[Xen-changelog] [xen master] x86/mm: fix #GP(0) in switch_cr3_cr4()

[patchbot@xxxxxxx]

commit fdc2056767ba74346dfd8bbe868bb22521ba1418
Author:     Jan Beulich <jbeulich@xxxxxxxx>
AuthorDate: Tue Mar 5 17:02:36 2019 +0100
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Tue Mar 5 17:02:36 2019 +0100

    x86/mm: fix #GP(0) in switch_cr3_cr4()
    
    With "pcid=no-xpti" and opposite XPTI settings in two 64-bit PV domains
    (achievable with one of "xpti=no-dom0" or "xpti=no-domu"), switching
    from a PCID-disabled to a PCID-enabled 64-bit PV domain fails to set
    CR4.PCIDE in time, as CR4.PGE would not be set in either (see
    pv_fixup_guest_cr4(), in particular as used by write_ptbase()), and
    hence the early CR4 write would be skipped.
    
    Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
    Reviewed-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
    Release-acked-by: Juergen Gross <jgross@xxxxxxxx>
---
 xen/arch/x86/flushtlb.c | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/xen/arch/x86/flushtlb.c b/xen/arch/x86/flushtlb.c
index eb222e3412..4004129c49 100644
--- a/xen/arch/x86/flushtlb.c
+++ b/xen/arch/x86/flushtlb.c
@@ -122,6 +122,7 @@ void switch_cr3_cr4(unsigned long cr3, unsigned long cr4)
         write_cr4(old_cr4);
     }
     else if ( use_invpcid )
+    {
         /*
          * Flushing the TLB via INVPCID is necessary only in case PCIDs are
          * in use, which is true only with INVPCID being available.
@@ -132,6 +133,19 @@ void switch_cr3_cr4(unsigned long cr3, unsigned long cr4)
          */
         invpcid_flush_all_nonglobals();
 
+        /*
+         * CR4.PCIDE needs to be set before the CR3 write below. Otherwise
+         * - the CR3 write will fault when CR3.NOFLUSH is set (which is the
+         *   case normally),
+         * - the subsequent CR4 write will fault if CR3.PCID != 0.
+         */
+        if ( (old_cr4 & X86_CR4_PCIDE) < (cr4 & X86_CR4_PCIDE) )
+        {
+            write_cr4(cr4);
+            old_cr4 = cr4;
+        }
+    }
+
     /*
      * If we don't change PCIDs, the CR3 write below needs to flush this very
      * PCID, even when a full flush was performed above, as we are currently
--
generated by git-patchbot for /home/xen/git/xen.git#master

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