linux-kernel - Re: [PATCH v1 2/2] signal: add procfd

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Date:   Mon, 19 Nov 2018 07:57:42 -0800
From:   Daniel Colascione <dancol@...gle.com>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Christian Brauner <christian@...uner.io>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        LKML <linux-kernel@...r.kernel.org>,
        "Serge E. Hallyn" <serge@...lyn.com>, Jann Horn <jannh@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Oleg Nesterov <oleg@...hat.com>,
        Aleksa Sarai <cyphar@...har.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Linux FS Devel <linux-fsdevel@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>,
        Tim Murray <timmurray@...gle.com>,
        linux-man <linux-man@...r.kernel.org>,
        Kees Cook <keescook@...omium.org>
Subject: Re: [PATCH v1 2/2] signal: add procfd_signal() syscall

On Mon, Nov 19, 2018 at 7:45 AM, Andy Lutomirski <luto@...nel.org> wrote:
> On Mon, Nov 19, 2018 at 2:33 AM Christian Brauner <christian@...uner.io> wrote:
>>
>> The kill() syscall operates on process identifiers. After a process has
>> exited its pid can be reused by another process. If a caller sends a signal
>> to a reused pid it will end up signaling the wrong process. This issue has
>> often surfaced and there has been a push [1] to address this problem.
>>
>> A prior patch has introduced the ability to get a file descriptor
>> referencing struct pid by opening /proc/<pid>. This guarantees a stable
>> handle on a process which can be used to send signals to the referenced
>> process. Discussion has shown that a dedicated syscall is preferable over
>> ioctl()s. Thus, the  new syscall procfd_signal() is introduced to solve
>> this problem. It operates on a process file descriptor.
>> The syscall takes an additional siginfo_t and flags argument. If siginfo_t
>> is NULL then procfd_signal() behaves like kill() if it is not NULL it
>> behaves like rt_sigqueueinfo.
>> The flags argument is added to allow for future extensions of this syscall.
>> It currently needs to be passed as 0.
>
> A few questions.  First: you've made this work on /proc/PID, but
> should it also work on /proc/PID/task/TID to send signals to a
> specific thread?

+1

>> +       if (info) {
>> +               ret = __copy_siginfo_from_user(sig, &kinfo, info);
>> +               if (unlikely(ret))
>> +                       goto err;
>> +               /*
>> +                * Not even root can pretend to send signals from the kernel.
>> +                * Nor can they impersonate a kill()/tgkill(), which adds
>> +                * source info.
>> +                */
>> +               ret = -EPERM;
>> +               if ((kinfo.si_code >= 0 || kinfo.si_code == SI_TKILL) &&
>> +                   (task_pid(current) != pid))
>> +                       goto err;
>
> Is the exception for signaling yourself actually useful here?

All the signal functions exempt the current process from access
checks. Whether that's useful or not (and I think it is), being
inconsistent here would be wrong.