svn commit: r320804 - head/sys/i386/i386

[Konstantin Belousov]

Author: kib
Date: Sat Jul  8 11:07:39 2017
New Revision: 320804
URL: https://svnweb.freebsd.org/changeset/base/320804

Log:
  Fix handling of one more possible exception on return to usermode.
  
  If %ss is loaded with a segment pointing to a non-present descriptor
  by the IRETD instruction, a kernel-mode #SS exception is generated.
  Resulting T_STKFLT trap must be checked against doreti_iret_fault
  location and handled, otherwise userspace may panic the kernel.
  
  Note that this is i386 variant of FreeBSD-SA-15:21.amd64, but unlike
  amd64, there is no swapgs on i386 and the issue is arguably not
  exploitable.
  
  Reported by:	Maxime Villard <max at m00nbsd.net>
  Tested by:	pho
  Sponsored by:	The FreeBSD Foundation
  MFC after:	1 week

Modified:
  head/sys/i386/i386/trap.c

Modified: head/sys/i386/i386/trap.c
==============================================================================
--- head/sys/i386/i386/trap.c	Sat Jul  8 11:06:27 2017	(r320803)
+++ head/sys/i386/i386/trap.c	Sat Jul  8 11:07:39 2017	(r320804)
@@ -550,11 +550,7 @@ user_trctrap_out:
 					vm86_trap((struct vm86frame *)frame);
 				goto out;
 			}
-			if (type == T_STKFLT)
-				break;
-
 			/* FALL THROUGH */
-
 		case T_SEGNPFLT:	/* segment not present fault */
 			if (curpcb->pcb_flags & PCB_VM86CALL)
 				break;
@@ -595,6 +591,9 @@ user_trctrap_out:
 				frame->tf_eip = (int)doreti_iret_fault;
 				goto out;
 			}
+			if (type == T_STKFLT)
+				break;
+
 			if (frame->tf_eip == (int)doreti_popl_ds) {
 				frame->tf_eip = (int)doreti_popl_ds_fault;
 				goto out;