[Xen-changelog] [xen stable-4.12] x86/cpuid: leak OSXSAVE only when XSAVE is not clear in policy

[patchbot@xxxxxxx]

commit 675ccffbb20c111db2c2703c26e8e151f993e2e5
Author:     Igor Druzhinin <igor.druzhinin@xxxxxxxxxx>
AuthorDate: Fri Jul 5 10:24:30 2019 +0200
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Fri Jul 5 10:24:30 2019 +0200

    x86/cpuid: leak OSXSAVE only when XSAVE is not clear in policy
    
    This fixes booting of old non-PV-OPS kernels which historically
    looked for OSXSAVE instead of XSAVE bit in CPUID to check whether
    XSAVE feature is enabled. If such a guest appears to be started on
    an XSAVE enabled CPU and the feature is explicitly cleared in
    policy, leaked OSXSAVE bit from Xen will lead to guest crash early in
    boot.
    
    Signed-off-by: Igor Druzhinin <igor.druzhinin@xxxxxxxxxx>
    Reviewed-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
    master commit: 902888922e6feda2c485cc4bdeffd0d6e6c26e14
    master date: 2019-06-28 13:17:53 +0100
---
 xen/arch/x86/cpuid.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/xen/arch/x86/cpuid.c b/xen/arch/x86/cpuid.c
index 3efad9cf17..57e80694f2 100644
--- a/xen/arch/x86/cpuid.c
+++ b/xen/arch/x86/cpuid.c
@@ -781,7 +781,8 @@ void guest_cpuid(const struct vcpu *v, uint32_t leaf,
              *    damage itself.
              *
              * - Enlightened CPUID or CPUID faulting available:
-             *    Xen can fully control what is seen here.  Guest kernels need
+             *    Xen can fully control what is seen here.  When the guest has
+             *    been configured to have XSAVE available, guest kernels need
              *    to see the leaked OSXSAVE via the enlightened path, but
              *    guest userspace and the native is given architectural
              *    behaviour.
@@ -791,7 +792,8 @@ void guest_cpuid(const struct vcpu *v, uint32_t leaf,
              */
             /* OSXSAVE clear in policy.  Fast-forward CR4 back in. */
             if ( (v->arch.pv.ctrlreg[4] & X86_CR4_OSXSAVE) ||
-                 (regs->entry_vector == TRAP_invalid_op &&
+                 (p->basic.xsave &&
+                  regs->entry_vector == TRAP_invalid_op &&
                   guest_kernel_mode(v, regs) &&
                   (read_cr4() & X86_CR4_OSXSAVE)) )
                 res->c |= cpufeat_mask(X86_FEATURE_OSXSAVE);
--
generated by git-patchbot for /home/xen/git/xen.git#stable-4.12

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