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

On Feb 25, 2019, at 3:53 AM, Peter Zijlstra [off-list ref] wrote:

quoted

On Mon, Feb 25, 2019 at 11:51:44AM +0100, Peter Zijlstra wrote:

quoted

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.

quoted

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

quoted

*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();