
+	listener = user_trap_syscall(__NR_dup, SECCOMP_FILTER_FLAG_NEW_LISTENER);
+	ASSERT_GE(listener, 0);
+
+	pid = fork();
+	ASSERT_GE(pid, 0);
+
+	if (pid == 0) {
+		int dup_fd, pipe_fds[2];
+		pid_t self;
+
+		ret = pipe(pipe_fds);
+		if (ret < 0)
+			exit(1);
+
+		dup_fd = dup(pipe_fds[0]);
+		if (dup_fd < 0)
+			exit(1);
+
+		self = getpid();
+
+		ret = filecmp(self, self, pipe_fds[0], dup_fd);
+		if (ret)
+			exit(2);
+
+		exit(0);
+	}
+
+	pollfd.fd = listener;
+	pollfd.events = POLLIN | POLLOUT;
+
+	EXPECT_GT(poll(&pollfd, 1, -1), 0);
+	EXPECT_EQ(pollfd.revents, POLLIN);
+
+	EXPECT_EQ(ioctl(listener, SECCOMP_IOCTL_NOTIF_RECV, &req), 0);
+
+	pollfd.fd = listener;
+	pollfd.events = POLLIN | POLLOUT;
+
+	EXPECT_GT(poll(&pollfd, 1, -1), 0);
+	EXPECT_EQ(pollfd.revents, POLLOUT);
+
+	EXPECT_EQ(req.data.nr, __NR_dup);
+
+	resp.id = req.id;
+	resp.flags = SECCOMP_USER_NOTIF_FLAG_CONTINUE;
+
+	/*
+	 * Verify that setting SECCOMP_USER_NOTIF_FLAG_CONTINUE enforces other
+	 * args be set to 0.
+	 */
+	resp.error = 0;
+	resp.val = USER_NOTIF_MAGIC;
+	EXPECT_EQ(ioctl(listener, SECCOMP_IOCTL_NOTIF_SEND, &resp), -1);
+	EXPECT_EQ(errno, EINVAL);
+
+	resp.error = USER_NOTIF_MAGIC;
+	resp.val = 0;
+	EXPECT_EQ(ioctl(listener, SECCOMP_IOCTL_NOTIF_SEND, &resp), -1);
+	EXPECT_EQ(errno, EINVAL);
+
+	resp.error = 0;
+	resp.val = 0;
+	EXPECT_EQ(ioctl(listener, SECCOMP_IOCTL_NOTIF_SEND, &resp), 0) {
+		if (errno == EINVAL)
+			XFAIL(goto skip, "Kernel does not support SECCOMP_USER_NOTIF_FLAG_CONTINUE");
+	}
+
+skip:
+	EXPECT_EQ(waitpid(pid, &status, 0), pid);
+	EXPECT_EQ(true, WIFEXITED(status));
+	EXPECT_EQ(0, WEXITSTATUS(status)) {
+		if (WEXITSTATUS(status) == 2) {
+			XFAIL(return, "Kernel does not support kcmp() syscall");
+			return;
+		}
+	}
+}
+
 /*
  * TODO:
  * - add microbenchmarks
-- 
2.23.0

Re: [PATCH v2 2/3] seccomp: avoid overflow in implicit constant conversion

[Tycho Andersen]

On Fri, Sep 20, 2019 at 10:30:06AM +0200, Christian Brauner wrote:
> USER_NOTIF_MAGIC is assigned to int variables in this test so set it to INT_MAX
> to avoid warnings:
> 
> seccomp_bpf.c: In function ‘user_notification_continue’:
> seccomp_bpf.c:3088:26: warning: overflow in implicit constant conversion [-Woverflow]
>  #define USER_NOTIF_MAGIC 116983961184613L
>                           ^
> seccomp_bpf.c:3572:15: note: in expansion of macro ‘USER_NOTIF_MAGIC’
>   resp.error = USER_NOTIF_MAGIC;
>                ^~~~~~~~~~~~~~~~
> 
> Fixes: 6a21cc50f0c7 ("seccomp: add a return code to trap to userspace")
> Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
> Reviewed-by: Tyler Hicks <tyhicks@canonical.com>
> Cc: Kees Cook <keescook@chromium.org>
> Cc: Andy Lutomirski <luto@amacapital.net>
> Cc: Will Drewry <wad@chromium.org>
> Cc: Shuah Khan <shuah@kernel.org>
> Cc: Alexei Starovoitov <ast@kernel.org>
> Cc: Daniel Borkmann <daniel@iogearbox.net>
> Cc: Martin KaFai Lau <kafai@fb.com>
> Cc: Song Liu <songliubraving@fb.com>
> Cc: Yonghong Song <yhs@fb.com>
> Cc: Tycho Andersen <tycho@tycho.ws>

You can also add,

Reviewed-by: Tycho Andersen <tycho@tycho.ws>

for this one.

Tycho

Re: [PATCH v2 1/3] seccomp: add SECCOMP_USER_NOTIF_FLAG_CONTINUE

[Kees Cook]

On Fri, Sep 20, 2019 at 10:30:05AM +0200, Christian Brauner wrote:
> + * Similar precautions should be applied when stacking SECCOMP_RET_USER_NOTIF.
> + * For SECCOMP_RET_USER_NOTIF filters acting on the same syscall the uppermost
> + * filter takes precedence. This means that the uppermost
> + * SECCOMP_RET_USER_NOTIF filter can override any SECCOMP_IOCTL_NOTIF_SEND from
> + * lower filters essentially allowing all syscalls to pass by using
> + * SECCOMP_USER_NOTIF_FLAG_CONTINUE. Note that SECCOMP_RET_USER_NOTIF can
                                                          ^^^^^^^^^^^^^^
This is meant to read RET_TRACE, yes?

> + * equally be overriden by SECCOMP_USER_NOTIF_FLAG_CONTINUE.

I rewrote this paragraph with that corrected and swapping some
"upper/lower" to "most recently added" etc:

+ * Similar precautions should be applied when stacking SECCOMP_RET_USER_NOTIF
+ * or SECCOMP_RET_TRACE. For SECCOMP_RET_USER_NOTIF filters acting on the
+ * same syscall, the most recently added filter takes precedence. This means
+ * that the new SECCOMP_RET_USER_NOTIF filter can override any
+ * SECCOMP_IOCTL_NOTIF_SEND from earlier filters, essentially allowing all
+ * such filtered syscalls to be executed by sending the response
+ * SECCOMP_USER_NOTIF_FLAG_CONTINUE. Note that SECCOMP_RET_TRACE can equally
+ * be overriden by SECCOMP_USER_NOTIF_FLAG_CONTINUE.


Ultimately, I think this caveat is fine. RET_USER_NOTIF and RET_TRACE are
both used from the "process manager" use-case. The benefits of "continue"
semantics here outweighs the RET_USER_NOTIF and RET_TRACE "bypass". If
we end up in a situation where we need to deal with some kind of
nesting where this is a problem in practice, we can revisit this.

Applied to my for-next/seccomp. Thanks!

-- 
Kees Cook

Re: [PATCH v2 1/3] seccomp: add SECCOMP_USER_NOTIF_FLAG_CONTINUE

[Christian Brauner]

On Thu, Oct 10, 2019 at 02:45:38PM -0700, Kees Cook wrote:
> On Fri, Sep 20, 2019 at 10:30:05AM +0200, Christian Brauner wrote:
> > + * Similar precautions should be applied when stacking SECCOMP_RET_USER_NOTIF.
> > + * For SECCOMP_RET_USER_NOTIF filters acting on the same syscall the uppermost
> > + * filter takes precedence. This means that the uppermost
> > + * SECCOMP_RET_USER_NOTIF filter can override any SECCOMP_IOCTL_NOTIF_SEND from
> > + * lower filters essentially allowing all syscalls to pass by using
> > + * SECCOMP_USER_NOTIF_FLAG_CONTINUE. Note that SECCOMP_RET_USER_NOTIF can
>                                                           ^^^^^^^^^^^^^^
> This is meant to read RET_TRACE, yes?

Yes. :)

> 
> > + * equally be overriden by SECCOMP_USER_NOTIF_FLAG_CONTINUE.
> 
> I rewrote this paragraph with that corrected and swapping some
> "upper/lower" to "most recently added" etc:
> 
> + * Similar precautions should be applied when stacking SECCOMP_RET_USER_NOTIF
> + * or SECCOMP_RET_TRACE. For SECCOMP_RET_USER_NOTIF filters acting on the
> + * same syscall, the most recently added filter takes precedence. This means
> + * that the new SECCOMP_RET_USER_NOTIF filter can override any
> + * SECCOMP_IOCTL_NOTIF_SEND from earlier filters, essentially allowing all
> + * such filtered syscalls to be executed by sending the response
> + * SECCOMP_USER_NOTIF_FLAG_CONTINUE. Note that SECCOMP_RET_TRACE can equally
> + * be overriden by SECCOMP_USER_NOTIF_FLAG_CONTINUE.
> 
> 
> Ultimately, I think this caveat is fine. RET_USER_NOTIF and RET_TRACE are
> both used from the "process manager" use-case. The benefits of "continue"
> semantics here outweighs the RET_USER_NOTIF and RET_TRACE "bypass". If
> we end up in a situation where we need to deal with some kind of
> nesting where this is a problem in practice, we can revisit this.
> 
> Applied to my for-next/seccomp. Thanks!

Thanks!
Christian