Re: [PATCH 1/2] signal: Don't always set SA_IMMUTABLE for forced signals

[Kees Cook <keescook@chromium.org>]

On Thu, Nov 18, 2021 at 04:04:58PM -0600, Eric W. Biederman wrote:
> 
> Recently to prevent issues with SECCOMP_RET_KILL and similar signals
> being changed before they are delivered SA_IMMUTABLE was added.
> 
> Unfortunately this broke debuggers[1][2] which reasonably expect to be
> able to trap synchronous SIGTRAP and SIGSEGV even when the target
> process is not configured to handle those signals.
> 
> Update force_sig_to_task to support both the case when we can
> allow the debugger to intercept and possibly ignore the
> signal and the case when it is not safe to let userspace
> known about the signal until the process has exited.
> 
> Reported-by: Kyle Huey <me@kylehuey.com>
> Reported-by: kernel test robot <oliver.sang@intel.com>
> Cc: stable@vger.kernel.org
> [1] https://lkml.kernel.org/r/CAP045AoMY4xf8aC_4QU_-j7obuEPYgTcnQQP3Yxk=2X90jtpjw@mail.gmail.com
> [2] https://lkml.kernel.org/r/20211117150258.GB5403@xsang-OptiPlex-902
> Fixes: 00b06da29cf9 ("signal: Add SA_IMMUTABLE to ensure forced siganls do not get changed")
> Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>

Thanks! This passes the seccomp self-tests.

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

-Kees

> ---
>  kernel/signal.c | 23 ++++++++++++++++-------
>  1 file changed, 16 insertions(+), 7 deletions(-)
> 
> diff --git a/kernel/signal.c b/kernel/signal.c
> index 7c4b7ae714d4..02058c983bd6 100644
> --- a/kernel/signal.c
> +++ b/kernel/signal.c
> @@ -1298,6 +1298,12 @@ int do_send_sig_info(int sig, struct kernel_siginfo *info, struct task_struct *p
>  	return ret;
>  }
>  
> +enum sig_handler {
> +	HANDLER_CURRENT, /* If reachable use the current handler */
> +	HANDLER_SIG_DFL, /* Always use SIG_DFL handler semantics */
> +	HANDLER_EXIT,	 /* Only visible as the proces exit code */
> +};
> +
>  /*
>   * Force a signal that the process can't ignore: if necessary
>   * we unblock the signal and change any SIG_IGN to SIG_DFL.
> @@ -1310,7 +1316,8 @@ int do_send_sig_info(int sig, struct kernel_siginfo *info, struct task_struct *p
>   * that is why we also clear SIGNAL_UNKILLABLE.
>   */
>  static int
> -force_sig_info_to_task(struct kernel_siginfo *info, struct task_struct *t, bool sigdfl)
> +force_sig_info_to_task(struct kernel_siginfo *info, struct task_struct *t,
> +	enum sig_handler handler)
>  {
>  	unsigned long int flags;
>  	int ret, blocked, ignored;
> @@ -1321,9 +1328,10 @@ force_sig_info_to_task(struct kernel_siginfo *info, struct task_struct *t, bool
>  	action = &t->sighand->action[sig-1];
>  	ignored = action->sa.sa_handler == SIG_IGN;
>  	blocked = sigismember(&t->blocked, sig);
> -	if (blocked || ignored || sigdfl) {
> +	if (blocked || ignored || (handler != HANDLER_CURRENT)) {
>  		action->sa.sa_handler = SIG_DFL;
> -		action->sa.sa_flags |= SA_IMMUTABLE;
> +		if (handler == HANDLER_EXIT)
> +			action->sa.sa_flags |= SA_IMMUTABLE;
>  		if (blocked) {
>  			sigdelset(&t->blocked, sig);
>  			recalc_sigpending_and_wake(t);
> @@ -1343,7 +1351,7 @@ force_sig_info_to_task(struct kernel_siginfo *info, struct task_struct *t, bool
>  
>  int force_sig_info(struct kernel_siginfo *info)
>  {
> -	return force_sig_info_to_task(info, current, false);
> +	return force_sig_info_to_task(info, current, HANDLER_CURRENT);
>  }
>  
>  /*
> @@ -1660,7 +1668,7 @@ void force_fatal_sig(int sig)
>  	info.si_code = SI_KERNEL;
>  	info.si_pid = 0;
>  	info.si_uid = 0;
> -	force_sig_info_to_task(&info, current, true);
> +	force_sig_info_to_task(&info, current, HANDLER_SIG_DFL);
>  }
>  
>  /*
> @@ -1693,7 +1701,7 @@ int force_sig_fault_to_task(int sig, int code, void __user *addr
>  	info.si_flags = flags;
>  	info.si_isr = isr;
>  #endif
> -	return force_sig_info_to_task(&info, t, false);
> +	return force_sig_info_to_task(&info, t, HANDLER_CURRENT);
>  }
>  
>  int force_sig_fault(int sig, int code, void __user *addr
> @@ -1813,7 +1821,8 @@ int force_sig_seccomp(int syscall, int reason, bool force_coredump)
>  	info.si_errno = reason;
>  	info.si_arch = syscall_get_arch(current);
>  	info.si_syscall = syscall;
> -	return force_sig_info_to_task(&info, current, force_coredump);
> +	return force_sig_info_to_task(&info, current,
> +		force_coredump ? HANDLER_EXIT : HANDLER_CURRENT);
>  }
>  
>  /* For the crazy architectures that include trap information in
> -- 
> 2.20.1

-- 
Kees Cook