x86: refine guest_mode()

The 2nd of the assertions as well as the macro's return value have been
assuming we're on the primary stack. While for most IST exceptions we
switch back to the main one when user mode was interrupted, for #DF we
intentionally never do, and hence a #DF actually triggering on a user
mode insn (which then is still a Xen bug) would in turn trigger this
assertion, rather than cleanly logging state.

Reported-by: Andrew Cooper <andrew.cooper3@citrix.com>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>

diff --git a/xen/include/asm-x86/regs.h b/xen/include/asm-x86/regs.h
index 8d984ea38826d0c79cca7c9efe382860aaa4810c..3fb94deedc7b9bc7ff3c88b28a450e9e7fbe3654 100644 (file)
--- a/xen/include/asm-x86/regs.h
+++ b/xen/include/asm-x86/regs.h
@@ -10,9 +10,10 @@
     /* Frame pointer must point into current CPU stack. */                    \
     ASSERT(diff < STACK_SIZE);                                                \
     /* If not a guest frame, it must be a hypervisor frame. */                \
-    ASSERT((diff == 0) || (r->cs == __HYPERVISOR_CS));                        \
+    if ( diff < PRIMARY_STACK_SIZE )                                          \
+        ASSERT(!diff || ((r)->cs == __HYPERVISOR_CS));                        \
     /* Return TRUE if it's a guest frame. */                                  \
-    (diff == 0);                                                              \
+    !diff || ((r)->cs != __HYPERVISOR_CS);                                    \
 })
 
 #define read_sreg(name) ({                           \