Re: [PATCH] signal: Add SA_IMMUTABLE to ensure forced siganls do not get changed

[Andrea Righi <andrea.righi@canonical.com>]

On Fri, Oct 29, 2021 at 10:09:04AM -0500, Eric W. Biederman wrote:
> 
> As Andy pointed out that there are races between
> force_sig_info_to_task and sigaction[1] when force_sig_info_task.  As
> Kees discovered[2] ptrace is also able to change these signals.
> 
> In the case of seeccomp killing a process with a signal it is a
> security violation to allow the signal to be caught or manipulated.
> 
> Solve this problem by introducing a new flag SA_IMMUTABLE that
> prevents sigaction and ptrace from modifying these forced signals.
> This flag is carefully made kernel internal so that no new ABI is
> introduced.
> 
> Longer term I think this can be solved by guaranteeing short circuit
> delivery of signals in this case.  Unfortunately reliable and
> guaranteed short circuit delivery of these signals is still a ways off
> from being implemented, tested, and merged.  So I have implemented a much
> simpler alternative for now.
> 
> [1] https://lkml.kernel.org/r/b5d52d25-7bde-4030-a7b1-7c6f8ab90660@www.fastmail.com
> [2] https://lkml.kernel.org/r/202110281136.5CE65399A7@keescook
> Cc: stable@vger.kernel.org
> Fixes: 307d522f5eb8 ("signal/seccomp: Refactor seccomp signal and coredump generation")
> Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
> ---

FWIW I've tested this patch and I confirm that it fixes the failure that
I reported with the seccomp_bpf selftest.

Tested-by: Andrea Righi <andrea.righi@canonical.com>

Thanks!
-Andrea

> 
> I have tested this patch and this changed works for me to fix the issue.
> 
> I believe this closes all of the races that force_sig_info_to_task
> has when sigdfl is specified.  So this should be enough for anything
> that needs a guaranteed that userspace can not race with the kernel
> is handled.
> 
> Can folks look this over and see if I missed something?
> Thank you,
> Eric
> 
> 
>  include/linux/signal_types.h           | 3 +++
>  include/uapi/asm-generic/signal-defs.h | 1 +
>  kernel/signal.c                        | 8 +++++++-
>  3 files changed, 11 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/signal_types.h b/include/linux/signal_types.h
> index 34cb28b8f16c..927f7c0e5bff 100644
> --- a/include/linux/signal_types.h
> +++ b/include/linux/signal_types.h
> @@ -70,6 +70,9 @@ struct ksignal {
>  	int sig;
>  };
>  
> +/* Used to kill the race between sigaction and forced signals */
> +#define SA_IMMUTABLE		0x008000000
> +
>  #ifndef __ARCH_UAPI_SA_FLAGS
>  #ifdef SA_RESTORER
>  #define __ARCH_UAPI_SA_FLAGS	SA_RESTORER
> diff --git a/include/uapi/asm-generic/signal-defs.h b/include/uapi/asm-generic/signal-defs.h
> index fe929e7b77ca..7572f2f46ee8 100644
> --- a/include/uapi/asm-generic/signal-defs.h
> +++ b/include/uapi/asm-generic/signal-defs.h
> @@ -45,6 +45,7 @@
>  #define SA_UNSUPPORTED	0x00000400
>  #define SA_EXPOSE_TAGBITS	0x00000800
>  /* 0x00010000 used on mips */
> +/* 0x00800000 used for internal SA_IMMUTABLE */
>  /* 0x01000000 used on x86 */
>  /* 0x02000000 used on x86 */
>  /*
> diff --git a/kernel/signal.c b/kernel/signal.c
> index 6a5e1802b9a2..056a107e3cbc 100644
> --- a/kernel/signal.c
> +++ b/kernel/signal.c
> @@ -1336,6 +1336,7 @@ force_sig_info_to_task(struct kernel_siginfo *info, struct task_struct *t, bool
>  	blocked = sigismember(&t->blocked, sig);
>  	if (blocked || ignored || sigdfl) {
>  		action->sa.sa_handler = SIG_DFL;
> +		action->sa.sa_flags |= SA_IMMUTABLE;
>  		if (blocked) {
>  			sigdelset(&t->blocked, sig);
>  			recalc_sigpending_and_wake(t);
> @@ -2760,7 +2761,8 @@ bool get_signal(struct ksignal *ksig)
>  		if (!signr)
>  			break; /* will return 0 */
>  
> -		if (unlikely(current->ptrace) && signr != SIGKILL) {
> +		if (unlikely(current->ptrace) && (signr != SIGKILL) &&
> +		    !(sighand->action[signr -1].sa.sa_flags & SA_IMMUTABLE)) {
>  			signr = ptrace_signal(signr, &ksig->info);
>  			if (!signr)
>  				continue;
> @@ -4110,6 +4112,10 @@ int do_sigaction(int sig, struct k_sigaction *act, struct k_sigaction *oact)
>  	k = &p->sighand->action[sig-1];
>  
>  	spin_lock_irq(&p->sighand->siglock);
> +	if (k->sa.sa_flags & SA_IMMUTABLE) {
> +		spin_unlock_irq(&p->sighand->siglock);
> +		return -EINVAL;
> +	}
>  	if (oact)
>  		*oact = *k;
>  
> -- 
> 2.20.1