x86/entry/32: Check for VM86 mode in slow-path check

The SWITCH_TO_KERNEL_STACK macro only checks for CPL == 0 to go down the
slow and paranoid entry path. The problem is that this check also returns
true when coming from VM86 mode. This is not a problem by itself, as the
paranoid path handles VM86 stack-frames just fine, but it is not necessary
as the normal code path handles VM86 mode as well (and faster).

Extend the check to include VM86 mode. This also makes an optimization of
the paranoid path possible.

Signed-off-by: Joerg Roedel <jroedel@suse.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: "H . Peter Anvin" <hpa@zytor.com>
Cc: linux-mm@kvack.org
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Dave Hansen <dave.hansen@intel.com>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Juergen Gross <jgross@suse.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Jiri Kosina <jkosina@suse.cz>
Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: David Laight <David.Laight@aculab.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: Eduardo Valentin <eduval@amazon.com>
Cc: Greg KH <gregkh@linuxfoundation.org>
Cc: Will Deacon <will.deacon@arm.com>
Cc: aliguori@amazon.com
Cc: daniel.gruss@iaik.tugraz.at
Cc: hughd@google.com
Cc: keescook@google.com
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Waiman Long <llong@redhat.com>
Cc: Pavel Machek <pavel@ucw.cz>
Cc: "David H . Gutteridge" <dhgutteridge@sympatico.ca>
Cc: Arnaldo Carvalho de Melo <acme@kernel.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: joro@8bytes.org
Link: https://lkml.kernel.org/r/1532103744-31902-3-git-send-email-joro@8bytes.org

arch/x86/entry/entry_32.S

@@ -414,8 +414,16 @@
 	andl	$(0x0000ffff), PT_CS(%esp)
 
 	/* Special case - entry from kernel mode via entry stack */
-	testl	$SEGMENT_RPL_MASK, PT_CS(%esp)
-	jz	.Lentry_from_kernel_\@
+#ifdef CONFIG_VM86
+	movl	PT_EFLAGS(%esp), %ecx		# mix EFLAGS and CS
+	movb	PT_CS(%esp), %cl
+	andl	$(X86_EFLAGS_VM | SEGMENT_RPL_MASK), %ecx
+#else
+	movl	PT_CS(%esp), %ecx
+	andl	$SEGMENT_RPL_MASK, %ecx
+#endif
+	cmpl	$USER_RPL, %ecx
+	jb	.Lentry_from_kernel_\@
 
 	/* Bytes to copy */
 	movl	$PTREGS_SIZE, %ecx