Re: [PATCH v2] signalfd: don't dequeue the forced fatal signals

[Kees Cook]

On Fri, Jun 19, 2026 at 04:03:12PM +0200, Oleg Nesterov wrote:
> It seems that this fix is going to be lost ;)

Eek; thanks for the reminder. If Christian doesn't beat me to it, I'll
grab this for -next after -rc2 so we can get some soak time.

-Kees

> 
> Can anyone pick it up?
> 
> Oleg.
> 
> On 04/06, Oleg Nesterov wrote:
> >
> > These signals should act like SIGKILL, in that userspace must never dequeue
> > them. But as Kusaram explains, io_uring-driven signalfd_read_iter() called
> > from get_signal() -> task_work_run() paths can do this before get_signal()
> > has a chance to dequeue such a signal and notice SA_IMMUTABLE.
> > 
> > Change signalfd_poll() and signalfd_dequeue() to add pending SA_IMMUTABLE
> > signals to ctx->sigmask.
> > 
> > TODO: we should probably change force_sig_info_to_task(HANDLER_EXIT) to
> > make fatal_signal_pending() true, or add a fatal_or_forced_signal_pending()
> > helper. Then signalfd_dequeue() could just return -EINTR in this case.
> > This also makes sense for get_signal(), which could prioritize a fatal
> > signal sent by (say) force_sig_seccomp(force_coredump => true), just like
> > it already prioritizes SIGKILL.
> > 
> > Cc: stable@xxxxxxxxxx
> > Reported-by: syzbot+0a4c46806941297fecb9@xxxxxxxxxxxxxxxxxxxxxxxxx
> > Closes: https://syzkaller.appspot.com/bug?extid=0a4c46806941297fecb9
> > Tested-by: syzbot+0a4c46806941297fecb9@xxxxxxxxxxxxxxxxxxxxxxxxx
> > Link: https://lore.kernel.org/all/69d122fd.050a0220.2dbe29.001c.GAE@xxxxxxxxxx/
> > Suggested-by: Kusaram Devineni <kusaram@xxxxxxxxxxx>
> > Signed-off-by: Oleg Nesterov <oleg@xxxxxxxxxx>
> > Reviewed-by: Kees Cook <kees@xxxxxxxxxx>
> > ---
> >  fs/signalfd.c | 28 ++++++++++++++++++++++------
> >  1 file changed, 22 insertions(+), 6 deletions(-)
> > 
> > diff --git a/fs/signalfd.c b/fs/signalfd.c
> > index dff53745e352..22bc0870a824 100644
> > --- a/fs/signalfd.c
> > +++ b/fs/signalfd.c
> > @@ -48,17 +48,30 @@ static int signalfd_release(struct inode *inode, struct file *file)
> >  	return 0;
> >  }
> >  
> > +static void refine_sigmask(struct signalfd_ctx *ctx, sigset_t *sigmask)
> > +{
> > +	struct k_sigaction *k = current->sighand->action;
> > +	int n;
> > +
> > +	*sigmask = ctx->sigmask;
> > +	for (n = 1; n <= _NSIG; ++n, ++k) {
> > +		if (k->sa.sa_flags & SA_IMMUTABLE)
> > +			sigaddset(sigmask, n);
> > +	}
> > +}
> > +
> >  static __poll_t signalfd_poll(struct file *file, poll_table *wait)
> >  {
> >  	struct signalfd_ctx *ctx = file->private_data;
> >  	__poll_t events = 0;
> > +	sigset_t sigmask;
> >  
> >  	poll_wait(file, &current->sighand->signalfd_wqh, wait);
> >  
> >  	spin_lock_irq(&current->sighand->siglock);
> > -	if (next_signal(&current->pending, &ctx->sigmask) ||
> > -	    next_signal(&current->signal->shared_pending,
> > -			&ctx->sigmask))
> > +	refine_sigmask(ctx, &sigmask);
> > +	if (next_signal(&current->pending, &sigmask) ||
> > +	    next_signal(&current->signal->shared_pending, &sigmask))
> >  		events |= EPOLLIN;
> >  	spin_unlock_irq(&current->sighand->siglock);
> >  
> > @@ -155,11 +168,13 @@ 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);
> > +	sigset_t sigmask;
> > +	ssize_t ret;
> >  
> >  	spin_lock_irq(&current->sighand->siglock);
> > -	ret = dequeue_signal(&ctx->sigmask, info, &type);
> > +	refine_sigmask(ctx, &sigmask);
> > +	ret = dequeue_signal(&sigmask, info, &type);
> >  	switch (ret) {
> >  	case 0:
> >  		if (!nonblock)
> > @@ -174,7 +189,7 @@ 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(&ctx->sigmask, info, &type);
> > +		ret = dequeue_signal(&sigmask, info, &type);
> >  		if (ret != 0)
> >  			break;
> >  		if (signal_pending(current)) {
> > @@ -184,6 +199,7 @@ static ssize_t signalfd_dequeue(struct signalfd_ctx *ctx, kernel_siginfo_t *info
> >  		spin_unlock_irq(&current->sighand->siglock);
> >  		schedule();
> >  		spin_lock_irq(&current->sighand->siglock);
> > +		refine_sigmask(ctx, &sigmask);
> >  	}
> >  	spin_unlock_irq(&current->sighand->siglock);
> >  
> > -- 
> > 2.52.0
> > 
> 

-- 
Kees Cook