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Date:   Thu, 10 Feb 2022 16:48:30 -0600
From:   "Eric W. Biederman" <ebiederm@...ssion.com>
To:     Kees Cook <keescook@...omium.org>
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

Kees Cook <keescook@...omium.org> writes:

> 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. :)

Speaking of patches I am just about to send out.

> 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.

Yes.  It is really just the HANDLER_EXIT case where this is interesting.

>
> 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".

Yes.  I see that now.  I had not had an occasion to look at the order
all of these were called in before and my mental model was wrong.

It makes a certain kind of sense that the per syscall work happens
before we do additional things like process signals.  It simply
had not realized that was happening in that order until now.


> (an aside: where to PF_IO_WORKER threads die?)

They are calling do_exit explicitly.

>> 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"...

The conflation lead me to believe that we could simply and safely cause
seccomp to use normal signal delivery to kill the process.  The first
part of the conflation I sorted out by introducing HANDLER_EXIT.  The
user visible part of the change I am not yet certain what to do with.

My gut reaction is does it matter?  Can you escape the seccomp filter
with a stop?  Does it break userspace?

I realize the outcome of that question is that it does matter so we
probably need to find a way to supress that situation for HANDLER_EXIT.
Both force_exit_sig and force_sig_seccomp appear to be using dumpable
signals which makes the problem doubly tricky.

The first tricky bit is fatal_signal_pending isn't set because a
coredump is possible, so something else is needed to detect this
condition.

The second part is what to do when we detect the condition.


The only solution I can think of quickly is to modify
force_sig_info_to_task clear TIF_SYSCALL_WORK on the architectures where
that is used and to clear SYSCALL_WORK_EXIT on x86 and s390, and to do
whatever the architecture appropriate thing is on the other
architectures.

It might be easier once I have cleaned some more code up but as long as
we can limit the logic to force_sig_info_to_task for the HANDLER_EXIT
case I don't see any advantage in the cleanups I have planned for this
case.  This may be incentive to make the architecture code more uniform
in this area.

Anyway I am off to finish fixing some other bugs.

Eric


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