xen: temporary disable SMAP when forwarding hypercalls from user-space

The Xen page-table walker used to resolve the virtual addresses in the
hypercalls will refuse to access user-space pages when SMAP is enabled
unless the AC flag in EFLAGS is set (just like normal hardware with
SMAP support would do).

Since privcmd allows forwarding hypercalls (and buffers) from
user-space into Xen make sure SMAP is temporary disabled for the
duration of the hypercall from user-space.

Approved by:		re (gjb)
Sponsored by:		Citrix Systems R&D

1 files changed, 13 insertions, 1 deletions

diff --git a/sys/dev/xen/privcmd/privcmd.c b/sys/dev/xen/privcmd/privcmd.c
index 246390e3b7c2..e09886f42adb 100644
--- a/sys/dev/xen/privcmd/privcmd.c
+++ b/sys/dev/xen/privcmd/privcmd.c
@@ -232,9 +232,21 @@ privcmd_ioctl(struct cdev *dev, unsigned long cmd, caddr_t arg,
 		struct ioctl_privcmd_hypercall *hcall;
 
 		hcall = (struct ioctl_privcmd_hypercall *)arg;
-
+#ifdef __amd64__
+		/*
+		 * The hypervisor page table walker will refuse to access
+		 * user-space pages if SMAP is enabled, so temporary disable it
+		 * while performing the hypercall.
+		 */
+		if (cpu_stdext_feature & CPUID_STDEXT_SMAP)
+			stac();
+#endif
 		error = privcmd_hypercall(hcall->op, hcall->arg[0],
 		    hcall->arg[1], hcall->arg[2], hcall->arg[3], hcall->arg[4]);
+#ifdef __amd64__
+		if (cpu_stdext_feature & CPUID_STDEXT_SMAP)
+			clac();
+#endif
 		if (error >= 0) {
 			hcall->retval = error;
 			error = 0;