Re: [RFC][PATCH] objtool: STAC/CLAC validation

[Peter Zijlstra]

On Mon, Feb 25, 2019 at 11:51:44AM +0100, Peter Zijlstra wrote:
> On Fri, Feb 22, 2019 at 03:55:25PM -0800, Andy Lutomirski wrote:
> > I'm wondering if we can just change the code that does getreg() and
> > load_gs_index() so it doesn't do it with AC set.  Also, what about
> > paravirt kernels?  They'll call into PV code for load_gs_index() with
> > AC set.
> 
> Paravirt can go bugger off. There's no sane way to fix that.

> I don't fully understand that code at all; I also have no clue why GS
> has paravirt bits on but the other segments do not. 

*sigh* SWAPGS

> *thought*... we could delay the actual set_user_seg() thing until after
> the get_user_catch(), would that work?


How horrible / broken is this?

---

diff --git a/arch/x86/ia32/ia32_signal.c b/arch/x86/ia32/ia32_signal.c
index 321fe5f5d0e9..67c866943102 100644
--- a/arch/x86/ia32/ia32_signal.c
+++ b/arch/x86/ia32/ia32_signal.c
@@ -60,17 +60,21 @@
 	regs->seg = GET_SEG(seg) | 3;			\
 } while (0)
 
-#define RELOAD_SEG(seg)		{		\
-	unsigned int pre = GET_SEG(seg);	\
-	unsigned int cur = get_user_seg(seg);	\
-	pre |= 3;				\
-	if (pre != cur)				\
-		set_user_seg(seg, pre);		\
+#define LOAD_SEG(seg)		{			\
+	pre_##seg = 3 | GET_SEG(seg);			\
+	cur_##seg = get_user_seg(seg);			\
+}
+
+#define RELOAD_SEG(seg)		{			\
+	if (pre_##seg != cur_##seg)			\
+		set_user_seg(seg, pre_##seg);		\
 }
 
 static int ia32_restore_sigcontext(struct pt_regs *regs,
 				   struct sigcontext_32 __user *sc)
 {
+	u16 pre_gs, pre_fs, pre_ds, pre_es;
+	u16 cur_gs, cur_fs, cur_ds, cur_es;
 	unsigned int tmpflags, err = 0;
 	void __user *buf;
 	u32 tmp;
@@ -85,10 +89,10 @@ static int ia32_restore_sigcontext(struct pt_regs *regs,
 		 * 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);
+		LOAD_SEG(gs);
+		LOAD_SEG(fs);
+		LOAD_SEG(ds);
+		LOAD_SEG(es);
 
 		COPY(di); COPY(si); COPY(bp); COPY(sp); COPY(bx);
 		COPY(dx); COPY(cx); COPY(ip); COPY(ax);
@@ -106,6 +110,11 @@ static int ia32_restore_sigcontext(struct pt_regs *regs,
 		buf = compat_ptr(tmp);
 	} get_user_catch(err);
 
+	RELOAD_SEG(gs);
+	RELOAD_SEG(fs);
+	RELOAD_SEG(ds);
+	RELOAD_SEG(es);
+
 	err |= fpu__restore_sig(buf, 1);
 
 	force_iret();