Kees Cook <[email protected]> writes: > On Wed, Jul 30, 2025 at 06:14:43PM -0600, Abhinav Saxena wrote: >> TIOCSTI is a TTY ioctl command that allows inserting characters into >> the terminal input queue, making it appear as if the user typed those >> characters. This functionality has behavior that varies based on system >> configuration and process credentials. >> >> The dev.tty.legacy_tiocsti sysctl introduced in commit 83efeeeb3d04 >> (“tty: Allow TIOCSTI to be disabled”) controls TIOCSTI usage. When >> disabled, TIOCSTI requires CAP_SYS_ADMIN capability. >> >> The current implementation checks the current process’s credentials via >> capable(CAP_SYS_ADMIN), but does not validate against the file opener’s >> credentials stored in file->f_cred. This creates different behavior when >> file descriptors are passed between processes via SCM_RIGHTS. >> >> Add a test suite with 16 test variants using fixture variants to verify >> TIOCSTI behavior when dev.tty.legacy_tiocsti is enabled/disabled: >> >> - Basic TIOCSTI tests (8 variants): Direct testing with different >> capability and controlling terminal combinations >> - FD passing tests (8 variants): Test behavior when file descriptors >> are passed between processes with different capabilities >> >> The FD passing tests document this behavior - some tests show different >> results than expected based on file opener credentials, demonstrating >> that TIOCSTI uses current process credentials rather than file opener >> credentials. >> >> The tests validate proper enforcement of the legacy_tiocsti sysctl. Test >> implementation uses openpty(3) with TIOCSCTTY for isolated PTY >> environments. See tty_ioctl(4) for details on TIOCSTI behavior and >> security requirements. > > This is looking really nice! Notes below… > >> >> Signed-off-by: Abhinav Saxena <[email protected]> >> — >> To run all tests: >> $ sudo ./tools/testing/selftests/tty/tty_tiocsti_test >> >> Test Results: >> - PASSED: 13/16 tests >> - Different behavior: 3/16 tests (documenting credential checking behavior) >> >> All tests validated using: >> - scripts/checkpatch.pl –strict (clean output) >> - Functional testing on kernel v6.16-rc2 >> >> Changes in v3: >> - Replaced all printf() calls with TH_LOG() for proper test logging (Kees >> Cook) >> - Added struct __test_metadata parameter to helper functions >> - Moved common legacy_tiocsti availability check to FIXTURE_SETUP() >> - Implemented sysctl modification/restoration in FIXTURE_SETUP/TEARDOWN >> - Used openpty() with TIOCSCTTY for reliable PTY testing environment >> - Fixed child/parent synchronization in FD passing tests >> - Replaced manual _exit(1) handling with proper ASSERT statements >> - Switched // comments to /* */ format throughout >> - Expanded to 16 test variants using fixture variants >> - Enhanced error handling and test reliability >> - Link to v2: >> <https://lore.kernel.org/r/[email protected]> >> - Link to v1: >> <https://lore.kernel.org/r/[email protected]> >> >> References: >> - tty_ioctl(4) - documents TIOCSTI ioctl and capability requirements >> - openpty(3) - pseudo-terminal creation and management >> - commit 83efeeeb3d04 (“tty: Allow TIOCSTI to be disabled”) >> - Documentation/security/credentials.rst >> - <https://github.com/KSPP/linux/issues/156> >> - <https://lore.kernel.org/linux-hardening/Y0m9l52AKmw6Yxi1@hostpad/> >> - drivers/tty/Kconfig >> - Documentation/driver-api/tty/ >> — >> tools/testing/selftests/tty/Makefile | 6 +- >> tools/testing/selftests/tty/config | 1 + >> tools/testing/selftests/tty/tty_tiocsti_test.c | 650 >> +++++++++++++++++++++++++ >> 3 files changed, 656 insertions(+), 1 deletion(-) >> >> diff –git a/tools/testing/selftests/tty/Makefile >> b/tools/testing/selftests/tty/Makefile >> index 50d7027b2ae3..7f6fbe5a0cd5 100644 >> — a/tools/testing/selftests/tty/Makefile >> +++ b/tools/testing/selftests/tty/Makefile >> @@ -1,5 +1,9 @@ >> # SPDX-License-Identifier: GPL-2.0 >> CFLAGS = -O2 -Wall >> -TEST_GEN_PROGS := tty_tstamp_update >> +TEST_GEN_PROGS := tty_tstamp_update tty_tiocsti_test >> +LDLIBS += -lcap >> >> include ../lib.mk >> + >> +# Add libcap for TIOCSTI test >> +$(OUTPUT)/tty_tiocsti_test: LDLIBS += -lcap >> diff –git a/tools/testing/selftests/tty/config >> b/tools/testing/selftests/tty/config >> new file mode 100644 >> index 000000000000..c6373aba6636 >> — /dev/null >> +++ b/tools/testing/selftests/tty/config >> @@ -0,0 +1 @@ >> +CONFIG_LEGACY_TIOCSTI=y >> diff –git a/tools/testing/selftests/tty/tty_tiocsti_test.c >> b/tools/testing/selftests/tty/tty_tiocsti_test.c >> new file mode 100644 >> index 000000000000..1eafef6e36fa >> — /dev/null >> +++ b/tools/testing/selftests/tty/tty_tiocsti_test.c >> @@ -0,0 +1,650 @@ >> +// SPDX-License-Identifier: GPL-2.0 >> +/* >> + * TTY Tests - TIOCSTI >> + * >> + * Copyright © 2025 Abhinav Saxena <[email protected]> >> + */ >> + >> +#include <stdio.h> >> +#include <stdlib.h> >> +#include <unistd.h> >> +#include <fcntl.h> >> +#include <sys/ioctl.h> >> +#include <errno.h> >> +#include <stdbool.h> >> +#include <string.h> >> +#include <sys/socket.h> >> +#include <sys/wait.h> >> +#include <pwd.h> >> +#include <termios.h> >> +#include <grp.h> >> +#include <sys/capability.h> >> +#include <sys/prctl.h> >> +#include <pty.h> >> +#include <utmp.h> >> + >> +#include “../kselftest_harness.h” >> + >> +enum test_type { >> + TEST_PTY_TIOCSTI_BASIC, >> + TEST_PTY_TIOCSTI_FD_PASSING, >> + /* other tests cases such as serial may be added. */ >> +}; >> + >> +/* >> + * Test Strategy: >> + * - Basic tests: Use PTY with/without TIOCSCTTY (controlling terminal for >> + * current process) >> + * - FD passing tests: Child creates PTY, parent receives FD (demonstrates >> + * security issue) >> + * >> + * SECURITY VULNERABILITY DEMONSTRATION: >> + * FD passing tests show that TIOCSTI uses CURRENT process credentials, not >> + * opener credentials. This means privileged processes can be given FDs from >> + * unprivileged processes and successfully perform TIOCSTI operations that >> the >> + * unprivileged process couldn’t do directly. >> + * >> + * Attack scenario: >> + * 1. Unprivileged process opens TTY (direct TIOCSTI fails due to lack of >> + * privileges) >> + * 2. Unprivileged process passes FD to privileged process via SCM_RIGHTS >> + * 3. Privileged process can use TIOCSTI on the FD (succeeds due to its >> + * privileges) >> + * 4. Result: Effective privilege escalation via file descriptor passing >> + * >> + * This matches the kernel logic in tiocsti(): >> + * 1. if (!tty_legacy_tiocsti && !capable(CAP_SYS_ADMIN)) return -EIO; >> + * 2. if ((current->signal->tty != tty) && !capable(CAP_SYS_ADMIN)) >> + * return -EPERM; >> + * Note: Both checks use capable() on CURRENT process, not FD opener! >> + * >> + * If the file credentials were also checked along with the capable() checks >> + * then the results for FD pass tests would be consistent with the basic >> tests. >> + */ >> + >> +FIXTURE(tiocsti) >> +{ >> + int pty_master_fd; /* PTY - for basic tests */ >> + int pty_slave_fd; >> + bool has_pty; >> + bool initial_cap_sys_admin; >> + int original_legacy_tiocsti_setting; >> + bool can_modify_sysctl; >> +}; >> + >> +FIXTURE_VARIANT(tiocsti) >> +{ >> + const enum test_type test_type; >> + const bool controlling_tty; /* true=current->signal->tty `= tty */ >> + const int legacy_tiocsti; /* 0=restricted, 1=permissive */ >> + const bool requires_cap; /* true=with CAP_SYS_ADMIN, false=without */ >> + const int expected_success; /* 0=success, -EIO/-EPERM=specific error */ >> +}; >> + >> +/* >> + * Tests Controlling Terminal Variants (current->signal->tty =' tty) >> + * >> + * TIOCSTI Test Matrix: >> + * >> + * | legacy_tiocsti | CAP_SYS_ADMIN | Expected Result | Error | >> + * |—————-|—————|—————–|——-| >> + * | 1 (permissive) | true | SUCCESS | - | >> + * | 1 (permissive) | false | SUCCESS | - | >> + * | 0 (restricted) | true | SUCCESS | - | >> + * | 0 (restricted) | false | FAILURE | -EIO | >> + */ >> + >> +/* clang-format off */ >> +FIXTURE_VARIANT_ADD(tiocsti, basic_pty_permissive_withcap) { >> + .test_type = TEST_PTY_TIOCSTI_BASIC, >> + .controlling_tty = true, >> + .legacy_tiocsti = 1, >> + .requires_cap = true, >> + .expected_success = 0, >> +}; >> + >> +FIXTURE_VARIANT_ADD(tiocsti, basic_pty_permissive_nocap) { >> + .test_type = TEST_PTY_TIOCSTI_BASIC, >> + .controlling_tty = true, >> + .legacy_tiocsti = 1, >> + .requires_cap = false, >> + .expected_success = 0, >> +}; >> + >> +FIXTURE_VARIANT_ADD(tiocsti, basic_pty_restricted_withcap) { >> + .test_type = TEST_PTY_TIOCSTI_BASIC, >> + .controlling_tty = true, >> + .legacy_tiocsti = 0, >> + .requires_cap = true, >> + .expected_success = 0, >> +}; >> + >> +FIXTURE_VARIANT_ADD(tiocsti, basic_pty_restricted_nocap) { >> + .test_type = TEST_PTY_TIOCSTI_BASIC, >> + .controlling_tty = true, >> + .legacy_tiocsti = 0, >> + .requires_cap = false, >> + .expected_success = -EIO, /* FAILURE: legacy restriction */ >> +}; /* clang-format on */ >> + >> +/* >> + * Note for FD Passing Test Variants >> + * Since we’re testing the scenario where an unprivileged process pass an FD >> + * to a privileged one, .requires_cap here means the caps of the child >> process. >> + * Not the parent; parent would always be privileged. >> + */ >> + >> +/* clang-format off */ >> +FIXTURE_VARIANT_ADD(tiocsti, fdpass_pty_permissive_withcap) { >> + .test_type = TEST_PTY_TIOCSTI_FD_PASSING, >> + .controlling_tty = true, >> + .legacy_tiocsti = 1, >> + .requires_cap = true, >> + .expected_success = 0, >> +}; >> + >> +FIXTURE_VARIANT_ADD(tiocsti, fdpass_pty_permissive_nocap) { >> + .test_type = TEST_PTY_TIOCSTI_FD_PASSING, >> + .controlling_tty = true, >> + .legacy_tiocsti = 1, >> + .requires_cap = false, >> + .expected_success = 0, >> +}; >> + >> +FIXTURE_VARIANT_ADD(tiocsti, fdpass_pty_restricted_withcap) { >> + .test_type = TEST_PTY_TIOCSTI_FD_PASSING, >> + .controlling_tty = true, >> + .legacy_tiocsti = 0, >> + .requires_cap = true, >> + .expected_success = 0, >> +}; >> + >> +FIXTURE_VARIANT_ADD(tiocsti, fdpass_pty_restricted_nocap) { >> + .test_type = TEST_PTY_TIOCSTI_FD_PASSING, >> + .controlling_tty = true, >> + .legacy_tiocsti = 0, >> + .requires_cap = false, >> + .expected_success = -EIO, >> +}; /* clang-format on */ >> + >> +/* >> + * Non-Controlling Terminal Variants (current->signal->tty != tty) >> + * >> + * TIOCSTI Test Matrix: >> + * >> + * | legacy_tiocsti | CAP_SYS_ADMIN | Expected Result | Error | >> + * |—————-|—————|—————–|——-| >> + * | 1 (permissive) | true | SUCCESS | - | >> + * | 1 (permissive) | false | FAILURE | -EPERM| >> + * | 0 (restricted) | true | SUCCESS | - | >> + * | 0 (restricted) | false | FAILURE | -EIO | >> + */ >> + >> +/* clang-format off */ >> +FIXTURE_VARIANT_ADD(tiocsti, basic_nopty_permissive_withcap) { >> + .test_type = TEST_PTY_TIOCSTI_BASIC, >> + .controlling_tty = false, >> + .legacy_tiocsti = 1, >> + .requires_cap = true, >> + .expected_success = 0, >> +}; >> + >> +FIXTURE_VARIANT_ADD(tiocsti, basic_nopty_permissive_nocap) { >> + .test_type = TEST_PTY_TIOCSTI_BASIC, >> + .controlling_tty = false, >> + .legacy_tiocsti = 1, >> + .requires_cap = false, >> + .expected_success = -EPERM, >> +}; >> + >> +FIXTURE_VARIANT_ADD(tiocsti, basic_nopty_restricted_withcap) { >> + .test_type = TEST_PTY_TIOCSTI_BASIC, >> + .controlling_tty = false, >> + .legacy_tiocsti = 0, >> + .requires_cap = true, >> + .expected_success = 0, >> +}; >> + >> +FIXTURE_VARIANT_ADD(tiocsti, basic_nopty_restricted_nocap) { >> + .test_type = TEST_PTY_TIOCSTI_BASIC, >> + .controlling_tty = false, >> + .legacy_tiocsti = 0, >> + .requires_cap = false, >> + .expected_success = -EIO, >> +}; >> + >> +FIXTURE_VARIANT_ADD(tiocsti, fdpass_nopty_permissive_withcap) { >> + .test_type = TEST_PTY_TIOCSTI_FD_PASSING, >> + .controlling_tty = false, >> + .legacy_tiocsti = 1, >> + .requires_cap = true, >> + .expected_success = 0, >> +}; >> + >> +FIXTURE_VARIANT_ADD(tiocsti, fdpass_nopty_permissive_nocap) { >> + .test_type = TEST_PTY_TIOCSTI_FD_PASSING, >> + .controlling_tty = false, >> + .legacy_tiocsti = 1, >> + .requires_cap = false, >> + .expected_success = -EPERM, >> +}; >> + >> +FIXTURE_VARIANT_ADD(tiocsti, fdpass_nopty_restricted_withcap) { >> + .test_type = TEST_PTY_TIOCSTI_FD_PASSING, >> + .controlling_tty = false, >> + .legacy_tiocsti = 0, >> + .requires_cap = true, >> + .expected_success = 0, >> +}; >> + >> +FIXTURE_VARIANT_ADD(tiocsti, fdpass_nopty_restricted_nocap) { >> + .test_type = TEST_PTY_TIOCSTI_FD_PASSING, >> + .controlling_tty = false, >> + .legacy_tiocsti = 0, >> + .requires_cap = false, >> + .expected_success = -EIO, >> +}; /* clang-format on */ >> + >> +/* Helper function to send FD via SCM_RIGHTS */ >> +static int send_fd_via_socket(int socket_fd, int fd_to_send) >> +{ >> + struct msghdr msg = { 0 }; >> + struct cmsghdr *cmsg; >> + char cmsg_buf[CMSG_SPACE(sizeof(int))]; >> + char dummy_data = ’F’; >> + struct iovec iov = { .iov_base = &dummy_data, .iov_len = 1 }; >> + >> + msg.msg_iov = &iov; >> + msg.msg_iovlen = 1; >> + msg.msg_control = cmsg_buf; >> + msg.msg_controllen = sizeof(cmsg_buf); >> + >> + cmsg = CMSG_FIRSTHDR(&msg); >> + cmsg->cmsg_level = SOL_SOCKET; >> + cmsg->cmsg_type = SCM_RIGHTS; >> + cmsg->cmsg_len = CMSG_LEN(sizeof(int)); >> + >> + memcpy(CMSG_DATA(cmsg), &fd_to_send, sizeof(int)); >> + >> + return sendmsg(socket_fd, &msg, 0) < 0 ? -1 : 0; >> +} >> + >> +/* Helper function to receive FD via SCM_RIGHTS */ >> +static int recv_fd_via_socket(int socket_fd) >> +{ >> + struct msghdr msg = { 0 }; >> + struct cmsghdr *cmsg; >> + char cmsg_buf[CMSG_SPACE(sizeof(int))]; >> + char dummy_data; >> + struct iovec iov = { .iov_base = &dummy_data, .iov_len = 1 }; >> + int received_fd = -1; >> + >> + msg.msg_iov = &iov; >> + msg.msg_iovlen = 1; >> + msg.msg_control = cmsg_buf; >> + msg.msg_controllen = sizeof(cmsg_buf); >> + >> + if (recvmsg(socket_fd, &msg, 0) < 0) >> + return -1; >> + >> + for (cmsg = CMSG_FIRSTHDR(&msg); cmsg; cmsg = CMSG_NXTHDR(&msg, cmsg)) { >> + if (cmsg->cmsg_level `= SOL_SOCKET && >> + cmsg->cmsg_type =' SCM_RIGHTS) { >> + memcpy(&received_fd, CMSG_DATA(cmsg), sizeof(int)); >> + break; >> + } >> + } >> + >> + return received_fd; >> +} >> + >> +static inline bool has_cap_sys_admin(void) >> +{ >> + cap_t caps = cap_get_proc(); >> + >> + if (!caps) >> + return false; >> + >> + cap_flag_value_t cap_val; >> + bool has_cap = (cap_get_flag(caps, CAP_SYS_ADMIN, CAP_EFFECTIVE, >> + &cap_val) `= 0) && >> + (cap_val =' CAP_SET); >> + >> + cap_free(caps); >> + return has_cap; >> +} >> + >> +/* >> + * Drop to nobody user (uid/gid 65534) to lose all capabilities >> + */ >> +static inline bool drop_to_nobody(struct __test_metadata *_metadata) >> +{ >> + ASSERT_EQ(setgroups(0, NULL), 0); >> + ASSERT_EQ(setgid(65534), 0); >> + ASSERT_EQ(setuid(65534), 0); >> + >> + ASSERT_FALSE(has_cap_sys_admin()); >> + return true; >> +} >> + >> +static inline int get_legacy_tiocsti_setting(struct __test_metadata >> *_metadata) >> +{ >> + FILE *fp; >> + int value = -1; >> + >> + fp = fopen(“/proc/sys/dev/tty/legacy_tiocsti”, “r”); >> + if (!fp) { >> + /* legacy_tiocsti sysctl not available (kernel < 6.2) */ >> + return -1; >> + } >> + >> + if (fscanf(fp, “%d”, &value) == 1) { >> + if (value < 0 || value > 1) >> + value = -1; /* Invalid value */ >> + } else { >> + value = -1; /* Failed to parse */ >> + } >> + >> + fclose(fp); >> + return value; >> +} >> + >> +static inline bool set_legacy_tiocsti_setting(struct __test_metadata >> *_metadata, >> + int value) >> +{ >> + FILE *fp; >> + bool success = false; >> + >> + /* Sanity-check the value */ >> + ASSERT_GE(value, 0); >> + ASSERT_LE(value, 1); >> + >> + /* >> + * Try to open for writing; if we lack permission, return false so >> + * the test harness will skip variants that need to change it >> + */ >> + fp = fopen(“/proc/sys/dev/tty/legacy_tiocsti”, “w”); >> + if (!fp) >> + return false; >> + >> + /* Write the new setting */ >> + if (fprintf(fp, “%d\n”, value) > 0) >> + success = true; >> + else >> + TH_LOG(“Failed to write legacy_tiocsti: %s”, strerror(errno)); >> + >> + fclose(fp); >> + return success; > > It’s not very obvious, but actually some write failures are delayed > until the close. From the man page: > > ERRORS > … > The fclose() function may also fail and set errno for any of the > errors specified for the routines close(2), write(2), or fflush(3). > > You’ll need to check both the fprintf and the fclose. Probably as: > > if (fprintf(fp, “%d\n”, value) > 0 && fclose(fp)) > success = true; > else > TH_LOG(“Failed to write legacy_tiocsti: %s”, strerror(errno)); > return success; > >> +} >> + >> +/* >> + * TIOCSTI injection test function >> + * @tty_fd: TTY slave file descriptor to test TIOCSTI on >> + * Returns: 0 on success, -errno on failure >> + */ >> +static inline int test_tiocsti_injection(struct __test_metadata *_metadata, >> + int tty_fd) >> +{ >> + int ret; >> + char inject_char = ’V’; >> + >> + errno = 0; >> + ret = ioctl(tty_fd, TIOCSTI, &inject_char); >> + return ret == 0 ? 0 : -errno; >> +} >> + >> +FIXTURE_SETUP(tiocsti) >> +{ >> + /* Create PTY pair for basic tests */ >> + self->has_pty = (openpty(&self->pty_master_fd, &self->pty_slave_fd, >> + NULL, NULL, NULL) == 0); >> + if (!self->has_pty) { >> + self->pty_master_fd = -1; >> + self->pty_slave_fd = -1; >> + } >> + >> + self->initial_cap_sys_admin = has_cap_sys_admin(); >> + self->original_legacy_tiocsti_setting = >> + get_legacy_tiocsti_setting(_metadata); >> + >> + if (self->original_legacy_tiocsti_setting < 0) >> + SKIP(return, “legacy_tiocsti sysctl not available (kernel < >> 6.2)”); >> + >> + /* Test if we can modify the sysctl (requires appropriate privileges) */ >> + self->can_modify_sysctl = set_legacy_tiocsti_setting(_metadata, >> + >> self->original_legacy_tiocsti_setting); >> + if (!self->can_modify_sysctl) >> + TH_LOG(“Warning: Cannot modify legacy_tiocsti sysctl - will >> skip mismatched variants”); >> +} >> + >> +FIXTURE_TEARDOWN(tiocsti) >> +{ >> + /* >> + * Backup restoration - >> + * each test should restore its own sysctl changes >> + */ >> + if (self->can_modify_sysctl && >> + self->original_legacy_tiocsti_setting >= 0) { >> + int current_value = get_legacy_tiocsti_setting(_metadata); >> + >> + if (current_value != self->original_legacy_tiocsti_setting) { >> + TH_LOG(“Backup: Restoring legacy_tiocsti from %d to %d”, >> + current_value, >> + self->original_legacy_tiocsti_setting); >> + set_legacy_tiocsti_setting(_metadata, >> + self->original_legacy_tiocsti_setting); >> + } > > This “set the value if it’s different” logic here is mostly duplicated > below; probably better to have a helper for doing this. It can also be > aware of the value (since you read it during SETUP), so you may not need > the triple read (once in SETUP, once in test, once in TEARDOWN). Though > it doesn’t really hurt anything to do the read/check/write cycle here > either. Up to you! > >> + } >> + >> + if (self->has_pty) { >> + if (self->pty_master_fd >= 0) >> + close(self->pty_master_fd); >> + if (self->pty_slave_fd >= 0) >> + close(self->pty_slave_fd); >> + } >> +} >> + >> +TEST_F(tiocsti, test) >> +{ >> + int saved_legacy_tiocsti = get_legacy_tiocsti_setting(_metadata); >> + bool need_restore = false; >> + int status; >> + pid_t child_pid; >> + >> + /* Set legacy_tiocsti sysctl to match variant requirement */ >> + if (self->can_modify_sysctl) { >> + if (saved_legacy_tiocsti != variant->legacy_tiocsti) { >> + if (!set_legacy_tiocsti_setting(_metadata, >> + variant->legacy_tiocsti)) { >> + SKIP(return, >> + “Failed to set legacy_tiocsti sysctl”); >> + } >> + need_restore = true; > > You don’t need to handle the restore since TEARDOWN will do it, yes? > >> + } >> + } else { >> + /* >> + * Can’t modify sysctl >> + * - check if current value matches variant >> + */ >> + if (self->original_legacy_tiocsti_setting != >> + variant->legacy_tiocsti) { >> + SKIP(return, >> + “legacy_tiocsti setting mismatch and cannot modify >> sysctl”); >> + } > > I feel like both the set and this check should be part of SETUP instead? > All variants have a legacy_tiocsti setting, so better to put common > setup code in the SETUP. > >> + } >> + >> + /* Common skip conditions */ >> + if (variant->test_type `= TEST_PTY_TIOCSTI_BASIC && !self->has_pty) { >> + SKIP(goto restore_sysctl, >> + "PTY not available for controlling terminal test"); >> + } >> + >> + if (variant->test_type =' TEST_PTY_TIOCSTI_FD_PASSING && >> + !self->initial_cap_sys_admin) { >> + SKIP(goto restore_sysctl, >> + “FD Pass tests require CAP_SYS_ADMIN”); >> + } >> + >> + if (variant->requires_cap && !self->initial_cap_sys_admin) { >> + SKIP(goto restore_sysctl, >> + “Test requires initial CAP_SYS_ADMIN”); >> + } > > Same for all of these: they do a skip, which should work from SETUP. And > they can be done before sysctl changing, so they can just do a return. > (A skipped SETUP will, I think, still call TEARDOWN.) > >> + if (variant->test_type == TEST_PTY_TIOCSTI_BASIC) { > > variants within variants. ;) > > I would lift the fork logic out of the if/else, and then put the bodies > of each in a separate function (pass in the metadata and the pid). > That’ll make these more readable instead of be heavily indented. > > >> + /* `===' BASIC TIOCSTI TEST `===' */ >> + child_pid = fork(); >> + ASSERT_GE(child_pid, 0); >> + >> + if (child_pid == 0) { >> + /* Child process - perform the actual test */ >> + >> + /* Handle capability requirements */ >> + if (self->initial_cap_sys_admin && >> + !variant->requires_cap) >> + ASSERT_TRUE(drop_to_nobody(_metadata)); >> + >> + if (variant->controlling_tty) { >> + /* >> + * Create new session and set PTY as >> + * controlling terminal >> + */ >> + pid_t sid = setsid(); >> + >> + ASSERT_GE(sid, 0); >> + ASSERT_EQ(ioctl(self->pty_slave_fd, TIOCSCTTY, >> + 0), >> + 0); > > which avoids this kind of weird “oh no I’m almost at 80 characters” > stuff above. > >> + } >> + >> + /* >> + * Validate test environment setup and verify final >> + * capability state matches expectation >> + * after potential drop. >> + * >> + */ >> + ASSERT_TRUE(self->has_pty); >> + ASSERT_EQ(has_cap_sys_admin(), variant->requires_cap); >> + >> + /* Test TIOCSTI and validate result */ >> + int result = test_tiocsti_injection(_metadata, >> + self->pty_slave_fd); >> + >> + /* Check against expected result from variant */ >> + EXPECT_EQ(result, variant->expected_success); >> + _exit(0); >> + } >> + >> + } else { >> + /* `===' FD PASSING SECURITY TEST `===' */ >> + int sockpair[2]; >> + >> + ASSERT_EQ(socketpair(AF_UNIX, SOCK_STREAM, 0, sockpair), 0); >> + >> + child_pid = fork(); >> + ASSERT_GE(child_pid, 0); >> + >> + if (child_pid == 0) { >> + /* Child process - create PTY and send FD */ >> + close(sockpair[0]); >> + signal(SIGHUP, SIG_IGN); >> + >> + /* Handle privilege dropping */ >> + if (!variant->requires_cap && has_cap_sys_admin()) >> + ASSERT_TRUE(drop_to_nobody(_metadata)); >> + >> + /* Create child’s PTY */ >> + int child_master_fd, child_slave_fd; >> + >> + ASSERT_EQ(openpty(&child_master_fd, &child_slave_fd, >> + NULL, NULL, NULL), >> + 0); >> + >> + if (variant->controlling_tty) { >> + pid_t sid = setsid(); >> + >> + ASSERT_GE(sid, 0); >> + ASSERT_EQ(ioctl(child_slave_fd, TIOCSCTTY, 0), >> + 0); >> + } >> + >> + /* Test child’s direct TIOCSTI for reference */ >> + int direct_result = test_tiocsti_injection(_metadata, >> + >> child_slave_fd); >> + EXPECT_EQ(direct_result, variant->expected_success); >> + >> + /* Send FD to parent */ >> + ASSERT_EQ(send_fd_via_socket(sockpair[1], >> + child_slave_fd), >> + 0); >> + >> + /* Wait for parent completion signal */ >> + char sync_byte; >> + ssize_t bytes_read = read(sockpair[1], &sync_byte, 1); >> + >> + ASSERT_EQ(bytes_read, 1); >> + >> + close(child_master_fd); >> + close(child_slave_fd); >> + close(sockpair[1]); >> + _exit(0); >> + } >> + >> + /* Parent process - receive FD and test TIOCSTI */ >> + close(sockpair[1]); >> + >> + int received_fd = recv_fd_via_socket(sockpair[0]); >> + >> + ASSERT_GE(received_fd, 0); >> + >> + bool parent_has_cap = self->initial_cap_sys_admin; >> + >> + TH_LOG(“`=' TIOCSTI FD Passing Test Context `='”); >> + TH_LOG(“legacy_tiocsti: %d, Parent CAP_SYS_ADMIN: %s, Child: >> %s”, >> + variant->legacy_tiocsti, parent_has_cap ? “yes” : “no”, >> + variant->requires_cap ? “kept” : “dropped”); >> + >> + /* SECURITY TEST: Try TIOCSTI with FD opened by child */ >> + int result = test_tiocsti_injection(_metadata, received_fd); >> + >> + /* Log security concern if demonstrated */ >> + if (result == 0 && !variant->requires_cap) { >> + TH_LOG("*** SECURITY CONCERN DEMONSTRATED ***“); >> + TH_LOG(”Privileged parent can use TIOCSTI on FD from >> unprivileged child“); >> + TH_LOG(”This shows current process credentials are >> used, not opener credentials“); >> + } >> + >> + EXPECT_EQ(result, variant->expected_success) >> + { >> + TH_LOG(”FD passing: expected error %d, got %d“, >> + variant->expected_success, result); >> + } >> + >> + /* Signal child completion */ >> + char sync_byte = ‘D’; >> + ssize_t bytes_written = write(sockpair[0], &sync_byte, 1); >> + >> + ASSERT_EQ(bytes_written, 1); >> + >> + close(received_fd); >> + close(sockpair[0]); >> + } >> + >> + /* Common child process cleanup for both test types */ >> + ASSERT_EQ(waitpid(child_pid, &status, 0), child_pid); >> + >> + if (WIFSIGNALED(status)) { >> + TH_LOG(”Child terminated by signal %d“, WTERMSIG(status)); >> + ASSERT_FALSE(WIFSIGNALED(status)) >> + { >> + TH_LOG(”Child process failed assertion"); >> + } >> + } else { >> + EXPECT_EQ(WEXITSTATUS(status), 0); >> + } >> + >> +restore_sysctl: >> + if (need_restore) >> + set_legacy_tiocsti_setting(_metadata, saved_legacy_tiocsti); >> +} >> + >> +TEST_HARNESS_MAIN > > Thanks for chipping away at this! :) > > -Kees
Thanks for the feedback! I wasn’t sure if you could use fixture data inside setup and teardown. But apparently we can[1]. I have addressed the fclose() error handling, moved setup skip logic to FIXTURE_SETUP() and addressed all other changes in v4. -Abhinav [1] - tools/testing/selftests/kselftest_harness/harness-selftest.c
