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authorPaolo Bonzini <pbonzini@redhat.com>2016-03-08 12:13:39 +0100
committerGreg Kroah-Hartman <gregkh@linuxfoundation.org>2016-03-16 08:42:58 -0700
commit68ed2ca153c74438cff147eeabc3306c07d730bb (patch)
tree598b9b8c990c44a30879d530a87e7e23b077889f /Documentation
parent1c463a390a89beb929ed2750c79d6eb6d06c7cdd (diff)
KVM: MMU: fix ept=0/pte.u=1/pte.w=0/CR0.WP=0/CR4.SMEP=1/EFER.NX=0 combo
commit 844a5fe219cf472060315971e15cbf97674a3324 upstream. 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: Andy Lutomirski <luto@amacapital.net> Reviewed-by: Xiao Guangrong <guangrong.xiao@linux.intel.com> Fixes: f6577a5fa15d82217ca73c74cd2dcbc0f6c781dd Signed-off-by: Paolo Bonzini <pbonzini@redhat.com> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Diffstat (limited to 'Documentation')
-rw-r--r--Documentation/virtual/kvm/mmu.txt3
1 files changed, 2 insertions, 1 deletions
diff --git a/Documentation/virtual/kvm/mmu.txt b/Documentation/virtual/kvm/mmu.txt
index 3a4d681c3e98..b653641d4261 100644
--- a/Documentation/virtual/kvm/mmu.txt
+++ b/Documentation/virtual/kvm/mmu.txt
@@ -358,7 +358,8 @@ In the first case there are two additional complications:
- if CR4.SMEP is enabled: since we've turned the page into a kernel page,
the kernel may now execute it. We handle this by also setting spte.nx.
If we get a user fetch or read fault, we'll change spte.u=1 and
- spte.nx=gpte.nx back.
+ spte.nx=gpte.nx back. For this to work, KVM forces EFER.NX to 1 when
+ shadow paging is in use.
- if CR4.SMAP is disabled: since the page has been changed to a kernel
page, it can not be reused when CR4.SMAP is enabled. We set
CR4.SMAP && !CR0.WP into shadow page's role to avoid this case. Note,