Quoting Eric W. Biederman (ebiederm@xxxxxxxxxxxx):
>
> Kenton Varda <kenton@xxxxxxxxxxxx> discovered that by remounting a
> read-only bind mount read-only in a user namespace the
> MNT_LOCK_READONLY bit would be cleared, allowing an unprivileged user
> to the remount a read-only mount read-write.
>
> Upon review of the code in remount it was discovered that the code allowed
> nosuid, noexec, and nodev to be cleared. It was also discovered that
> the code was allowing the per mount atime flags to be changed.
>
> The first naive patch to fix these issues contained the flaw that using
> default atime settings when remounting a filesystem could be disallowed.
>
> To avoid this problems in the future add tests to ensure unprivileged
> remounts are succeeding and failing at the appropriate times.
>
> Cc: stable@xxxxxxxxxxxxxxx
one nit below
Acked-by: Serge E. Hallyn <serge.hallyn@xxxxxxxxxx>
> Signed-off-by: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
> ---
> tools/testing/selftests/Makefile | 1 +
> tools/testing/selftests/mount/Makefile | 17 ++
> .../selftests/mount/unprivileged-remount-test.c | 242 +++++++++++++++++++++
> 3 files changed, 260 insertions(+)
> create mode 100644 tools/testing/selftests/mount/Makefile
> create mode 100644 tools/testing/selftests/mount/unprivileged-remount-test.c
>
> diff --git a/tools/testing/selftests/Makefile b/tools/testing/selftests/Makefile
> index e66e710cc595..0a8a9db43d34 100644
> --- a/tools/testing/selftests/Makefile
> +++ b/tools/testing/selftests/Makefile
> @@ -4,6 +4,7 @@ TARGETS += efivarfs
> TARGETS += kcmp
> TARGETS += memory-hotplug
> TARGETS += mqueue
> +TARGETS += mount
> TARGETS += net
> TARGETS += ptrace
> TARGETS += timers
> diff --git a/tools/testing/selftests/mount/Makefile b/tools/testing/selftests/mount/Makefile
> new file mode 100644
> index 000000000000..337d853c2b72
> --- /dev/null
> +++ b/tools/testing/selftests/mount/Makefile
> @@ -0,0 +1,17 @@
> +# Makefile for mount selftests.
> +
> +all: unprivileged-remount-test
> +
> +unprivileged-remount-test: unprivileged-remount-test.c
> + gcc -Wall -O2 unprivileged-remount-test.c -o unprivileged-remount-test
> +
> +# Allow specific tests to be selected.
> +test_unprivileged_remount: unprivileged-remount-test
> + @if [ -f /proc/self/uid_map ] ; then ./unprivileged-remount-test ; fi
> +
> +run_tests: all test_unprivileged_remount
> +
> +clean:
> + rm -f unprivileged-remount-test
> +
> +.PHONY: all test_unprivileged_remount
> diff --git a/tools/testing/selftests/mount/unprivileged-remount-test.c b/tools/testing/selftests/mount/unprivileged-remount-test.c
> new file mode 100644
> index 000000000000..c165752e66f6
> --- /dev/null
> +++ b/tools/testing/selftests/mount/unprivileged-remount-test.c
> @@ -0,0 +1,242 @@
> +#define _GNU_SOURCE
> +#include <sched.h>
> +#include <stdio.h>
> +#include <errno.h>
> +#include <string.h>
> +#include <sys/types.h>
> +#include <sys/mount.h>
> +#include <sys/wait.h>
> +#include <stdlib.h>
> +#include <unistd.h>
> +#include <fcntl.h>
> +#include <grp.h>
> +#include <stdbool.h>
> +#include <stdarg.h>
> +
> +#ifndef CLONE_NEWSNS
Could cause build error in some places... missspelled NEW S NS above.
> +# define CLONE_NEWNS 0x00020000
> +#endif
> +#ifndef CLONE_NEWUTS
> +# define CLONE_NEWUTS 0x04000000
> +#endif
> +#ifndef CLONE_NEWIPC
> +# define CLONE_NEWIPC 0x08000000
> +#endif
> +#ifndef CLONE_NEWNET
> +# define CLONE_NEWNET 0x40000000
> +#endif
> +#ifndef CLONE_NEWUSER
> +# define CLONE_NEWUSER 0x10000000
> +#endif
> +#ifndef CLONE_NEWPID
> +# define CLONE_NEWPID 0x20000000
> +#endif
> +
> +#ifndef MS_RELATIME
> +#define MS_RELATIME (1 << 21)
> +#endif
> +#ifndef MS_STRICTATIME
> +#define MS_STRICTATIME (1 << 24)
> +#endif
> +
> +static void die(char *fmt, ...)
> +{
> + va_list ap;
> + va_start(ap, fmt);
> + vfprintf(stderr, fmt, ap);
> + va_end(ap);
> + exit(EXIT_FAILURE);
> +}
> +
> +static void write_file(char *filename, char *fmt, ...)
> +{
> + char buf[4096];
> + int fd;
> + ssize_t written;
> + int buf_len;
> + va_list ap;
> +
> + va_start(ap, fmt);
> + buf_len = vsnprintf(buf, sizeof(buf), fmt, ap);
> + va_end(ap);
> + if (buf_len < 0) {
> + die("vsnprintf failed: %s\n",
> + strerror(errno));
> + }
> + if (buf_len >= sizeof(buf)) {
> + die("vsnprintf output truncated\n");
> + }
> +
> + fd = open(filename, O_WRONLY);
> + if (fd < 0) {
> + die("open of %s failed: %s\n",
> + filename, strerror(errno));
> + }
> + written = write(fd, buf, buf_len);
> + if (written != buf_len) {
> + if (written >= 0) {
> + die("short write to %s\n", filename);
> + } else {
> + die("write to %s failed: %s\n",
> + filename, strerror(errno));
> + }
> + }
> + if (close(fd) != 0) {
> + die("close of %s failed: %s\n",
> + filename, strerror(errno));
> + }
> +}
> +
> +static void create_and_enter_userns(void)
> +{
> + uid_t uid;
> + gid_t gid;
> +
> + uid = getuid();
> + gid = getgid();
> +
> + if (unshare(CLONE_NEWUSER) !=0) {
> + die("unshare(CLONE_NEWUSER) failed: %s\n",
> + strerror(errno));
> + }
> +
> + write_file("/proc/self/uid_map", "0 %d 1", uid);
> + write_file("/proc/self/gid_map", "0 %d 1", gid);
> +
> + if (setgroups(0, NULL) != 0) {
> + die("setgroups failed: %s\n",
> + strerror(errno));
> + }
> + if (setgid(0) != 0) {
> + die ("setgid(0) failed %s\n",
> + strerror(errno));
> + }
> + if (setuid(0) != 0) {
> + die("setuid(0) failed %s\n",
> + strerror(errno));
> + }
> +}
> +
> +static
> +bool test_unpriv_remount(int mount_flags, int remount_flags, int invalid_flags)
> +{
> + pid_t child;
> +
> + child = fork();
> + if (child == -1) {
> + die("fork failed: %s\n",
> + strerror(errno));
> + }
> + if (child != 0) { /* parent */
> + pid_t pid;
> + int status;
> + pid = waitpid(child, &status, 0);
> + if (pid == -1) {
> + die("waitpid failed: %s\n",
> + strerror(errno));
> + }
> + if (pid != child) {
> + die("waited for %d got %d\n",
> + child, pid);
> + }
> + if (!WIFEXITED(status)) {
> + die("child did not terminate cleanly\n");
> + }
> + return WEXITSTATUS(status) == EXIT_SUCCESS ? true : false;
> + }
> +
> + create_and_enter_userns();
> + if (unshare(CLONE_NEWNS) != 0) {
> + die("unshare(CLONE_NEWNS) failed: %s\n",
> + strerror(errno));
> + }
> +
> + if (mount("testing", "/tmp", "ramfs", mount_flags, NULL) != 0) {
> + die("mount of /tmp failed: %s\n",
> + strerror(errno));
> + }
> +
> + create_and_enter_userns();
> +
> + if (unshare(CLONE_NEWNS) != 0) {
> + die("unshare(CLONE_NEWNS) failed: %s\n",
> + strerror(errno));
> + }
> +
> + if (mount("/tmp", "/tmp", "none",
> + MS_REMOUNT | MS_BIND | remount_flags, NULL) != 0) {
> + /* system("cat /proc/self/mounts"); */
> + die("remount of /tmp failed: %s\n",
> + strerror(errno));
> + }
> +
> + if (mount("/tmp", "/tmp", "none",
> + MS_REMOUNT | MS_BIND | invalid_flags, NULL) == 0) {
> + /* system("cat /proc/self/mounts"); */
> + die("remount of /tmp with invalid flags "
> + "succeeded unexpectedly\n");
> + }
> + exit(EXIT_SUCCESS);
> +}
> +
> +static bool test_unpriv_remount_simple(int mount_flags)
> +{
> + return test_unpriv_remount(mount_flags, mount_flags, 0);
> +}
> +
> +static bool test_unpriv_remount_atime(int mount_flags, int invalid_flags)
> +{
> + return test_unpriv_remount(mount_flags, mount_flags, invalid_flags);
> +}
> +
> +int main(int argc, char **argv)
> +{
> + if (!test_unpriv_remount_simple(MS_RDONLY|MS_NODEV)) {
> + die("MS_RDONLY malfunctions\n");
> + }
> + if (!test_unpriv_remount_simple(MS_NODEV)) {
> + die("MS_NODEV malfunctions\n");
> + }
> + if (!test_unpriv_remount_simple(MS_NOSUID|MS_NODEV)) {
> + die("MS_NOSUID malfunctions\n");
> + }
> + if (!test_unpriv_remount_simple(MS_NOEXEC|MS_NODEV)) {
> + die("MS_NOEXEC malfunctions\n");
> + }
> + if (!test_unpriv_remount_atime(MS_RELATIME|MS_NODEV,
> + MS_NOATIME|MS_NODEV))
> + {
> + die("MS_RELATIME malfunctions\n");
> + }
> + if (!test_unpriv_remount_atime(MS_STRICTATIME|MS_NODEV,
> + MS_NOATIME|MS_NODEV))
> + {
> + die("MS_STRICTATIME malfunctions\n");
> + }
> + if (!test_unpriv_remount_atime(MS_NOATIME|MS_NODEV,
> + MS_STRICTATIME|MS_NODEV))
> + {
> + die("MS_RELATIME malfunctions\n");
> + }
> + if (!test_unpriv_remount_atime(MS_RELATIME|MS_NODIRATIME|MS_NODEV,
> + MS_NOATIME|MS_NODEV))
> + {
> + die("MS_RELATIME malfunctions\n");
> + }
> + if (!test_unpriv_remount_atime(MS_STRICTATIME|MS_NODIRATIME|MS_NODEV,
> + MS_NOATIME|MS_NODEV))
> + {
> + die("MS_RELATIME malfunctions\n");
> + }
> + if (!test_unpriv_remount_atime(MS_NOATIME|MS_NODIRATIME|MS_NODEV,
> + MS_STRICTATIME|MS_NODEV))
> + {
> + die("MS_RELATIME malfunctions\n");
> + }
> + if (!test_unpriv_remount(MS_STRICTATIME|MS_NODEV, MS_NODEV,
> + MS_NOATIME|MS_NODEV))
> + {
> + die("Default atime malfunctions\n");
> + }
> + return EXIT_SUCCESS;
> +}
> --
> 1.9.1
>
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