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Yes, all of these are needed. :) This is admittedly a bit odd, but
kvm-unit-tests access.flat tests this if you run it with "-cpu host"
and of course ept=0.
KVM runs the guest with CR0.WP=1, so it must handle supervisor writes
specially when pte.u=1/pte.w=0/CR0.WP=0. Such writes cause a fault
when U=1 and W=0 in the SPTE, but they must succeed because CR0.WP=0.
When KVM gets the fault, it sets U=0 and W=1 in the shadow PTE and
restarts execution. This will still cause a user write to fault, while
supervisor writes will succeed. User reads will fault spuriously now,
and KVM will then flip U and W again in the SPTE (U=1, W=0). User reads
will be enabled and supervisor writes disabled, going back to the
originary situation where supervisor writes fault spuriously.
When SMEP is in effect, however, U=0 will enable kernel execution of
this page. To avoid this, KVM also sets NX=1 in the shadow PTE together
with U=0. If the guest has not enabled NX, the result is a continuous
stream of page faults due to the NX bit being reserved.
The fix is to force EFER.NX=1 even if the CPU is taking care of the EFER
switch. (All machines with SMEP have the CPU_LOAD_IA32_EFER vm-entry
control, so they do not use user-return notifiers for EFER---if they did,
EFER.NX would be forced to the same value as the host).
There is another bug in the reserved bit check, which I've split to a
separate patch for easier application to stable kernels.
Cc: stable@vger.kernel.org
Cc: Andy Lutomirski <luto@amacapital.net>
Reviewed-by: Xiao Guangrong <guangrong.xiao@linux.intel.com>
Fixes:
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| paravirt_ops.txt | ||