linux-kernel - Re: [PATCH] signal: Make the si_code check in rt

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Message-ID: <20151013145439.GA19175@redhat.com>
Date:	Tue, 13 Oct 2015 16:54:39 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	"Amanieu d'Antras" <amanieu@...il.com>
Cc:	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: [PATCH] signal: Make the si_code check in rt_[tg]sigqueueinfo
	stricter

On 10/12, Amanieu d'Antras wrote:
>
> On Mon, Oct 12, 2015 at 4:54 PM, Oleg Nesterov <oleg@...hat.com> wrote:
> > Yes, copy_siginfo_to_user() does __put_user((short)from->si_code).
> > But SI_FROMUSER/SI_FROMKERNEL are internal kernel checks, we mostly
> > use them in copy_siginfo_to_user().
> >
> > And note that if ->si_code < 0 we simply do __copy_to_user(), so
> > userspace can't see something which looks like "from kernel", in
> > this case we do not truncate ->si_code.
>
> Ah my bad, I seem to have missed that case. So copy_siginfo_to_user
> seems to be safe, but it is still an issue for copy_siginfo_to_user32
> which doesn't have this check.
>
> Maybe this should be fixed in the compat code instead?

I agree, this doesn't look right... But I'm afraid it is too late to
change this, this logic predates the git history.

And unless I missed something copy_siginfo_to_user32() assumes that
"from" was filled by copy_siginfo_from_user32(), but this is not true
if the sender is 64bit task. And this all doesn't match the behaviour
with 32bit kernel.

Personally, I think we should leave this ancient code alone. But I
agree that something like below looks right... I dunno.

And I have no idea why copy_siginfo_to_user32() abuses '>> 16', it
seems that "si_code & __SI_MASK" could work just fine.

Oleg.

--- x/arch/x86/kernel/signal_compat.c
+++ x/arch/x86/kernel/signal_compat.c
@@ -17,13 +17,14 @@
 		   3 ints plus the relevant union member.  */
 		put_user_ex(from->si_signo, &to->si_signo);
 		put_user_ex(from->si_errno, &to->si_errno);
-		put_user_ex((short)from->si_code, &to->si_code);
 
 		if (from->si_code < 0) {
+			put_user_ex(from->si_code, &to->si_code);
 			put_user_ex(from->si_pid, &to->si_pid);
 			put_user_ex(from->si_uid, &to->si_uid);
 			put_user_ex(ptr_to_compat(from->si_ptr), &to->si_ptr);
 		} else {
+			put_user_ex((short)from->si_code, &to->si_code);
 			/*
 			 * First 32bits of unions are always present:
 			 * si_pid === si_band === si_tid === si_addr(LS half)

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