Re: [PATCH] KVM: fix sil/dil/bpl/spl in the mod/rm fields

From: Avi Kivity
Date: Mon Jun 03 2013 - 11:42:21 EST

On Thu, May 30, 2013 at 7:34 PM, Paolo Bonzini <pbonzini@xxxxxxxxxx> wrote:
> Il 30/05/2013 17:34, Paolo Bonzini ha scritto:
>> Il 30/05/2013 16:35, Paolo Bonzini ha scritto:
>>> The x86-64 extended low-byte registers were fetched correctly from reg,
>>> but not from mod/rm.
>>> This fixes another bug in the boot of RHEL5.9 64-bit, but it is still
>>> not enough.
>> Well, it is enough but it takes 2 minutes to reach the point where
>> hardware virtualization is used. It is doing a lot of stuff in
>> emulation mode because FS and GS have leftovers from the A20 test:
>> FS =0000 0000000000000000 0000ffff 00009300 DPL=0 DS16 [-WA]
>> GS =ffff 00000000000ffff0 0000ffff 00009300 DPL=0 DS16 [-WA]
>> 0x00000000000113be: in $0x92,%al
>> 0x00000000000113c0: or $0x2,%al
>> 0x00000000000113c2: out %al,$0x92
>> 0x00000000000113c4: xor %ax,%ax
>> 0x00000000000113c6: mov %ax,%fs
>> 0x00000000000113c8: dec %ax
>> 0x00000000000113c9: mov %ax,%gs
>> 0x00000000000113cb: inc %ax
>> 0x00000000000113cc: mov %ax,%fs:0x200
>> 0x00000000000113d0: cmp %gs:0x210,%ax
>> 0x00000000000113d5: je 0x113cb
>> The DPL < RPL test fails. Any ideas? Should we introduce a new
>> intermediate value for emulate_invalid_guest_state (0=none, 1=some, 2=full)?
> One idea could be to replace invalid descriptors with NULL ones. Then
> you can intercept this in the #GP handler and trigger emulation for that
> instruction only.

Won't work, vmx won't let you enter in such a configuration.

Maybe you can detect the exact code sequence (%eip, some instructions,
register state) and clear %fs and %gs.
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