[xen stable-4.12] x86: validate VM assist value in arch_set_info_guest()

[patchbot@xxxxxxx]

commit d32cbbc141837600aa74f331c31a06df3777a2fb
Author:     Jan Beulich <jbeulich@xxxxxxxx>
AuthorDate: Mon Apr 27 15:57:13 2020 +0200
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Mon Apr 27 15:57:13 2020 +0200

    x86: validate VM assist value in arch_set_info_guest()
    
    While I can't spot anything that would go wrong, just like the
    respective hypercall only permits applicable bits to be set, we should
    also do so when loading guest context.
    
    Reported-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
    Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
    Acked-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
    master commit: a62c6fe05c4ae905b7d4cb0ca946508b7f96d522
    master date: 2020-04-22 13:01:10 +0200
---
 xen/arch/x86/domain.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/xen/arch/x86/domain.c b/xen/arch/x86/domain.c
index 745da387f6..6b0cb689a1 100644
--- a/xen/arch/x86/domain.c
+++ b/xen/arch/x86/domain.c
@@ -868,6 +868,9 @@ int arch_set_info_guest(
         }
     }
 
+    if ( v->vcpu_id == 0 && (c(vm_assist) & ~arch_vm_assist_valid_mask(d)) )
+        return -EINVAL;
+
     if ( is_hvm_domain(d) )
     {
         for ( i = 0; i < ARRAY_SIZE(v->arch.dr); ++i )
--
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