Re: [PATCH 2/3] signal: Requeue signals in the appropriate queue

Kees Cook
2021-11-16T18:30:53Z

On Mon, Nov 15, 2021 at 11:33:19PM -0600, Eric W. Biederman wrote:
>
> In the event that a tracer changes which signal needs to be delivered
> and that signal is currently blocked then the signal needs to be
> requeued for later delivery.
>
> With the advent of CLONE_THREAD the kernel has 2 signal queues per
> task. The per process queue and the per task queue. Update the code
> so that if the signal is removed from the per process queue it is
> requeued on the per process queue. This is necessary to make it
> appear the signal was never dequeued.
>
> The rr debugger reasonably believes that the state of the process from
> the last ptrace_stop it observed until PTRACE_EVENT_EXIT can be recreated
> by simply letting a process run. If a SIGKILL interrupts a ptrace_stop
> this is not true today.
>
> So return signals to their original queue in ptrace_signal so that
> signals that are not delivered appear like they were never dequeued.

The only comment I have on this is that it seems like many callers
totally ignore the result store in "type" (signalfd_dequeue,
kernel_dequeue_signal, do_sigtimedwait), which would imply a different
API might be desirable. That said, it's also not a big deal.

>
> Fixes: 794aa320b79d ("[PATCH] sigfix-2.5.40-D6")
> History Tree: https://git.kernel.org/pub/scm/linux/kernel/git/tglx/history.gi
> Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>

Reviewed-by: Kees Cook <keescook@chromium.org>

> ---
> fs/signalfd.c | 5 +++--
> include/linux/sched/signal.h | 7 ++++---
> kernel/signal.c | 21 ++++++++++++++-------
> 3 files changed, 21 insertions(+), 12 deletions(-)
>
> diff --git a/fs/signalfd.c b/fs/signalfd.c
> index 040e1cf90528..74f134cd1ff6 100644
> --- a/fs/signalfd.c
> +++ b/fs/signalfd.c
> @@ -165,11 +165,12 @@ static int signalfd_copyinfo(struct signalfd_siginfo __user *uinfo,
> static ssize_t signalfd_dequeue(struct signalfd_ctx *ctx, kernel_siginfo_t *info,
> int nonblock)
> {
> + enum pid_type type;
> ssize_t ret;
> DECLARE_WAITQUEUE(wait, current);
>
> spin_lock_irq(&current->sighand->siglock);
> - ret = dequeue_signal(current, &ctx->sigmask, info);
> + ret = dequeue_signal(current, &ctx->sigmask, info, &type);
> switch (ret) {
> case 0:
> if (!nonblock)
> @@ -184,7 +185,7 @@ static ssize_t signalfd_dequeue(struct signalfd_ctx *ctx, kernel_siginfo_t *info
> add_wait_queue(&current->sighand->signalfd_wqh, &wait);
> for (;;) {
> set_current_state(TASK_INTERRUPTIBLE);
> - ret = dequeue_signal(current, &ctx->sigmask, info);
> + ret = dequeue_signal(current, &ctx->sigmask, info, &type);
> if (ret != 0)
> break;
> if (signal_pending(current)) {
> diff --git a/include/linux/sched/signal.h b/include/linux/sched/signal.h
> index 23505394ef70..167995d471da 100644
> --- a/include/linux/sched/signal.h
> +++ b/include/linux/sched/signal.h
> @@ -286,17 +286,18 @@ static inline int signal_group_exit(const struct signal_struct *sig)
> extern void flush_signals(struct task_struct *);
> extern void ignore_signals(struct task_struct *);
> extern void flush_signal_handlers(struct task_struct *, int force_default);
> -extern int dequeue_signal(struct task_struct *task,
> - sigset_t *mask, kernel_siginfo_t *info);
> +extern int dequeue_signal(struct task_struct *task, sigset_t *mask,
> + kernel_siginfo_t *info, enum pid_type *type);
>
> static inline int kernel_dequeue_signal(void)
> {
> struct task_struct *task = current;
> kernel_siginfo_t __info;
> + enum pid_type __type;
> int ret;
>
> spin_lock_irq(&task->sighand->siglock);
> - ret = dequeue_signal(task, &task->blocked, &__info);
> + ret = dequeue_signal(task, &task->blocked, &__info, &__type);
> spin_unlock_irq(&task->sighand->siglock);
>
> return ret;
> diff --git a/kernel/signal.c b/kernel/signal.c
> index 986fa69c15c5..43e8b7e362b0 100644
> --- a/kernel/signal.c
> +++ b/kernel/signal.c
> @@ -626,7 +626,8 @@ static int __dequeue_signal(struct sigpending *pending, sigset_t *mask,
> *
> * All callers have to hold the siglock.
> */
> -int dequeue_signal(struct task_struct *tsk, sigset_t *mask, kernel_siginfo_t *info)
> +int dequeue_signal(struct task_struct *tsk, sigset_t *mask,
> + kernel_siginfo_t *info, enum pid_type *type)
> {
> bool resched_timer = false;
> int signr;
> @@ -634,8 +635,10 @@ int dequeue_signal(struct task_struct *tsk, sigset_t *mask, kernel_siginfo_t *in
> /* We only dequeue private signals from ourselves, we don't let
> * signalfd steal them
> */
> + *type = PIDTYPE_PID;
> signr = __dequeue_signal(&tsk->pending, mask, info, &resched_timer);
> if (!signr) {
> + *type = PIDTYPE_TGID;
> signr = __dequeue_signal(&tsk->signal->shared_pending,
> mask, info, &resched_timer);
> #ifdef CONFIG_POSIX_TIMERS
> @@ -2522,7 +2525,7 @@ static void do_freezer_trap(void)
> freezable_schedule();
> }
>
> -static int ptrace_signal(int signr, kernel_siginfo_t *info)
> +static int ptrace_signal(int signr, kernel_siginfo_t *info, enum pid_type type)
> {
> /*
> * We do not check sig_kernel_stop(signr) but set this marker
> @@ -2563,7 +2566,7 @@ static int ptrace_signal(int signr, kernel_siginfo_t *info)
>
> /* If the (new) signal is now blocked, requeue it. */
> if (sigismember(&current->blocked, signr)) {
> - send_signal(signr, info, current, PIDTYPE_PID);
> + send_signal(signr, info, current, type);
> signr = 0;
> }
>
> @@ -2664,6 +2667,7 @@ bool get_signal(struct ksignal *ksig)
>
> for (;;) {
> struct k_sigaction *ka;
> + enum pid_type type;
>
> /* Has this task already been marked for death? */
> if (signal_group_exit(signal)) {
> @@ -2706,16 +2710,18 @@ bool get_signal(struct ksignal *ksig)
> * so that the instruction pointer in the signal stack
> * frame points to the faulting instruction.
> */
> + type = PIDTYPE_PID;
> signr = dequeue_synchronous_signal(&ksig->info);
> if (!signr)
> - signr = dequeue_signal(current, &current->blocked, &ksig->info);
> + signr = dequeue_signal(current, &current->blocked,
> + &ksig->info, &type);
>
> if (!signr)
> break; /* will return 0 */
>
> if (unlikely(current->ptrace) && (signr != SIGKILL) &&
> !(sighand->action[signr -1].sa.sa_flags & SA_IMMUTABLE)) {
> - signr = ptrace_signal(signr, &ksig->info);
> + signr = ptrace_signal(signr, &ksig->info, type);
> if (!signr)
> continue;
> }
> @@ -3540,6 +3546,7 @@ static int do_sigtimedwait(const sigset_t *which, kernel_siginfo_t *info,
> ktime_t *to = NULL, timeout = KTIME_MAX;
> struct task_struct *tsk = current;
> sigset_t mask = *which;
> + enum pid_type type;
> int sig, ret = 0;
>
> if (ts) {
> @@ -3556,7 +3563,7 @@ static int do_sigtimedwait(const sigset_t *which, kernel_siginfo_t *info,
> signotset(&mask);
>
> spin_lock_irq(&tsk->sighand->siglock);
> - sig = dequeue_signal(tsk, &mask, info);
> + sig = dequeue_signal(tsk, &mask, info, &type);
> if (!sig && timeout) {
> /*
> * None ready, temporarily unblock those we're interested
> @@ -3575,7 +3582,7 @@ static int do_sigtimedwait(const sigset_t *which, kernel_siginfo_t *info,
> spin_lock_irq(&tsk->sighand->siglock);
> __set_task_blocked(tsk, &tsk->real_blocked);
> sigemptyset(&tsk->real_blocked);
> - sig = dequeue_signal(tsk, &mask, info);
> + sig = dequeue_signal(tsk, &mask, info, &type);
> }
> spin_unlock_irq(&tsk->sighand->siglock);
>
> --
> 2.20.1
>

--
Kees Cook