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charset="us-ascii" Content-Transfer-Encoding: 7bit Errors-To: ltp-bounces+ltp=archiver.kernel.org@lists.linux.it Sender: "ltp" From: Andrea Cervesato Verify that unprivileged fchroot() into failfs is refused with a shared fs_struct: a CLONE_FS sibling without no_new_privs could execute a setuid binary with the failfs root, so entry requires fs->users == 1, the same restriction setns() applies for the mount namespace. The sibling bumps fs->users to 2, making the syscall fail with EINVAL. Signed-off-by: Andrea Cervesato --- runtest/staging | 1 + testcases/kernel/syscalls/fchroot/.gitignore | 1 + testcases/kernel/syscalls/fchroot/fchroot12.c | 85 +++++++++++++++++++++++++++ 3 files changed, 87 insertions(+) diff --git a/runtest/staging b/runtest/staging index 137e3f99a..1655b6813 100644 --- a/runtest/staging +++ b/runtest/staging @@ -11,3 +11,4 @@ fchroot08 fchroot08 fchroot09 fchroot09 fchroot10 fchroot10 fchroot11 fchroot11 +fchroot12 fchroot12 diff --git a/testcases/kernel/syscalls/fchroot/.gitignore b/testcases/kernel/syscalls/fchroot/.gitignore index 0eca20548..de8ca0688 100644 --- a/testcases/kernel/syscalls/fchroot/.gitignore +++ b/testcases/kernel/syscalls/fchroot/.gitignore @@ -9,3 +9,4 @@ fchroot08 fchroot09 fchroot10 fchroot11 +fchroot12 diff --git a/testcases/kernel/syscalls/fchroot/fchroot12.c b/testcases/kernel/syscalls/fchroot/fchroot12.c new file mode 100644 index 000000000..c08144ecc --- /dev/null +++ b/testcases/kernel/syscalls/fchroot/fchroot12.c @@ -0,0 +1,85 @@ +// SPDX-License-Identifier: GPL-2.0-or-later +/* + * Copyright (C) 2026 SUSE LLC Andrea Cervesato + */ + +/*\ + * Test that unprivileged :manpage:`fchroot(2)` into failfs is refused + * with a shared ``fs_struct``. + * + * ``no_new_privs`` is checked on the calling thread, but the root lives in + * the fs_struct shared with a :manpage:`clone(2)` ``CLONE_FS`` sibling. A + * sibling without no_new_privs could execute a setuid binary with the + * failfs root, so entry requires ``fs->users == 1``, the same restriction + * :manpage:`setns(2)` applies for the mount namespace. The sibling bumps + * ``fs->users`` to 2, making the syscall fail with ``EINVAL``. + * + * Root is required to drop to an unprivileged user in the forked child. + */ + +#define _GNU_SOURCE +#include +#include +#include +#include +#include +#include "tst_test.h" +#include "lapi/fcntl.h" +#include "lapi/prctl.h" +#include "lapi/sched.h" +#include "lapi/syscalls.h" + +static struct passwd *ltpuser; + +static void run(void) +{ + if (SAFE_FORK()) + return; + + struct tst_clone_args args = { + .flags = CLONE_FS, + .exit_signal = SIGCHLD, + }; + pid_t parent = getpid(); + pid_t sib; + + SAFE_SETRESUID(ltpuser->pw_uid, ltpuser->pw_uid, + ltpuser->pw_uid); + + sib = SAFE_CLONE(&args); + if (!sib) { + /* + * Parked sibling sharing the fs_struct with the + * caller. It must die with the parent, otherwise it + * would outlive the test. + */ + SAFE_PRCTL(PR_SET_PDEATHSIG, SIGKILL, 0, 0, 0); + if (getppid() == parent) + pause(); + exit(0); + } + + SAFE_PRCTL(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0); + + TST_EXP_FAIL(tst_syscall(__NR_fchroot, FD_FAILFS_ROOT, 0), + EINVAL, "fchroot() with a shared fs_struct"); + + SAFE_KILL(sib, SIGKILL); + SAFE_WAITPID(sib, NULL, 0); + + exit(0); +} + +static void setup(void) +{ + clone3_supported_by_kernel(); + + ltpuser = SAFE_GETPWNAM("nobody"); +} + +static struct tst_test test = { + .setup = setup, + .test_all = run, + .needs_root = 1, + .forks_child = 1, +}; -- 2.51.0 -- Mailing list info: https://lists.linux.it/listinfo/ltp