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OpenBSD 6.1 errata 031, October 4th, 2017:

Do not leak kernel address when restoring an invalid FPU context.

Apply by doing:
    signify -Vep /etc/signify/openbsd-61-base.pub -x 031_xrstor_resume.patch.sig \
        -m - | (cd /usr/src && patch -p0)

And then rebuild and install a new kernel:
    KK=`sysctl -n kern.osversion | cut -d# -f1`
    cd /usr/src/sys/arch/`machine`/compile/$KK
    make obj
    make config
    make
    make install

Index: sys/arch/amd64/amd64/fpu.c
===================================================================
RCS file: /data/mirror/openbsd/cvs/src/sys/arch/amd64/amd64/fpu.c,v
retrieving revision 1.33.6.2
diff -u -p -r1.33.6.2 fpu.c
--- sys/arch/amd64/amd64/fpu.c	3 Oct 2017 22:11:56 -0000	1.33.6.2
+++ sys/arch/amd64/amd64/fpu.c	4 Oct 2017 13:20:39 -0000
@@ -56,7 +56,8 @@
 
 #include <dev/isa/isavar.h>
 
-void	xrstor_user(struct savefpu *_addr, uint64_t _mask);
+int	xrstor_user(struct savefpu *_addr, uint64_t _mask);
+void	trap(struct trapframe *);
 
 /*
  * We do lazy initialization and switching using the TS bit in cr0 and the
@@ -81,7 +82,7 @@ void	xrstor_user(struct savefpu *_addr, 
  */
 uint64_t	xsave_mask;
 
-void fpudna(struct cpu_info *);
+void fpudna(struct cpu_info *, struct trapframe *);
 static int x86fpflags_to_siginfo(u_int32_t);
 
 /*
@@ -195,7 +196,7 @@ x86fpflags_to_siginfo(u_int32_t flags)
  * saved state.
  */
 void
-fpudna(struct cpu_info *ci)
+fpudna(struct cpu_info *ci, struct trapframe *frame)
 {
 	struct savefpu *sfp;
 	struct proc *p;
@@ -256,7 +257,12 @@ fpudna(struct cpu_info *ci)
 		p->p_md.md_flags |= MDP_USEDFPU;
 	} else {
 		if (xsave_mask) {
-			xrstor_user(sfp, xsave_mask);
+			if (xrstor_user(sfp, xsave_mask)) {
+				fpusave_proc(p, 0);	/* faulted */
+				frame->tf_trapno = T_PROTFLT;
+				trap(frame);
+				return;
+			}
 		} else {
 			static double	zero = 0.0;
 
Index: sys/arch/amd64/amd64/locore.S
===================================================================
RCS file: /data/mirror/openbsd/cvs/src/sys/arch/amd64/amd64/locore.S,v
retrieving revision 1.84.4.1
diff -u -p -r1.84.4.1 locore.S
--- sys/arch/amd64/amd64/locore.S	3 Oct 2017 22:11:56 -0000	1.84.4.1
+++ sys/arch/amd64/amd64/locore.S	4 Oct 2017 13:20:39 -0000
@@ -1210,6 +1210,10 @@ ENTRY(xrstor_user)
 	.globl	xrstor_fault
 xrstor_fault:
 	xrstor	(%rdi)
+	xorl	%eax, %eax
+	ret
+ENTRY(xrstor_resume)
+	movl	$1, %eax
 	ret
 
 ENTRY(pagezero)
Index: sys/arch/amd64/amd64/trap.c
===================================================================
RCS file: /data/mirror/openbsd/cvs/src/sys/arch/amd64/amd64/trap.c,v
retrieving revision 1.53.4.2
diff -u -p -r1.53.4.2 trap.c
--- sys/arch/amd64/amd64/trap.c	3 Oct 2017 22:11:56 -0000	1.53.4.2
+++ sys/arch/amd64/amd64/trap.c	4 Oct 2017 13:20:39 -0000
@@ -148,7 +148,8 @@ trap(struct trapframe *frame)
 	struct proc *p = curproc;
 	int type = (int)frame->tf_trapno;
 	struct pcb *pcb;
-	extern char doreti_iret[], resume_iret[], xrstor_fault[];
+	extern char doreti_iret[], resume_iret[];
+	extern char xrstor_fault[], xrstor_resume[];
 	caddr_t onfault;
 	int error;
 	uint64_t cr2;
@@ -228,8 +229,8 @@ trap(struct trapframe *frame)
 		 * instruction that faulted.
 		 */
 		if (frame->tf_rip == (u_int64_t)xrstor_fault && p != NULL) {
-			fpusave_proc(p, 0);
-			goto user_trap;
+			frame->tf_rip = (u_int64_t)xrstor_resume;
+			return;
 		}
 	case T_SEGNPFLT:
 	case T_ALIGNFLT:
@@ -260,7 +261,6 @@ copyfault:
 	case T_TSSFLT|T_USER:
 	case T_SEGNPFLT|T_USER:
 	case T_STKFLT|T_USER:
-user_trap:
 #ifdef TRAP_SIGDEBUG
 		printf("pid %d (%s): %s at rip %llx addr %llx\n",
 		    p->p_p->ps_pid, p->p_p->ps_comm, "BUS",
Index: sys/arch/amd64/amd64/vector.S
===================================================================
RCS file: /data/mirror/openbsd/cvs/src/sys/arch/amd64/amd64/vector.S,v
retrieving revision 1.47.4.1
diff -u -p -r1.47.4.1 vector.S
--- sys/arch/amd64/amd64/vector.S	26 Aug 2017 00:14:20 -0000	1.47.4.1
+++ sys/arch/amd64/amd64/vector.S	4 Oct 2017 13:20:39 -0000
@@ -127,6 +127,7 @@ IDTVEC(trap07)
 	cld
 	SMAP_CLAC
 	movq	CPUVAR(SELF),%rdi
+	movq	%rsp, %rsi
 	call	_C_LABEL(fpudna)
 	INTRFASTEXIT
 IDTVEC(trap08)