x86, 64-bit: swapgs pvop with a user-stack can never be called

It's never safe to call a swapgs pvop when the user stack is current -
it must be inline replaced.  Rather than making a call, the
SWAPGS_UNSAFE_STACK pvop always just puts "swapgs" as a placeholder,
which must either be replaced inline or trap'n'emulated (somehow).

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Cc: xen-devel <xen-devel@lists.xensource.com>
Cc: Stephen Tweedie <sct@redhat.com>
Cc: Eduardo Habkost <ehabkost@redhat.com>
Cc: Mark McLoughlin <markmc@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

2 files changed, 11 insertions, 1 deletions

diff --git a/include/asm-x86/irqflags.h b/include/asm-x86/irqflags.h
index ea9bd2635d59..d17e1f623dbb 100644
--- a/include/asm-x86/irqflags.h
+++ b/include/asm-x86/irqflags.h
@@ -111,6 +111,7 @@ static inline unsigned long __raw_local_irq_save(void)
 #define DISABLE_INTERRUPTS(x)   cli
 
 #ifdef CONFIG_X86_64
+#define SWAPGS_UNSAFE_STACK     swapgs
 #define INTERRUPT_RETURN        iretq
 #define USERGS_SYSRET64                         \
         swapgs;                                 \
@@ -185,7 +186,6 @@ static inline void trace_hardirqs_fixup(void)
  * Either way, this is a good way to document that we don't
  * have a reliable stack. x86_64 only.
  */
-#define SWAPGS_UNSAFE_STACK     swapgs
 #define ARCH_TRACE_IRQS_ON              call trace_hardirqs_on_thunk
 #define ARCH_TRACE_IRQS_OFF             call trace_hardirqs_off_thunk
 #define ARCH_LOCKDEP_SYS_EXIT           call lockdep_sys_exit_thunk
diff --git a/include/asm-x86/paravirt.h b/include/asm-x86/paravirt.h
index 33f72f8fe757..3286a0c63b42 100644
--- a/include/asm-x86/paravirt.h
+++ b/include/asm-x86/paravirt.h
@@ -1522,6 +1522,16 @@ static inline unsigned long __raw_local_irq_save(void)
 
 
 #else   /* !CONFIG_X86_32 */
+
+/*
+ * If swapgs is used while the userspace stack is still current,
+ * there's no way to call a pvop.  The PV replacement *must* be
+ * inlined, or the swapgs instruction must be trapped and emulated.
+ */
+#define SWAPGS_UNSAFE_STACK                                             \
+        PARA_SITE(PARA_PATCH(pv_cpu_ops, PV_CPU_swapgs), CLBR_NONE,     \
+                  swapgs)
+
 #define SWAPGS                                                          \
         PARA_SITE(PARA_PATCH(pv_cpu_ops, PV_CPU_swapgs), CLBR_NONE,     \
                   PV_SAVE_REGS;                                         \