linux-hardening - Re: [PATCH 0/3] signal: HANDLER_EXIT should clear SIGNAL

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Date:   Thu, 10 Feb 2022 12:43:14 -0800
From:   Kees Cook <keescook@...omium.org>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     Robert Święcki <robert@...ecki.net>,
        Andy Lutomirski <luto@...capital.net>,
        Will Drewry <wad@...omium.org>, linux-kernel@...r.kernel.org,
        linux-hardening@...r.kernel.org
Subject: Re: [PATCH 0/3] signal: HANDLER_EXIT should clear SIGNAL_UNKILLABLE

On Thu, Feb 10, 2022 at 12:58:07PM -0600, Eric W. Biederman wrote:
> Kees Cook <keescook@...omium.org> writes:
> 
> > On Thu, Feb 10, 2022 at 12:17:50PM -0600, Eric W. Biederman wrote:
> >> Kees Cook <keescook@...omium.org> writes:
> >> 
> >> > Hi,
> >> >
> >> > This fixes the signal refactoring to actually kill unkillable processes
> >> > when receiving a fatal SIGSYS from seccomp. Thanks to Robert for the
> >> > report and Eric for the fix! I've also tweaked seccomp internal a bit to
> >> > fail more safely. This was a partial seccomp bypass, in the sense that
> >> > SECCOMP_RET_KILL_* didn't kill the process, but it didn't bypass other
> >> > aspects of the filters. (i.e. the syscall was still blocked, etc.)
> >> 
> >> Any luck on figuring out how to suppress the extra event?
> >
> > I haven't found a good single indicator of a process being in an "I am dying"
> > state, and even if I did, it seems every architecture's exit path would
> > need to add a new test.
> 
> The "I am dying" state for a task is fatal_signal_pending, at least
> before get_signal is reached, for a process there is SIGNAL_GROUP_EXIT.
> Something I am busily cleaning up and making more reliable at the
> moment.

The state I need to catch is "I am dying and this syscall was
interrupted". fatal_signal_pending() is kind of only the first half
(though it doesn't cover fatal SIGSYS?)

For example, if a process hits a BUG() in the middle of running a
syscall, that syscall isn't expected to "exit" from the perspective of
userspace. This is similarly true for seccomp's fatal SIGSYS.

> What is the event that is happening?  Is it
> tracehook_report_syscall_exit or something else?

Yes, but in more completely, it's these three, which are called in
various fashions from architecture syscall exit code:

	audit_syscall_exit()		(audit)
	trace_sys_exit()		(see "TRACE_EVENT_FN(sys_exit,")
	tracehook_report_syscall_exit()	(ptrace)

> From the bits I have seen it seems like something else.

But yes, the place Robert and I both noticed it was with ptrace from
tracehook_report_syscall_exit(), which is rather poorly named. :)

Looking at the results, audit_syscall_exit() and trace_sys_exit() need
to be skipped too, since they would each be reporting potential nonsense.

> > The best approach seems to be clearing the TIF_*WORK* bits, but that's
> > still a bit arch-specific. And I'm not sure which layer would do that.
> > At what point have we decided the process will not continue? More
> > than seccomp was calling do_exit() in the middle of a syscall, but those
> > appear to have all been either SIGKILL or SIGSEGV?
> 
> This is where I get confused what TIF_WORK bits matter?

This is where I wish all the architectures were using the common syscall
code. The old do_exit() path would completely skip _everything_ in the
exit path, so it was like never calling anything after the syscall
dispatch table. The only userspace visible things in there are triggered
from having TIF_WORK... flags (but again, it's kind of a per-arch mess).

Skipping the entire exit path makes a fair bit of sense. For example,
rseq_syscall() is redundant (forcing SIGSEGV).

Regardless, at least the three places above need to be skipped.

But just testing fatal_signal_pending() seems wrong: a normal syscall
could be finishing just fine, it just happens to have a fatal signal
ready to be processed.

Here's the ordering after a syscall on x86 from do_syscall_64():

do_syscall_x64()
	sys_call_table[...](regs)
syscall_exit_to_user_mode()
	__syscall_exit_to_user_mode_work()
		syscall_exit_to_user_mode_prepare()
			syscall_exit_work()
				arch_syscall_exit_tracehook()
					tracehook_report_syscall_exit()
	exit_to_user_mode_prepare()
		exit_to_user_mode_loop()
			handle_signal_work()
				arch_do_signal_or_restart()
					get_signal()
						do_group_exit()

Here's arm64 from el0_svc():

do_el0_svc()
	el0_svc_common()
		invoke_syscall()
			syscall_table[...](regs)
		syscall_trace_exit()
			tracehook_report_syscall()
				tracehook_report_syscall_exit()
exit_to_user_mode()
	prepare_exit_to_user_mode()
		do_notify_resume()
			do_signal()
				get_signal()
					do_group_exit()

In the past, any do_exit() would short circuit everything after the
syscall table. Now, we do all the exit work before starting the return
to user mode which is what processes the signals. So I guess there's
more precisely a difference between "visible to userspace" and "return
to userspace".

(an aside: where to PF_IO_WORKER threads die?)

> I expect if anything else mattered we would need to change it to
> HANDLER_EXIT.
> 
> I made a mistake conflating to cases and I want to make certain I
> successfully separate those two cases at the end of the day.

For skipping the exit work, I'm not sure it matters, since all the
signal stuff is "too late"...

-- 
Kees Cook