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Date: Wed, 18 Mar 2015 19:31:33 +0100
From: Oleg Nesterov <oleg@redhat.com>
To: Cyrill Gorcunov <gorcunov@gmail.com>
Cc: Andrey Wagin <avagin@gmail.com>, Andy Lutomirski <luto@kernel.org>,
        Ingo Molnar <mingo@kernel.org>, Andi Kleen <andi@firstfloor.org>,
        "H. Peter Anvin" <hpa@zytor.com>, Al Viro <viro@zeniv.linux.org.uk>,
        X86 ML <x86@kernel.org>, LKML <linux-kernel@vger.kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Borislav Petkov <bp@alien8.de>, Andy Lutomirski <luto@amacapital.net>,
        Pavel Emelyanov <xemul@openvz.org>
Subject: Re: [PATCH v3 1/2] x86_64,signal: Fix SS handling for signals
	delivered to 64-bit programs
Message-ID: <20150318183133.GB1832@redhat.com>
References: <cover.1426193719.git.luto@kernel.org> <405594361340a2ec32f8e2b115c142df0e180d8e.1426193719.git.luto@kernel.org> <CANaxB-yz9BZbU1TLB-kGfR21L_ZUkni9bqL96mK5jgsXAka+KA@mail.gmail.com> <20150318174843.GA32238@redhat.com> <20150318181306.GF2255@moon>
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On 03/18, Cyrill Gorcunov wrote:
>
> On Wed, Mar 18, 2015 at 06:48:43PM +0100, Oleg Nesterov wrote:
> >
> > Shot in a dark afer a quick grep: restore_gpregs() should initialize ->ss?
>
> It hasn't been needed earlier, if this would help it means abi is broken, no? :)
> Otherwise I don't understand what's happening.

until this commit the kernel simply forgot to restore ->ss in sigreturn().

after this commit, if your rt_sigframe has garbage in ->ss then SIGSEGV
is clear.

> > perhaps something like below... obviously uncompiled/untested.
> >
> > And my grep can't find the definition of UserX86RegsEntry in crtools...
> > Perhaps the change below needs CPREG1(ss, anothername).
> >
> > Seriously, where is UserX86RegsEntry?
>
> It's in protobif/core-x86.proto, welcome to protobuf hell.

Still can't find UserX86RegsEntry... OK, perhaps it comes from
user_x86_regs_entry in protobuf/core-x86.proto. Then I guess the patch I sent
can be compiled at least ;)

>
> >
> > Oleg.
> >
> >
> > --- a/arch/x86/crtools.c
> > +++ b/arch/x86/crtools.c
> > @@ -475,6 +475,7 @@ int restore_gpregs(struct rt_sigframe *f, UserX86RegsEntry *r)
> >  	CPREG2(rip, ip);
> >  	CPREG2(eflags, flags);
> >  	CPREG1(cs);
> > +	CPREG1(ss);
> >  	CPREG1(gs);
> >  	CPREG1(fs);
> >
> > diff --git a/arch/x86/include/asm/restorer.h b/arch/x86/include/asm/restorer.h
> > index 70199fb..c04fb94 100644
> > --- a/arch/x86/include/asm/restorer.h
> > +++ b/arch/x86/include/asm/restorer.h
> > @@ -53,7 +53,7 @@ struct rt_sigcontext {
> >  	unsigned short			cs;
> >  	unsigned short			gs;
> >  	unsigned short			fs;
> > -	unsigned short			__pad0;
> > +	unsigned short			ss;
> >  	unsigned long			err;
> >  	unsigned long			trapno;
> >  	unsigned long			oldmask;
> >
>
> 	Cyrill