Re: [PATCH] kernel/signal: Signal-based pre-coredump notification

[Enke Chen]

Jann,

Thanks for the feedback. I will post a revised patch shortly.

On the related topic of "pdeath_signal", there are several inconsistencies
by preserving the flag across execve(2). The flag is cleared under several
conditions in different places. I will start a separate thread to see if
it can still be cleaned up.

     PR_SET_PDEATHSIG (since Linux 2.1.57)
              Set the parent death signal of the calling process to arg2
              (either a signal value in the range 1..maxsig, or 0 to clear).
              This is the signal that the calling process will get when its
              parent dies.  This value is cleared for the child of a fork(2)
              and (since Linux 2.4.36 / 2.6.23) when executing a set-user-ID
              or set-group-ID binary, or a binary that has associated
              capabilities (see capabilities(7)).  This value is preserved
              across execve(2).

-- Enke

On 10/22/18 8:40 AM, Jann Horn wrote:
> On Sat, Oct 20, 2018 at 1:01 AM Enke Chen <enkechen@xxxxxxxxx> wrote:
>> Regarding the security considerations, it seems simpler and more secure to
>> just clear the "pre-coredump signal" cross execve(2), and let the new program
>> decide for itself.  What do you think?
> 
> I don't have a problem with these semantics.
> 
> I could imagine someone being unhappy about the theoretical race
> window if they want to perform an in-place reexecution of a running
> service, but I don't know whether anyone actually cares about that.
> 
>> Changes to prctl(2):
>>
>> DESCRIPTION
>>
>>        PR_SET_PREDUMP_SIG (since Linux 4.20.x)
>>               This allows the calling process to receive a signal (arg2,
>>               if nonzero) from a child process prior to the coredump of
>>               the child process. arg2 must be SIGUSR1, or SIGUSR2, or
>>               SIGCHLD, or 0 (for clear).
>>
>>               When SIGCHLD is specified, the signal code is set to
>>               CLD_PREDUMP in such an SIGCHLD signal.
>>
>>               The value of the pre-coredump signal is cleared across
>>               execve(2), or for the child of a fork(2).
>>
>>        PR_GET_PREDUMP_SIG (since Linux 4.20.x)
>>               Return the current value of the pre-coredump signal for the
>>               calling process, in the location pointed to by (int *) arg2.