From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751750AbbCKIdB (ORCPT ); Wed, 11 Mar 2015 04:33:01 -0400 Received: from mail.skyhub.de ([78.46.96.112]:37116 "EHLO mail.skyhub.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751502AbbCKIc4 (ORCPT ); Wed, 11 Mar 2015 04:32:56 -0400 Date: Wed, 11 Mar 2015 09:31:31 +0100 From: Borislav Petkov To: Andy Lutomirski Cc: Ingo Molnar , Andi Kleen , "H. Peter Anvin" , Al Viro , x86@kernel.org, linux-kernel@vger.kernel.org, Linus Torvalds Subject: Re: [PATCH v2 1/2] x86_64,signal: Fix SS handling for signals delivered to 64-bit programs Message-ID: <20150311083131.GA16419@pd.tnic> References: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Mar 10, 2015 at 07:03:24AM -0700, Andy Lutomirski wrote: > The comment in the signal code says that apps can save/restore other > segments on their own. It's true that apps can *save* SS on their > own, but there's no way for apps to restore it: SYSCALL effectively > resets SS to __USER_DS, so any value that user code tries to load > into SS gets lost on entry to sigreturn. > > This recycles two padding bytes in the segment selector area for SS. > > While we're at it, we need a second change to make this useful. If > the signal we're delivering is caused by a bad SS value, saving that > value isn't enough. We need to remove that bad value from the regs > before we try to deliver the signal. Oddly, x32 already got this > right. Are we at least reporting the bad SS value when delivering the signal so that userpsace knows why it got the signal? > I suspect that 64-bit programs that try to run 16-bit code and uses > signals will have a lot of trouble without this. Do we even have software doing that? Maybe we should search for similar bug reports... > Signed-off-by: Andy Lutomirski > --- > arch/x86/include/asm/sigcontext.h | 2 +- > arch/x86/include/uapi/asm/sigcontext.h | 2 +- > arch/x86/kernel/signal.c | 20 +++++++++++--------- > 3 files changed, 13 insertions(+), 11 deletions(-) > > diff --git a/arch/x86/include/asm/sigcontext.h b/arch/x86/include/asm/sigcontext.h > index 9dfce4e0417d..f910cdcb71fd 100644 > --- a/arch/x86/include/asm/sigcontext.h > +++ b/arch/x86/include/asm/sigcontext.h > @@ -59,7 +59,7 @@ struct sigcontext { > unsigned short cs; > unsigned short gs; > unsigned short fs; > - unsigned short __pad0; This __pad0 thing has been there since the beginning, according to my git history dive. > + unsigned short ss; > unsigned long err; > unsigned long trapno; > unsigned long oldmask; > diff --git a/arch/x86/include/uapi/asm/sigcontext.h b/arch/x86/include/uapi/asm/sigcontext.h > index d8b9f9081e86..076b11fd6fa1 100644 > --- a/arch/x86/include/uapi/asm/sigcontext.h > +++ b/arch/x86/include/uapi/asm/sigcontext.h > @@ -179,7 +179,7 @@ struct sigcontext { > __u16 cs; > __u16 gs; > __u16 fs; > - __u16 __pad0; > + __u16 ss; > __u64 err; > __u64 trapno; > __u64 oldmask; > diff --git a/arch/x86/kernel/signal.c b/arch/x86/kernel/signal.c > index ed37a768d0fc..40f34574fb36 100644 > --- a/arch/x86/kernel/signal.c > +++ b/arch/x86/kernel/signal.c > @@ -94,15 +94,8 @@ int restore_sigcontext(struct pt_regs *regs, struct sigcontext __user *sc, > COPY(r15); > #endif /* CONFIG_X86_64 */ > > -#ifdef CONFIG_X86_32 > COPY_SEG_CPL3(cs); > COPY_SEG_CPL3(ss); > -#else /* !CONFIG_X86_32 */ > - /* Kernel saves and restores only the CS segment register on signals, > - * which is the bare minimum needed to allow mixed 32/64-bit code. > - * App's signal handler can save/restore other segments if needed. */ > - COPY_SEG_CPL3(cs); > -#endif /* CONFIG_X86_32 */ > > get_user_ex(tmpflags, &sc->flags); > regs->flags = (regs->flags & ~FIX_EFLAGS) | (tmpflags & FIX_EFLAGS); > @@ -164,6 +157,7 @@ int setup_sigcontext(struct sigcontext __user *sc, void __user *fpstate, > put_user_ex(regs->cs, &sc->cs); > put_user_ex(0, &sc->gs); > put_user_ex(0, &sc->fs); > + put_user_ex(regs->ss, &sc->ss); > #endif /* CONFIG_X86_32 */ > > put_user_ex(fpstate, &sc->fpstate); > @@ -457,9 +451,17 @@ static int __setup_rt_frame(int sig, struct ksignal *ksig, > > regs->sp = (unsigned long)frame; > > - /* Set up the CS register to run signal handlers in 64-bit mode, > - even if the handler happens to be interrupting 32-bit code. */ > + /* Set up the CS and SS registers to run signal handlers in > + 64-bit mode, even if the handler happens to be interrupting > + 32-bit or 16-bit code. > + > + SS is subtle. In 64-bit mode, we don't need any particular > + SS descriptor, but we do need SS to be valid. It's possible > + that the old SS is entirely bogus -- this can happen if the > + signal we're trying to deliver is #GP or #SS caused by a bad > + SS value. */ Kernel comment style please: /* * Andy likes to go and play 16-bit games on 64-bit linux. We all are * having lotsa fun. */ :-D -- Regards/Gruss, Boris. ECO tip #101: Trim your mails when you reply. --