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Subject[PATCH 4.4 073/107] seccomp: Add filter flag to opt-out of SSB mitigation
Date
4.4-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Kees Cook <keescook@chromium.org>

commit 00a02d0c502a06d15e07b857f8ff921e3e402675 upstream

If a seccomp user is not interested in Speculative Store Bypass mitigation
by default, it can set the new SECCOMP_FILTER_FLAG_SPEC_ALLOW flag when
adding filters.

Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Srivatsa S. Bhat <srivatsa@csail.mit.edu>
Reviewed-by: Matt Helsley (VMware) <matt.helsley@gmail.com>
Reviewed-by: Alexey Makhalov <amakhalov@vmware.com>
Reviewed-by: Bo Gan <ganb@vmware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---

include/linux/seccomp.h | 3 -
include/uapi/linux/seccomp.h | 4 +
kernel/seccomp.c | 19 +++---
tools/testing/selftests/seccomp/seccomp_bpf.c | 78 +++++++++++++++++++++++++-
4 files changed, 93 insertions(+), 11 deletions(-)

--- a/include/linux/seccomp.h
+++ b/include/linux/seccomp.h
@@ -3,7 +3,8 @@

#include <uapi/linux/seccomp.h>

-#define SECCOMP_FILTER_FLAG_MASK (SECCOMP_FILTER_FLAG_TSYNC)
+#define SECCOMP_FILTER_FLAG_MASK (SECCOMP_FILTER_FLAG_TSYNC | \
+ SECCOMP_FILTER_FLAG_SPEC_ALLOW)

#ifdef CONFIG_SECCOMP

--- a/include/uapi/linux/seccomp.h
+++ b/include/uapi/linux/seccomp.h
@@ -15,7 +15,9 @@
#define SECCOMP_SET_MODE_FILTER 1

/* Valid flags for SECCOMP_SET_MODE_FILTER */
-#define SECCOMP_FILTER_FLAG_TSYNC 1
+#define SECCOMP_FILTER_FLAG_TSYNC (1UL << 0)
+/* In v4.14+ SECCOMP_FILTER_FLAG_LOG is (1UL << 1) */
+#define SECCOMP_FILTER_FLAG_SPEC_ALLOW (1UL << 2)

/*
* All BPF programs must return a 32-bit value.
--- a/kernel/seccomp.c
+++ b/kernel/seccomp.c
@@ -230,7 +230,8 @@ static inline void spec_mitigate(struct
}

static inline void seccomp_assign_mode(struct task_struct *task,
- unsigned long seccomp_mode)
+ unsigned long seccomp_mode,
+ unsigned long flags)
{
assert_spin_locked(&task->sighand->siglock);

@@ -240,8 +241,9 @@ static inline void seccomp_assign_mode(s
* filter) is set.
*/
smp_mb__before_atomic();
- /* Assume seccomp processes want speculation flaw mitigation. */
- spec_mitigate(task, PR_SPEC_STORE_BYPASS);
+ /* Assume default seccomp processes want spec flaw mitigation. */
+ if ((flags & SECCOMP_FILTER_FLAG_SPEC_ALLOW) == 0)
+ spec_mitigate(task, PR_SPEC_STORE_BYPASS);
set_tsk_thread_flag(task, TIF_SECCOMP);
}

@@ -309,7 +311,7 @@ static inline pid_t seccomp_can_sync_thr
* without dropping the locks.
*
*/
-static inline void seccomp_sync_threads(void)
+static inline void seccomp_sync_threads(unsigned long flags)
{
struct task_struct *thread, *caller;

@@ -350,7 +352,8 @@ static inline void seccomp_sync_threads(
* allow one thread to transition the other.
*/
if (thread->seccomp.mode == SECCOMP_MODE_DISABLED)
- seccomp_assign_mode(thread, SECCOMP_MODE_FILTER);
+ seccomp_assign_mode(thread, SECCOMP_MODE_FILTER,
+ flags);
}
}

@@ -469,7 +472,7 @@ static long seccomp_attach_filter(unsign

/* Now that the new filter is in place, synchronize to all threads. */
if (flags & SECCOMP_FILTER_FLAG_TSYNC)
- seccomp_sync_threads();
+ seccomp_sync_threads(flags);

return 0;
}
@@ -764,7 +767,7 @@ static long seccomp_set_mode_strict(void
#ifdef TIF_NOTSC
disable_TSC();
#endif
- seccomp_assign_mode(current, seccomp_mode);
+ seccomp_assign_mode(current, seccomp_mode, 0);
ret = 0;

out:
@@ -822,7 +825,7 @@ static long seccomp_set_mode_filter(unsi
/* Do not free the successfully attached filter. */
prepared = NULL;

- seccomp_assign_mode(current, seccomp_mode);
+ seccomp_assign_mode(current, seccomp_mode, flags);
out:
spin_unlock_irq(&current->sighand->siglock);
if (flags & SECCOMP_FILTER_FLAG_TSYNC)
--- a/tools/testing/selftests/seccomp/seccomp_bpf.c
+++ b/tools/testing/selftests/seccomp/seccomp_bpf.c
@@ -1477,7 +1477,11 @@ TEST_F(TRACE_syscall, syscall_dropped)
#endif

#ifndef SECCOMP_FILTER_FLAG_TSYNC
-#define SECCOMP_FILTER_FLAG_TSYNC 1
+#define SECCOMP_FILTER_FLAG_TSYNC (1UL << 0)
+#endif
+
+#ifndef SECCOMP_FILTER_FLAG_SPEC_ALLOW
+#define SECCOMP_FILTER_FLAG_SPEC_ALLOW (1UL << 2)
#endif

#ifndef seccomp
@@ -1576,6 +1580,78 @@ TEST(seccomp_syscall_mode_lock)
}
}

+/*
+ * Test detection of known and unknown filter flags. Userspace needs to be able
+ * to check if a filter flag is supported by the current kernel and a good way
+ * of doing that is by attempting to enter filter mode, with the flag bit in
+ * question set, and a NULL pointer for the _args_ parameter. EFAULT indicates
+ * that the flag is valid and EINVAL indicates that the flag is invalid.
+ */
+TEST(detect_seccomp_filter_flags)
+{
+ unsigned int flags[] = { SECCOMP_FILTER_FLAG_TSYNC,
+ SECCOMP_FILTER_FLAG_SPEC_ALLOW };
+ unsigned int flag, all_flags;
+ int i;
+ long ret;
+
+ /* Test detection of known-good filter flags */
+ for (i = 0, all_flags = 0; i < ARRAY_SIZE(flags); i++) {
+ int bits = 0;
+
+ flag = flags[i];
+ /* Make sure the flag is a single bit! */
+ while (flag) {
+ if (flag & 0x1)
+ bits ++;
+ flag >>= 1;
+ }
+ ASSERT_EQ(1, bits);
+ flag = flags[i];
+
+ ret = seccomp(SECCOMP_SET_MODE_FILTER, flag, NULL);
+ ASSERT_NE(ENOSYS, errno) {
+ TH_LOG("Kernel does not support seccomp syscall!");
+ }
+ EXPECT_EQ(-1, ret);
+ EXPECT_EQ(EFAULT, errno) {
+ TH_LOG("Failed to detect that a known-good filter flag (0x%X) is supported!",
+ flag);
+ }
+
+ all_flags |= flag;
+ }
+
+ /* Test detection of all known-good filter flags */
+ ret = seccomp(SECCOMP_SET_MODE_FILTER, all_flags, NULL);
+ EXPECT_EQ(-1, ret);
+ EXPECT_EQ(EFAULT, errno) {
+ TH_LOG("Failed to detect that all known-good filter flags (0x%X) are supported!",
+ all_flags);
+ }
+
+ /* Test detection of an unknown filter flag */
+ flag = -1;
+ ret = seccomp(SECCOMP_SET_MODE_FILTER, flag, NULL);
+ EXPECT_EQ(-1, ret);
+ EXPECT_EQ(EINVAL, errno) {
+ TH_LOG("Failed to detect that an unknown filter flag (0x%X) is unsupported!",
+ flag);
+ }
+
+ /*
+ * Test detection of an unknown filter flag that may simply need to be
+ * added to this test
+ */
+ flag = flags[ARRAY_SIZE(flags) - 1] << 1;
+ ret = seccomp(SECCOMP_SET_MODE_FILTER, flag, NULL);
+ EXPECT_EQ(-1, ret);
+ EXPECT_EQ(EINVAL, errno) {
+ TH_LOG("Failed to detect that an unknown filter flag (0x%X) is unsupported! Does a new flag need to be added to this test?",
+ flag);
+ }
+}
+
TEST(TSYNC_first)
{
struct sock_filter filter[] = {

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