[PATCH v2 2/2] kselftest/arm64: Add testcase for SECCOMP_RET_TRACE orig_x0 bypass
Jinjie Ruan
ruanjinjie at huawei.com
Mon Jul 20 01:51:45 PDT 2026
Add a selftest that verifies the kernel re-evaluates a seccomp filter
with the correct (ptrace-modified) first argument after
a SECCOMP_RET_TRACE stop. On arm64, syscall_get_arguments() reads
the first argument from orig_x0, which may be stale if the tracer modified
regs->regs[0] but orig_x0 was not synced. This can cause the filter to
see an old argument and incorrectly allow a syscall that it should
have rejected.
The child installs a filter that:
- TRACEs write() when fd == 2
- returns ERRNO(EPERM) when fd == 1
The parent catches the SECCOMP event, changes x0 (fd) from 2 to 1,
and resumes the child.
If the seccomp re-evaluation sees the stale orig_x0 (fd=2) the filter
returns TRACE again and the kernel (with recheck_after_trace=true)
allows the syscall to proceed – write succeeds and the child exits 0.
If the seccomp re-evaluation sees the new value (fd=1) the filter
returns ERRNO(EPERM), write fails and the child exits non-zero.
The test passes only when the write fails (child exit != 0).
Before the fix:
# ./seccomp_ret_trace_x0_bypass
TAP version 13
1..1
not ok 1 write succeeded, orig_x0 bypass likely
# Totals: pass:0 fail:1 xfail:0 xpass:0 skip:0 error:0
After the fix:
# ./seccomp_ret_trace_x0_bypass
TAP version 13
1..1
ok 1 seccomp correctly denied modified syscall
# Totals: pass:1 fail:0 xfail:0 xpass:0 skip:0 error:0
Cc: Kees Cook <kees at kernel.org>
Cc: Will Deacon <will at kernel.org>
Cc: Catalin Marinas <catalin.marinas at arm.com>
Cc: Mark Rutland <mark.rutland at arm.com>
Link: https://lore.kernel.org/all/20260717182758.17111-1-will@kernel.org/
Link: https://lore.kernel.org/all/20260716120640.6590-1-will@kernel.org/
Link: https://lore.kernel.org/all/202607152004.DEA95D63@keescook/
Suggested-by: Kees Cook <kees at kernel.org>
Signed-off-by: Jinjie Ruan <ruanjinjie at huawei.com>
---
tools/testing/selftests/arm64/abi/.gitignore | 1 +
tools/testing/selftests/arm64/abi/Makefile | 2 +-
.../arm64/abi/seccomp_ret_trace_x0_bypass.c | 196 ++++++++++++++++++
3 files changed, 198 insertions(+), 1 deletion(-)
create mode 100644 tools/testing/selftests/arm64/abi/seccomp_ret_trace_x0_bypass.c
diff --git a/tools/testing/selftests/arm64/abi/.gitignore b/tools/testing/selftests/arm64/abi/.gitignore
index 39129a9907c7..491a80db9dff 100644
--- a/tools/testing/selftests/arm64/abi/.gitignore
+++ b/tools/testing/selftests/arm64/abi/.gitignore
@@ -1,5 +1,6 @@
hwcap
ptrace
seccomp_ptrace_x0_bypass
+seccomp_ret_trace_x0_bypass
syscall-abi
tpidr2
diff --git a/tools/testing/selftests/arm64/abi/Makefile b/tools/testing/selftests/arm64/abi/Makefile
index 5a16db379bd4..a01d3806eba8 100644
--- a/tools/testing/selftests/arm64/abi/Makefile
+++ b/tools/testing/selftests/arm64/abi/Makefile
@@ -1,7 +1,7 @@
# SPDX-License-Identifier: GPL-2.0
# Copyright (C) 2021 ARM Limited
-TEST_GEN_PROGS := hwcap ptrace syscall-abi tpidr2 seccomp_ptrace_x0_bypass
+TEST_GEN_PROGS := hwcap ptrace syscall-abi tpidr2 seccomp_ptrace_x0_bypass seccomp_ret_trace_x0_bypass
include ../../lib.mk
diff --git a/tools/testing/selftests/arm64/abi/seccomp_ret_trace_x0_bypass.c b/tools/testing/selftests/arm64/abi/seccomp_ret_trace_x0_bypass.c
new file mode 100644
index 000000000000..6887ec71aef1
--- /dev/null
+++ b/tools/testing/selftests/arm64/abi/seccomp_ret_trace_x0_bypass.c
@@ -0,0 +1,196 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Test for SECCOMP_RET_TRACE argument modification bypass
+ * via stale orig_x0 during filter re-evaluation.
+ *
+ * On arm64, syscall_get_arguments() reads the first argument from
+ * regs->orig_x0. When a seccomp filter returns SECCOMP_RET_TRACE,
+ * ptrace may modify regs->regs[0] while orig_x0 remains unchanged.
+ * The kernel then re-evaluates the filter; if it sees the stale
+ * orig_x0, it may incorrectly allow a syscall that the tracer intended
+ * to block.
+ *
+ * This test installs a filter that:
+ * - TRACEs write() when fd == 2
+ * - returns ERRNO(EPERM) when fd == 1
+ * - allows all other syscalls
+ *
+ * The child calls write(2, ...). The parent catches the SECCOMP stop,
+ * changes x0 (fd) from 2 to 1, and resumes the child.
+ *
+ * If re-evaluation sees the old fd=2 (stale orig_x0), the filter
+ * returns TRACE again; because recheck_after_trace is true, the kernel
+ * allows the syscall to proceed. write(1, ...) succeeds, child exits 0.
+ * -> test FAIL (bypass detected).
+ *
+ * If re-evaluation sees the new fd=1 (synced orig_x0), the filter
+ * returns ERRNO(EPERM), write fails, child exits 1.
+ * -> test PASS (no bypass).
+ *
+ * No special privileges required beyond CAP_SYS_PTRACE.
+ */
+#include <errno.h>
+#include <signal.h>
+#include <stddef.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <unistd.h>
+#include <sys/prctl.h>
+#include <sys/ptrace.h>
+#include <sys/uio.h>
+#include <sys/wait.h>
+#include <linux/elf.h>
+#include <linux/filter.h>
+#include <linux/seccomp.h>
+#include <linux/ptrace.h>
+
+#include "kselftest.h"
+
+#ifndef __NR_write
+#define __NR_write 64
+#endif
+
+#define PTRACE_EVENT_MASK(status) ((status) >> 16)
+
+static int do_child(void)
+{
+ long ret;
+
+ if (ptrace(PTRACE_TRACEME, 0, NULL, NULL))
+ _exit(2);
+
+ raise(SIGSTOP); /* synchronize with parent */
+
+ /*
+ * Filter:
+ * if syscall == write:
+ * if fd == 2 -> TRACE
+ * if fd == 1 -> ERRNO(EPERM)
+ * else -> ALLOW
+ * else -> ALLOW
+ */
+ struct sock_filter filter[] = {
+ /* Load syscall number */
+ BPF_STMT(BPF_LD | BPF_W | BPF_ABS,
+ offsetof(struct seccomp_data, nr)),
+ /* If not write, allow */
+ BPF_JUMP(BPF_JMP | BPF_JEQ | BPF_K, __NR_write, 0, 5),
+ /* Load first argument (fd) */
+ BPF_STMT(BPF_LD | BPF_W | BPF_ABS,
+ offsetof(struct seccomp_data, args[0])),
+ /* fd == 2 ? */
+ BPF_JUMP(BPF_JMP | BPF_JEQ | BPF_K, 2, 0, 2),
+ /* Yes: TRACE */
+ BPF_STMT(BPF_RET | BPF_K, SECCOMP_RET_TRACE),
+ /* fd == 1 ? */
+ BPF_JUMP(BPF_JMP | BPF_JEQ | BPF_K, 1, 0, 1),
+ /* Yes: ERRNO(EPERM) */
+ BPF_STMT(BPF_RET | BPF_K, SECCOMP_RET_ERRNO | (EPERM & SECCOMP_RET_DATA)),
+ /* Other fd: ALLOW */
+ BPF_STMT(BPF_RET | BPF_K, SECCOMP_RET_ALLOW),
+ };
+
+ struct sock_fprog prog = {
+ .len = ARRAY_SIZE(filter),
+ .filter = filter,
+ };
+
+ if (prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0))
+ _exit(3);
+ if (prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &prog))
+ _exit(4);
+
+ /*
+ * write(2, ...) triggers TRACE, parent changes fd to 1.
+ * If re-eval sees fd=1 -> ERRNO -> write fails, ret = -EPERM.
+ * If re-eval sees fd=2 -> TRACE again -> allowed -> write succeeds.
+ */
+ ret = syscall(__NR_write, 2, "", 0);
+ _exit(ret == 0 ? 0 : 1);
+}
+
+int main(void)
+{
+ struct user_pt_regs regs;
+ struct iovec iov = { .iov_base = ®s, .iov_len = sizeof(regs) };
+ pid_t child;
+ int status;
+
+ ksft_print_header();
+ ksft_set_plan(1);
+
+ child = fork();
+ if (!child)
+ return do_child();
+
+ /* 1. Wait for initial SIGSTOP */
+ if (waitpid(child, &status, 0) != child)
+ ksft_exit_fail_msg("waitpid SIGSTOP");
+ if (!WIFSTOPPED(status) || WSTOPSIG(status) != SIGSTOP)
+ ksft_exit_fail_msg("unexpected initial stop");
+
+ /* 2. Enable SECCOMP ptrace events */
+ if (ptrace(PTRACE_SETOPTIONS, child, 0, PTRACE_O_TRACESECCOMP))
+ ksft_exit_fail_msg("PTRACE_SETOPTIONS");
+
+ /* 3. Continue child to hit SECCOMP stop */
+ if (ptrace(PTRACE_CONT, child, 0, 0))
+ ksft_exit_fail_msg("PTRACE_CONT");
+
+ /* 4. Wait for SECCOMP stop */
+ while (1) {
+ if (waitpid(child, &status, 0) != child)
+ ksft_exit_fail_msg("waitpid SECCOMP");
+ if (WIFEXITED(status)) {
+ ksft_test_result_fail("child exited before SECCOMP stop\n");
+ goto out;
+ }
+ if (WIFSIGNALED(status)) {
+ ksft_test_result_fail("child killed unexpectedly\n");
+ goto out;
+ }
+ if (WIFSTOPPED(status) &&
+ WSTOPSIG(status) == SIGTRAP &&
+ PTRACE_EVENT_MASK(status) == PTRACE_EVENT_SECCOMP)
+ break;
+ ptrace(PTRACE_CONT, child, 0, WSTOPSIG(status));
+ }
+
+ /* 5. Modify x0 (fd) from 2 to 1 */
+ if (ptrace(PTRACE_GETREGSET, child, NT_PRSTATUS, &iov))
+ ksft_exit_fail_perror("GETREGSET");
+ if (regs.regs[8] != __NR_write || regs.regs[0] != 2) {
+ ksft_test_result_fail("unexpected regs: syscall=%llu, x0=%llu\n",
+ regs.regs[8], regs.regs[0]);
+ goto out;
+ }
+ regs.regs[0] = 1;
+ if (ptrace(PTRACE_SETREGSET, child, NT_PRSTATUS, &iov))
+ ksft_exit_fail_perror("SETREGSET");
+
+ /* 6. Resume child */
+ if (ptrace(PTRACE_CONT, child, 0, 0))
+ ksft_exit_fail_perror("PTRACE_CONT");
+
+ /* 7. Reap child – must exit normally */
+ if (waitpid(child, &status, 0) != child)
+ ksft_exit_fail_msg("final waitpid");
+
+ if (!WIFEXITED(status)) {
+ ksft_test_result_fail("child did not exit normally\n");
+ goto out;
+ }
+
+ if (WEXITSTATUS(status) != 0)
+ ksft_test_result_pass("seccomp correctly denied modified syscall\n");
+ else
+ ksft_test_result_fail("write succeeded, orig_x0 bypass likely\n");
+
+out:
+ if (child > 0) {
+ kill(child, SIGKILL);
+ waitpid(child, NULL, 0);
+ }
+ ksft_print_cnts();
+ return ksft_get_fail_cnt() ? EXIT_FAILURE : EXIT_SUCCESS;
+}
--
2.34.1
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