linux-hardening - Re: [RFC] Get siginfo from unreaped task

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Date:   Sat, 12 Feb 2022 18:32:08 -0800
From:   Andy Lutomirski <luto@...capital.net>
To:     Robert Święcki <robert@...ecki.net>
Cc:     Kees Cook <keescook@...omium.org>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Jann Horn <jannh@...gle.com>, Will Drewry <wad@...omium.org>,
        linux-api@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-hardening@...r.kernel.org
Subject: Re: [RFC] Get siginfo from unreaped task

> On Feb 12, 2022, at 3:24 AM, Robert Święcki <robert@...ecki.net> wrote:
> 
> sob., 12 lut 2022 o 05:28 Kees Cook <keescook@...omium.org> napisał(a):
>> 
>> Make siginfo available through PTRACE_GETSIGINFO after process death,
>> without needing to have already used PTRACE_ATTACH. Uses 48 more bytes
>> in task_struct, though I bet there might be somewhere else we could
>> stash a copy of it?
> 
> An alternative way of accessing this info could be abusing the
> waitid() interface, with some additional, custom to Linux, flag
> 
> waitid(P_ALL, 0, &si, __WCHILDSIGINFO);
> 
> which would change what is put into si.
> 
> But maybe ptrace() is better, because it's mostly incompatible with
> other OSes anyway on the behavior/flag level, while waitd() seems to
> be POSIX/BSD standard, even if Linux specifies some additional flags.
> 
> 

I had a kind of opposite thought, which is that it would be very nice to be able to get all the waitid() data without reaping a process or even necessarily being its parent.  Maybe these can be combined?  A new waitid() option like you’re suggesting could add siginfo (and might need permissions).  And we could have a different waitid() flag that says “maybe not my child, don’t reap” (and also needs permissions).

Although the “don’t reap” thing is fundamentally racy. What a sane process manager actually wants is an interface to read all this info from a pidfd, which means it all needs to get stuck in struct pid. And task_struct needs a completion or wait queue so you can actually wait for a pidfd to exit (unless someone already did this — I had patches a while back).  And this would be awesome.