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Date:   Mon, 25 Feb 2019 17:37:45 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Andy Lutomirski <luto@amacapital.net>
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Julien Thierry <julien.thierry@arm.com>,
        Will Deacon <will.deacon@arm.com>,
        Ingo Molnar <mingo@kernel.org>,
        Catalin Marinas <catalin.marinas@arm.com>,
        James Morse <james.morse@arm.com>, valentin.schneider@arm.com,
        Brian Gerst <brgerst@gmail.com>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Andrew Lutomirski <luto@kernel.org>,
        Borislav Petkov <bp@alien8.de>,
        Denys Vlasenko <dvlasenk@redhat.com>,
        LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 2/6] x86/ia32: Fix ia32_restore_sigcontext AC leak
Message-ID: <20190225163745.GF32494@hirez.programming.kicks-ass.net>
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On Mon, Feb 25, 2019 at 08:29:12AM -0800, Andy Lutomirski wrote:

> > diff --git a/arch/x86/ia32/ia32_signal.c b/arch/x86/ia32/ia32_signal.c
> > index 321fe5f5d0e9..e04eeeddcc35 100644
> > --- a/arch/x86/ia32/ia32_signal.c
> > +++ b/arch/x86/ia32/ia32_signal.c
> > @@ -53,17 +53,16 @@
> > #define GET_SEG(seg)        ({            \
> >    unsigned short tmp;                \
> >    get_user_ex(tmp, &sc->seg);            \
> > -    tmp;                        \
> > +    tmp | 3;                    \
> > })
> > 
> 
> Drop this part.
> 
> > #define COPY_SEG_CPL3(seg)    do {            \
> > -    regs->seg = GET_SEG(seg) | 3;            \
> > +    regs->seg = GET_SEG(seg);            \
> > } while (0)
> 
> And this.
> 
> Unfortunately, whether we want the | 3 varies by segment. For FS and
> GS, we definitely don’t want it, since 0 is a common and important
> value, and 3 is a deeply screwed up value.  (3 is legal to *write* to
> GS, and it sticks, but IRET silently changes it to 0, because the
> original 386 designers (I assume) confused themselves.

> > 
> > #define RELOAD_SEG(seg)        {        \
> > -    unsigned int pre = GET_SEG(seg);    \
> > +    unsigned int pre = (seg);        \
> >    unsigned int cur = get_user_seg(seg);    \
> > -    pre |= 3;                \

And here ?

> >    if (pre != cur)                \
> >        set_user_seg(seg, pre);        \
> > }

Thing is; afaict the current code _always_ does |3 on any GET_SET()
result.

If you want to change that; I'm fine with that, but lets not do that in
this same patch.

So then?

diff --git a/arch/x86/ia32/ia32_signal.c b/arch/x86/ia32/ia32_signal.c
index 321fe5f5d0e9..4d5fcd47ab75 100644
--- a/arch/x86/ia32/ia32_signal.c
+++ b/arch/x86/ia32/ia32_signal.c
@@ -61,9 +61,8 @@
 } while (0)
 
 #define RELOAD_SEG(seg)		{		\
-	unsigned int pre = GET_SEG(seg);	\
+	unsigned int pre = (seg) | 3;		\
 	unsigned int cur = get_user_seg(seg);	\
-	pre |= 3;				\
 	if (pre != cur)				\
 		set_user_seg(seg, pre);		\
 }
@@ -72,6 +71,7 @@ static int ia32_restore_sigcontext(struct pt_regs *regs,
 				   struct sigcontext_32 __user *sc)
 {
 	unsigned int tmpflags, err = 0;
+	u16 gs, fs, es, ds;
 	void __user *buf;
 	u32 tmp;
 
@@ -79,16 +79,10 @@ static int ia32_restore_sigcontext(struct pt_regs *regs,
 	current->restart_block.fn = do_no_restart_syscall;
 
 	get_user_try {
-		/*
-		 * Reload fs and gs if they have changed in the signal
-		 * handler.  This does not handle long fs/gs base changes in
-		 * the handler, but does not clobber them at least in the
-		 * normal case.
-		 */
-		RELOAD_SEG(gs);
-		RELOAD_SEG(fs);
-		RELOAD_SEG(ds);
-		RELOAD_SEG(es);
+		gs = GET_SEG(gs);
+		fs = GET_SEG(fs);
+		ds = GET_SEG(ds);
+		es = GET_SEG(es);
 
 		COPY(di); COPY(si); COPY(bp); COPY(sp); COPY(bx);
 		COPY(dx); COPY(cx); COPY(ip); COPY(ax);
@@ -106,6 +100,17 @@ static int ia32_restore_sigcontext(struct pt_regs *regs,
 		buf = compat_ptr(tmp);
 	} get_user_catch(err);
 
+	/*
+	 * Reload fs and gs if they have changed in the signal
+	 * handler.  This does not handle long fs/gs base changes in
+	 * the handler, but does not clobber them at least in the
+	 * normal case.
+	 */
+	RELOAD_SEG(gs);
+	RELOAD_SEG(fs);
+	RELOAD_SEG(ds);
+	RELOAD_SEG(es);
+
 	err |= fpu__restore_sig(buf, 1);
 
 	force_iret();