Date:   Tue, 16 Nov 2021 10:30:49 -0800
From:   Kees Cook <keescook@...omium.org>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     linux-kernel@...r.kernel.org, Kyle Huey <me@...ehuey.com>,
        Jens Axboe <axboe@...nel.dk>,
        Peter Zijlstra <peterz@...radead.org>,
        Marco Elver <elver@...gle.com>,
        Oleg Nesterov <oleg@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Collingbourne <pcc@...gle.com>,
        Alexey Gladkov <legion@...nel.org>,
        Robert O'Callahan <rocallahan@...il.com>,
        Marko Mäkelä <marko.makela@...iadb.com>,
        linux-api@...r.kernel.org, Al Viro <viro@...iv.linux.org.uk>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH 2/3] signal: Requeue signals in the appropriate queue

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@...ssion.com>

Reviewed-by: Kees Cook <keescook@...omium.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

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