x86: fix GS-base-dirty determination

load_segments() writes the two MSRs in their "canonical" positions
(GS_BASE for the user base, SHADOW_GS_BASE for the kernel one) and uses
SWAPGS to switch them around if the incoming vCPU is in kernel mode. In
order to not leave a stale kernel address in GS_BASE when the incoming
guest is in user mode, the check on the outgoing vCPU needs to be
dependent upon the mode it is currently in, rather than blindly looking
at the user base.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Release-acked-by: Julien Grall <julien.grall@linaro.org>

diff --git a/xen/arch/x86/domain.c b/xen/arch/x86/domain.c
index c28ac38fbe5c8298edcf0bd90e6138a5be4a85af..735f45c13320588fab18d53cb4239f68f09a5df1 100644 (file)
--- a/xen/arch/x86/domain.c
+++ b/xen/arch/x86/domain.c
@@ -1479,7 +1479,8 @@ static void save_segments(struct vcpu *v)
         if ( regs->gs & ~3 )
             v->arch.pv_vcpu.gs_base_user = 0;
     }
-    if ( v->arch.pv_vcpu.gs_base_user )
+    if ( v->arch.flags & TF_kernel_mode ? v->arch.pv_vcpu.gs_base_kernel
+                                        : v->arch.pv_vcpu.gs_base_user )
         dirty_segment_mask |= DIRTY_GS_BASE_USER;
 
     this_cpu(dirty_segment_mask) = dirty_segment_mask;