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From: Andrew Morton on 20 May 2010 15:50
On Mon, 17 May 2010 21:54:14 +0200
Oleg Nesterov <oleg(a)redhat.com> wrote:

> Andrew Tridgell reports that aio_read(SIGEV_SIGNAL) can fail if the
> the notification from the helper thread races with setresuid(), see
> http://samba.org/~tridge/junkcode/aio_uid.c
>
> This happens because check_kill_permission() doesn't allow to send
> a signal to the task with the different cred->xids. But there is no
> any security reason to check ->cred's when the task sends a signal
> (private or group-wide) to its sub-thread. Whatever we do, any thread
> can bypass all security checks and send SIGKILL to all threads, or
> it can block a signal SIG and do kill(gettid(), SIG) to deliver this
> signal to another sub-thread. Not to mention that CLONE_THREAD implies
> CLONE_VM.
>
> Change check_kill_permission() to avoid the credentials check when
> the sender and the target are from the same thread group.
>
> Also, move "cred = current_cred()" down to avoid calling get_current()
> twice.
>
> Note: David Howells pointed out we could relax this even more, the
> CLONE_SIGHAND (without CLONE_THREAD) case probably does not need
> these checks too.

So... which kernel(s) do we think this fix should be merged into?
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From: Roland McGrath on 20 May 2010 16:10
> So... which kernel(s) do we think this fix should be merged into?

I'd say all. The glibc (libpthread) that does set*id across threads has
been in use for a while (2.3.4?), probably in distro's using kernels as old
or older than any active -stable streams. In the race in question, this
kernel bug is breaking valid POSIX application expectations.


Thanks,
Roland
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