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From:   Jann Horn <jannh@google.com>
Date:   Mon, 2 Nov 2020 14:49:01 +0100
Message-ID: <CAG48ez0W2zye2KeNiVaKq9RPtUhcUtzP0zOjULRQZbOuRyz+9w@mail.gmail.com>
Subject: Re: For review: seccomp_user_notif(2) manual page [v2]
To:     "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com>
Cc:     Kees Cook <keescook@chromium.org>,
        Tycho Andersen <tycho@tycho.pizza>,
        Sargun Dhillon <sargun@sargun.me>,
        Christian Brauner <christian@brauner.io>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Giuseppe Scrivano <gscrivan@redhat.com>,
        Song Liu <songliubraving@fb.com>,
        Robert Sesek <rsesek@google.com>,
        Containers <containers@lists.linux-foundation.org>,
        linux-man <linux-man@vger.kernel.org>,
        lkml <linux-kernel@vger.kernel.org>,
        Aleksa Sarai <cyphar@cyphar.com>,
        Alexei Starovoitov <ast@kernel.org>,
        Will Drewry <wad@chromium.org>, bpf <bpf@vger.kernel.org>,
        Andy Lutomirski <luto@amacapital.net>
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On Sat, Oct 31, 2020 at 9:31 AM Michael Kerrisk (man-pages)
<mtk.manpages@gmail.com> wrote:
> On 10/30/20 8:14 PM, Jann Horn wrote:
> > With the caveat that a cancelled syscall
> > could've also led to the memory being munmap()ed, so the nread==0 case
> > could also happen legitimately - so you might want to move this check
> > up above the nread==0 (mm went away) and nread==-1 (mm still exists,
> > but read from address failed, errno EIO) checks if the error message
> > shouldn't appear spuriously.
>
> In any case, I've been refactoring (simplifying) that code a little.
> I haven't so far rearranged the order of the checks, but I already
> log message for the nread==0 case. (Instead, there will eventually
> be an error when the response is sent.)
>
> I also haven't exactly tested the scenario you describe in the
> seccomp unotify scenario, but I think the above is not correct. Here
> are two scenarios I did test, simply with mmap() and /proc/PID/mem
> (no seccomp involved):
>
> Scenario 1:
> A creates a mapping at address X
> B opens /proc/A/mem and and lseeks on resulting FD to offset X
> A terminates
> B reads from FD ==> read() returns 0 (EOF)
>
> Scenario 2:
> A creates a mapping at address X
> B opens /proc/A/mem and and lseeks on resulting FD to offset X
> A unmaps mapping at address X
> B reads from FD ==> read() returns -1 / EIO.
>
> That last scenario seems to contradict what you say, since I
> think you meant that in this case read() should return 0 in
> that case. Have I misunderstood you?

Sorry, I messed up the description when I wrote that. Yes, this looks
as expected - EIO if the VMA is gone, 0 if the mm_users of the
mm_struct have dropped to zero because all tasks that use the mm have
exited.