Re: [PATCH 6/6] KVM: x86: check DR6/7 high-bits are clear only on long-mode

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

 



On 6/16/14, 5:56 PM, Paolo Bonzini wrote:
Il 16/06/2014 13:53, Nadav Amit ha scritto:
On 6/16/14, 2:09 PM, Paolo Bonzini wrote:
Il 16/06/2014 12:33, Nadav Amit ha scritto:

Do you get this if the input register has bit 31 set?
No. To be frank, the scenario may be considered a bit synthetic: the
guest assigns a value to a general-purpose register in 64-bit mode,
setting the high 32-bits to some non-zero value. Then, later, in 32-bit
mode, the guest performs MOV DR instruction. In between the two
assignments, the general purpose register is unmodified, so the high
32-bits of the general purpose registers are still set.

Note that this scenario does not occur when MOV DR is emulated, but
when
handle_dr() is called. In this case, the entire 64-bits of the general
purpose register used for MOV DR are read, regardless to the execution
mode of the guest.

I wonder if the same bug happens elsewhere.  For example,
kvm_emulate_hypercall doesn't look at CS.L/CS.DB, which is really a
corner case but arguably also a bug.  kvm_hv_hypercall instead does it
right.

Perhaps we need a variant of kvm_register_read that (on 64-bit hosts)
checks EFER/CS.L/CS.DB and masks the returned value accordingly.  You
could call it kvm_register_readl.

There are two questions that come in mind:
1. Should we ignore CS.DB? It would make it consistent with
kvm_hv_hypercall and handle_dr. I think this is the proper behavior.

It depends on what you're using it for, but as a start yes.

2. Reading CS.L once and masking all the registers (i.e., changing the
is_long_mode in kvm_emulate_hypercall to is_64_bit_mode) is likely to be
more efficient.

Yes, for the case of kvm_emulate_hypercall.  Then you can build
kvm_register_readl on top of is_64bit_mode and fix this bug with that
function.  Did you check that handle_cr is unaffected?

handle_cr is affected, and there are some other instances of affected register reads. Actually, most of the vmx instruction exit handling routines ignore long-mode and cs.l when considering the register operand size.

I'll make the necessary changes and resubmit.

Nadav

--
To unsubscribe from this list: send the line "unsubscribe kvm" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html




[Index of Archives]     [KVM ARM]     [KVM ia64]     [KVM ppc]     [Virtualization Tools]     [Spice Development]     [Libvirt]     [Libvirt Users]     [Linux USB Devel]     [Linux Audio Users]     [Yosemite Questions]     [Linux Kernel]     [Linux SCSI]     [XFree86]
  Powered by Linux