[PATCH v2] x86: don't allow clearing of TF_kernel_mode for other than 64-bit PV

["Jan Beulich" <JBeulich@suse.com>]

The flag is really only meant for those, both HVM and 32-bit PV tell
kernel from user mode based on CPL/RPL. Remove the all-question-marks
comment and let's be on the safe side here and also suppress clearing
for 32-bit PV (this isn't a fast path after all).

Remove no longer necessary is_pv_32bit_*() from sh_update_cr3() and
sh_walk_guest_tables(). Note that shadow_one_bit_disable() already
assumes the new behavior.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
---
v2: Also adjust shadow code.

--- a/xen/arch/x86/domain.c
+++ b/xen/arch/x86/domain.c
@@ -843,9 +843,15 @@ int arch_set_info_guest(
             return -EINVAL;
     }
 
-    v->arch.flags &= ~TF_kernel_mode;
-    if ( (flags & VGCF_in_kernel) || is_hvm_domain(d)/*???*/ )
-        v->arch.flags |= TF_kernel_mode;
+    v->arch.flags |= TF_kernel_mode;
+    if ( unlikely(!(flags & VGCF_in_kernel)) &&
+         /*
+          * TF_kernel_mode is only allowed to be clear for 64-bit PV. See
+          * update_cr3(), sh_update_cr3(), sh_walk_guest_tables(), and
+          * shadow_one_bit_disable() for why that is.
+          */
+         !is_hvm_domain(d) && !is_pv_32bit_domain(d) )
+        v->arch.flags &= ~TF_kernel_mode;
 
     v->arch.vgc_flags = flags;
 
--- a/xen/arch/x86/mm/shadow/multi.c
+++ b/xen/arch/x86/mm/shadow/multi.c
@@ -180,7 +180,7 @@ sh_walk_guest_tables(struct vcpu *v, uns
                              INVALID_MFN, v->arch.paging.shadow.gl3e);
 #else /* 32 or 64 */
     const struct domain *d = v->domain;
-    mfn_t root_mfn = ((v->arch.flags & TF_kernel_mode) || is_pv_32bit_domain(d)
+    mfn_t root_mfn = (v->arch.flags & TF_kernel_mode
                       ? pagetable_get_mfn(v->arch.guest_table)
                       : pagetable_get_mfn(v->arch.guest_table_user));
     void *root_map = map_domain_page(root_mfn);
@@ -4025,7 +4025,7 @@ sh_update_cr3(struct vcpu *v, int do_loc
                   v, (unsigned long)pagetable_get_pfn(v->arch.guest_table));
 
 #if GUEST_PAGING_LEVELS == 4
-    if ( !(v->arch.flags & TF_kernel_mode) && !is_pv_32bit_domain(d) )
+    if ( !(v->arch.flags & TF_kernel_mode) )
         gmfn = pagetable_get_mfn(v->arch.guest_table_user);
     else
 #endif





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