Calculate automatically the padding necessary for lining up the

iretq instruction used when Meltdown mitigation is effect.  It got
pushed off when an lfence was added in locore.S rev 1.107, resulting
in two signals being sent instead of one when iretq faulted, and
neither signal had the correct sigcontext info.  Update the makefile
rule for locore.o to verify that things are correct.

ok mlarkin@

diff --git a/sys/arch/amd64/amd64/locore.S b/sys/arch/amd64/amd64/locore.S
index 62ec812..ecc9eed 100644 (file)
--- a/sys/arch/amd64/amd64/locore.S
+++ b/sys/arch/amd64/amd64/locore.S
@@ -1,4 +1,4 @@
-/*     $OpenBSD: locore.S,v 1.107 2018/07/24 02:42:25 guenther Exp $   */
+/*     $OpenBSD: locore.S,v 1.108 2018/09/09 22:09:36 guenther Exp $   */
 /*     $NetBSD: locore.S,v 1.13 2004/03/25 18:33:17 drochner Exp $     */
 
 /*
@@ -860,10 +860,31 @@ intr_user_exit_post_ast:
        movq    CPUVAR(USER_CR3),%rax
        movq    %rax,%cr3
 Xiretq_trampback:
+KTEXT_PAGE_END
+/* the movq %cr3 switches to this "KUTEXT" page */
+KUTEXT_PAGE_START
+       .space  (Xiretq_trampback - Xsyscall_meltdown) - \
+               (. - XUsyscall_meltdown), 0xcc
+       movq    CPUVAR(SCRATCH),%rax
+.Liretq_swapgs:
+       swapgs
+doreti_iret_meltdown:
+       iretq
+KUTEXT_PAGE_END
+/*
+ * Back to the "KTEXT" page to fill in the speculation trap and the
+ * swapgs+iretq used for non-Meltdown kernels.  This switching back
+ * and forth between segments is so that we can do the .space
+ * calculation below to guarantee the iretq's above and below line
+ * up, so the 'doreti_iret' label lines up with the iretq whether
+ * the CPU is affected by Meltdown or not.
+ */
+KTEXT_PAGE_START
 0:     pause
        lfence
        jmp     0b
-       .space  5,0xcc          /* pad to match "movq CPUVAR(SCRATCH),%rax" */
+       .space  (.Liretq_swapgs - XUsyscall_meltdown) - \
+               (. - Xsyscall_meltdown), 0xcc
        CODEPATCH_END(CPTAG_MELTDOWN_NOP)
        swapgs
 
@@ -872,14 +893,6 @@ _C_LABEL(doreti_iret):
        iretq
 KTEXT_PAGE_END
 
-KUTEXT_PAGE_START
-       .space  (Xiretq_trampback - Xsyscall_meltdown) - \
-               (. - XUsyscall_meltdown), 0xcc
-       movq    CPUVAR(SCRATCH),%rax
-       swapgs
-       iretq
-KUTEXT_PAGE_END
-
        .text
        _ALIGN_TRAPS
 .Lintr_restore_xstate:         /* CPU doesn't have curproc's xstate */

diff --git a/sys/arch/amd64/conf/Makefile.amd64 b/sys/arch/amd64/conf/Makefile.amd64
index bd1d63d..270559f 100644 (file)
--- a/sys/arch/amd64/conf/Makefile.amd64
+++ b/sys/arch/amd64/conf/Makefile.amd64
@@ -1,4 +1,4 @@
-#      $OpenBSD: Makefile.amd64,v 1.101 2018/08/21 18:06:12 anton Exp $
+#      $OpenBSD: Makefile.amd64,v 1.102 2018/09/09 22:09:36 guenther Exp $
 
 # For instructions on building kernels consult the config(8) and options(4)
 # manual pages.
@@ -125,6 +125,12 @@ mcount.o: $S/lib/libkern/mcount.c Makefile
 ioconf.o: ioconf.c
        ${NORMAL_C}
 
+locore.o: assym.h
+       ${NORMAL_S}
+       @[[ -n `objdump -D $@ | grep -A1 doreti_iret | sort | uniq -d` ]] || \
+        { rm -f $@; echo "ERROR: overlaid iretq instructions don't line up"; \
+          echo "#GP-on-iretq fault handling would be broken"; exit 1; }
+
 ld.script: ${_machdir}/conf/ld.script
        cp ${_machdir}/conf/ld.script $@
 
@@ -155,7 +161,7 @@ cleandir: clean
 depend obj:
 
 locore0.o: ${_machdir}/${_mach}/locore0.S assym.h
-locore.o mutex.o vector.o copy.o spl.o: assym.h
+mutex.o vector.o copy.o spl.o: assym.h
 mptramp.o acpi_wakecode.o vmm_support.o: assym.h
 
 hardlink-obsd: