[PATCH v3 1/1] x86/ia32: Leave NULL selector values 0~3 as is

["Xin Li (Intel)" <xin@zytor.com>]

The first GDT descriptor is reserved as 'NULL descriptor'.  As bits 0
and 1 of a segment selector, i.e., the RPL bits, are NOT used to index
GDT, selector values 0~3 all point to the NULL descriptor, thus values
0, 1, 2 and 3 are all valid NULL selector values.

When a NULL selector value is to be loaded into a segment register,
reload_segments() sets its RPL bits.  Later IRET zeros ES, FS, GS, and
DS segment registers if any of them is found to have any nonzero NULL
selector value.  The two operations offset each other to actually effect
a nop.

Besides, zeroing of RPL in NULL selector values is an information leak
in pre-FRED systems as userspace can spot any interrupt/exception by
loading a nonzero NULL selector, and waiting for it to drop to zero.
But there is nothing software can do to prevent it before FRED.

ERETU, the only legit instruction to return to userspace from kernel
under FRED, by design does NOT zero any segment register to avoid this
problem behavior.

As such, leave NULL selector values 0~3 as is.

Do the same on 32-bit kernel as well.

Signed-off-by: Xin Li (Intel) <xin@zytor.com>
---

Changes since v2:
* No, don't zero non-zero NULL selector values, essentially revert
  to v1 (Andrew Cooper).

Changes since v1:
* Normalize non-zero NULL selector values to 0 (Eric W. Biederman).
* Apply the same normalization logic in a 32bit kernel (Eric W.
  Biederman).
---
 arch/x86/kernel/signal_32.c | 47 +++++++++++++++++++++++++++----------
 1 file changed, 35 insertions(+), 12 deletions(-)

diff --git a/arch/x86/kernel/signal_32.c b/arch/x86/kernel/signal_32.c
index ef654530bf5a..23879f14aa51 100644
--- a/arch/x86/kernel/signal_32.c
+++ b/arch/x86/kernel/signal_32.c
@@ -33,6 +33,29 @@
 #include <asm/smap.h>
 #include <asm/gsseg.h>
 
+/*
+ * The first GDT descriptor is reserved as 'NULL descriptor'.  As bits 0
+ * and 1 of a segment selector, i.e., the RPL bits, are NOT used to index
+ * GDT, selector values 0~3 all point to the NULL descriptor, thus values
+ * 0, 1, 2 and 3 are all valid NULL selector values.
+ *
+ * However IRET zeros ES, FS, GS, and DS segment registers if any of them
+ * is found to have any nonzero NULL selector value, which can be used by
+ * userspace in pre-FRED systems to spot any interrupt/exception by loading
+ * a nonzero NULL selector and waiting for it to drop to zero.  Before FRED
+ * there is nothing we can do to prevent such an information leak.
+ *
+ * ERETU, the only legit instruction to return to userspace from kernel
+ * under FRED, by design does NOT zero any segment register to avoid this
+ * problem behavior.
+ *
+ * As such, leave NULL selector values 0~3 as is.
+ */
+static inline u16 usrseg(u16 sel)
+{
+	return sel <= 3 ? sel : sel | 3;
+}
+
 #ifdef CONFIG_IA32_EMULATION
 #include <asm/unistd_32_ia32.h>
 
@@ -41,17 +64,17 @@ static inline void reload_segments(struct sigcontext_32 *sc)
 	unsigned int cur;
 
 	savesegment(gs, cur);
-	if ((sc->gs | 0x03) != cur)
-		load_gs_index(sc->gs | 0x03);
+	if (usrseg(sc->gs) != cur)
+		load_gs_index(usrseg(sc->gs));
 	savesegment(fs, cur);
-	if ((sc->fs | 0x03) != cur)
-		loadsegment(fs, sc->fs | 0x03);
+	if (usrseg(sc->fs) != cur)
+		loadsegment(fs, usrseg(sc->fs));
 	savesegment(ds, cur);
-	if ((sc->ds | 0x03) != cur)
-		loadsegment(ds, sc->ds | 0x03);
+	if (usrseg(sc->ds) != cur)
+		loadsegment(ds, usrseg(sc->ds));
 	savesegment(es, cur);
-	if ((sc->es | 0x03) != cur)
-		loadsegment(es, sc->es | 0x03);
+	if (usrseg(sc->es) != cur)
+		loadsegment(es, usrseg(sc->es));
 }
 
 #define sigset32_t			compat_sigset_t
@@ -113,10 +136,10 @@ static bool ia32_restore_sigcontext(struct pt_regs *regs,
 	 */
 	reload_segments(&sc);
 #else
-	loadsegment(gs, sc.gs);
-	regs->fs = sc.fs;
-	regs->es = sc.es;
-	regs->ds = sc.ds;
+	loadsegment(gs, usrseg(sc.gs));
+	regs->fs = usrseg(sc.fs);
+	regs->es = usrseg(sc.es);
+	regs->ds = usrseg(sc.ds);
 #endif
 
 	return fpu__restore_sig(compat_ptr(sc.fpstate), 1);

base-commit: 25f324dbc68cd6ff972f5ab3bb07f1bf6200f0e0
-- 
2.47.0