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author | Paolo Bonzini <pbonzini@redhat.com> | 2016-03-08 12:13:39 +0100 |
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committer | Paolo Bonzini <pbonzini@redhat.com> | 2016-03-10 11:26:07 +0100 |
commit | 844a5fe219cf472060315971e15cbf97674a3324 (patch) | |
tree | ffc7eb5a0cd87af357eec478e12c9dc902c3223f /Documentation/virtual/kvm/mmu.txt | |
parent | 313f636d5c490c9741d3f750dc8da33029edbc6b (diff) | |
download | linux-stable-844a5fe219cf472060315971e15cbf97674a3324.tar.gz linux-stable-844a5fe219cf472060315971e15cbf97674a3324.tar.bz2 linux-stable-844a5fe219cf472060315971e15cbf97674a3324.zip |
KVM: MMU: fix ept=0/pte.u=1/pte.w=0/CR0.WP=0/CR4.SMEP=1/EFER.NX=0 combo
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: f6577a5fa15d82217ca73c74cd2dcbc0f6c781dd
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Diffstat (limited to 'Documentation/virtual/kvm/mmu.txt')
-rw-r--r-- | Documentation/virtual/kvm/mmu.txt | 3 |
1 files changed, 2 insertions, 1 deletions
diff --git a/Documentation/virtual/kvm/mmu.txt b/Documentation/virtual/kvm/mmu.txt index daf9c0f742d2..c81731096a43 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, |