Re: [PATCH 2/6] x86/ia32: Fix ia32_restore_sigcontext AC leak

[Peter Zijlstra]

On Mon, Feb 25, 2019 at 07:41:50AM -0800, Andy Lutomirski wrote:
> This is so tangled.
> 
> How about changing RELOAD_SEG to replace unsigned int pre =
> GET_SEG(seg); with unsigned int pre = (seg); to make it less magic.
> Then do:
> 
> unsigned int gs = GET_SEG(gs);
> 
> ...
> 
> RELOAD_SEG(gs);
> 
> And now the code actually does what it looks like it does.

Is this what you mean?

---
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;                                        \
 })
 
 #define COPY_SEG_CPL3(seg)     do {                    \
-       regs->seg = GET_SEG(seg) | 3;                   \
+       regs->seg = GET_SEG(seg);                       \
 } while (0)
 
 #define RELOAD_SEG(seg)                {               \
-       unsigned int pre = GET_SEG(seg);        \
+       unsigned int pre = (seg);               \
        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();