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lxc-attach: rework pty allocation

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Previously we implemented two ways to get a pty for lxc-attach:
	1. get a pty in the container
	2. get a pty on the host

Where 1. was the default and 2. was only tried after 1. failed.
For safety and simplicity reasons, we remove 1. and just keep 2. around.

Signed-off-by: Christian Brauner <christian.brauner@mailbox.org>
This commit is contained in:
Christian Brauner 2016-02-25 15:22:53 +01:00
parent fa79f0a4e3
commit 478dda766a
2 changed files with 20 additions and 179 deletions
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19
doc/lxc-attach.sgml.in
View File

@ -80,20 +80,17 @@ Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
</para>
<para>
Previous versions of <command>lxc-attach</command> simply attached to the
specified namespaces of a container and ran a shell or the specified
command without allocating a pseudo terminal. This made them vulnerable to
specified namespaces of a container and ran a shell or the specified command
without first allocating a pseudo terminal. This made them vulnerable to
input faking via a TIOCSTI <command>ioctl</command> call after switching
between userspace execution contexts with different privilege levels. Newer
versions of <command>lxc-attach</command> will try to allocate a pseudo
terminal master/slave pair and attach any standard file descriptors which
refer to a terminal to the slave side of the pseudo terminal before
executing a shell or command. <command>lxc-attach</command> will first try
to allocate a pseudo terminal in the container. Should this fail it will try
to allocate a pseudo terminal on the host before finally giving up. Note,
that if none of the standard file descriptors refer to a terminal
<command>lxc-attach</command> will not try to allocate a pseudo terminal.
Instead it will simply attach to the containers namespaces and run a shell
or the specified command.
terminal master/slave pair on the host and attach any standard file
descriptors which refer to a terminal to the slave side of the pseudo
terminal before executing a shell or command. Note, that if none of the
standard file descriptors refer to a terminal <command>lxc-attach</command>
will not try to allocate a pseudo terminal. Instead it will simply attach
to the containers namespaces and run a shell or the specified command.
</para>
</refsect1>

180
src/lxc/lxc_attach.c
View File

@ -205,7 +205,6 @@ Options :\n\
struct wrapargs {
lxc_attach_options_t *options;
lxc_attach_command_t *command;
struct lxc_tty_state *ts;
struct lxc_console *console;
int ptyfd;
};
@ -223,162 +222,7 @@ static int login_pty(int fd)
return 0;
}
/* Minimalistic forkpty() implementation. */
static pid_t fork_pty(int *masterfd)
{
int master, slave;
int ret = openpty(&master, &slave, NULL, NULL, NULL);
if (ret < 0)
return -1;
pid_t pid = fork();
if (pid < 0) {
close(master);
close(slave);
return -1;
} else if (pid == 0) {
close(master);
if (login_pty(slave) < 0)
_exit(-1); /* closes fds */
return 0;
} else {
*masterfd = master;
close(slave);
return pid;
}
}
/*
* This is probably redundant but just so that intentions can be checked against
* code for future modifications. Here is what this is supposed to achieve:
* Since we fork() in pty_in_container() the shell that is run on the slave side
* of the pty is the grandchild of c->attach() in main(). But what we probably
* are interested in is the exit code of the grandchild. So we wait for the
* grandchild to change status and return its exit code from this function. We
* thereby make the exit code of the grandchild the exit code of the child and
* allow the grandparent to see the exit code of the grandchild by waiting on
* the pid of the child to change status:
*
* grandchild: lxc_attach_run_command()/lxc_attach_run_shell()
*
* child: pty_in_container()
* - perform waitpid() on pid returned by lxc_attach_run_command() or
* lxc_attach_run_shell()
*
* grandparent: c->attach()
* - return pid of pty_in_container() in pid argument.
*
* main()
* - perform waitpid() on pid returned in pid argument of c->attach()
*/
static int pty_in_container(void *p)
{
int ret;
int pid = -1;
int master = -1;
struct wrapargs *args = p;
INFO("Trying to allocate a pty in the container.");
if (!isatty(args->ptyfd)) {
ERROR("stdin is not a tty");
return -1;
}
/* Get termios from one of the stdfds. */
struct termios oldtios;
ret = lxc_setup_tios(args->ptyfd, &oldtios);
if (ret < 0)
return -1;
/* Create master/slave fd pair for pty. */
pid = fork_pty(&master);
if (pid < 0)
goto err1;
/* Pass windowsize from one of the stdfds to the current masterfd. */
lxc_console_winsz(args->ptyfd, master);
/* Run shell/command on slave side of pty. */
if (pid == 0) {
if (args->command->program)
lxc_attach_run_command(args->command);
else
lxc_attach_run_shell(NULL);
return -1;
}
/*
* For future reference: Must use process_lock() when called from a
* threaded context.
*/
args->ts = lxc_console_sigwinch_init(args->ptyfd, master);
if (!args->ts) {
ret = -1;
goto err2;
}
args->ts->escape = -1;
args->ts->stdoutfd = STDOUT_FILENO;
args->ts->winch_proxy = NULL;
struct lxc_epoll_descr descr;
ret = lxc_mainloop_open(&descr);
if (ret) {
ERROR("failed to create mainloop");
goto err3;
}
/* Register sigwinch handler in mainloop. */
ret = lxc_mainloop_add_handler(&descr, args->ts->sigfd,
lxc_console_cb_sigwinch_fd, args->ts);
if (ret) {
ERROR("failed to add handler for SIGWINCH fd");
goto err4;
}
/* Register i/o callbacks in mainloop. */
ret = lxc_mainloop_add_handler(&descr, args->ts->stdinfd,
lxc_console_cb_tty_stdin, args->ts);
if (ret) {
ERROR("failed to add handler for stdinfd");
goto err4;
}
ret = lxc_mainloop_add_handler(&descr, args->ts->masterfd,
lxc_console_cb_tty_master, args->ts);
if (ret) {
ERROR("failed to add handler for masterfd");
goto err4;
}
ret = lxc_mainloop(&descr, -1);
if (ret) {
ERROR("mainloop returned an error");
goto err4;
}
err4:
lxc_mainloop_close(&descr);
err3:
lxc_console_sigwinch_fini(args->ts);
err2:
close(args->ts->masterfd);
err1:
tcsetattr(args->ptyfd, TCSAFLUSH, &oldtios);
ret = lxc_wait_for_pid_status(pid);
if (ret < 0)
return ret;
if (WIFEXITED(ret))
return WEXITSTATUS(ret);
return -1;
}
static int pty_on_host_callback(void *p)
static int get_pty_on_host_callback(void *p)
{
struct wrapargs *wrap = p;
@ -393,7 +237,7 @@ static int pty_on_host_callback(void *p)
return -1;
}
static int pty_on_host(struct lxc_container *c, struct wrapargs *wrap, int *pid)
static int get_pty_on_host(struct lxc_container *c, struct wrapargs *wrap, int *pid)
{
int ret = -1;
struct wrapargs *args = wrap;
@ -422,7 +266,7 @@ static int pty_on_host(struct lxc_container *c, struct wrapargs *wrap, int *pid)
/* Send wrapper function on its way. */
wrap->console = &conf->console;
if (c->attach(c, pty_on_host_callback, wrap, wrap->options, pid) < 0)
if (c->attach(c, get_pty_on_host_callback, wrap, wrap->options, pid) < 0)
goto err1;
close(conf->console.slave); /* Close slave side. */
@ -475,6 +319,7 @@ err1:
int main(int argc, char *argv[])
{
int ret = -1;
int wexit = 0;
pid_t pid;
lxc_attach_options_t attach_options = LXC_ATTACH_OPTIONS_DEFAULT;
lxc_attach_command_t command = (lxc_attach_command_t){.program = NULL};
@ -544,9 +389,7 @@ int main(int argc, char *argv[])
wrap.ptyfd = STDOUT_FILENO;
else if (isatty(STDERR_FILENO))
wrap.ptyfd = STDERR_FILENO;
ret = c->attach(c, pty_in_container, &wrap, &attach_options, &pid);
if (ret < 0)
ret = pty_on_host(c, &wrap, &pid);
ret = get_pty_on_host(c, &wrap, &pid);
} else {
if (my_args.argc > 1)
ret = c->attach(c, lxc_attach_run_command, &command, &attach_options, &pid);
@ -554,17 +397,18 @@ int main(int argc, char *argv[])
ret = c->attach(c, lxc_attach_run_shell, NULL, &attach_options, &pid);
}
lxc_container_put(c);
if (ret < 0)
exit(EXIT_FAILURE);
goto out;
ret = lxc_wait_for_pid_status(pid);
if (ret < 0)
exit(EXIT_FAILURE);
goto out;
if (WIFEXITED(ret))
return WEXITSTATUS(ret);
wexit = WEXITSTATUS(ret);
out:
lxc_container_put(c);
if (ret >= 0)
exit(wexit);
exit(EXIT_FAILURE);
}