linux-kernel - Re: [PATCH] KVM: X86: Fix the decoding of segment overrides in 64bit mode

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Date:   Thu, 22 Mar 2018 10:19:41 +0000
From:   Andrew Cooper <andrew.cooper3@...rix.com>
To:     Paolo Bonzini <pbonzini@...hat.com>,
        Wanpeng Li <kernellwp@...il.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Cc:     Radim Krčmář <rkrcmar@...hat.com>
Subject: Re: [PATCH] KVM: X86: Fix the decoding of segment overrides in 64bit
 mode

On 22/03/2018 10:07, Paolo Bonzini wrote:
> On 22/03/2018 09:34, Wanpeng Li wrote:
>> From: Wanpeng Li <wanpengli@...cent.com>
>>
>> Explicit segment overides other than %fs and %gs are documented as ignored by
>> both Intel and AMD.
>>
>> In practice, this means that:
>>
>>  * Explicit uses of %ss don't actually yield #SS[0] for non-canonical
>>    memory references.
>>  * Explicit uses of %{e,c,d}s don't override %rbp/%rsp-based memory references
>>    to yield #GP[0] for non-canonical memory references.
>>
>> Cc: Paolo Bonzini <pbonzini@...hat.com>
>> Cc: Radim Krčmář <rkrcmar@...hat.com>
>> Signed-off-by: Wanpeng Li <wanpengli@...cent.com>

When porting fixes from other projects, it is customary to identify so
in the commit message.  In this case, the fix you've ported is
http://xenbits.xen.org/gitweb/?p=xen.git;a=commitdiff;h=b7dce29d9faf3597d009c853ed1fcbed9f7a7f68

Here is an example of how Xen ports fixes from Linux for the drivers
that we share. 
http://xenbits.xen.org/gitweb/?p=xen.git;a=commitdiff;h=4e131596f1defec9407b6e60d584a696beaf5d7e


>> ---
>>  arch/x86/kvm/emulate.c | 6 ++++--
>>  1 file changed, 4 insertions(+), 2 deletions(-)
>>
>> diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
>> index dd88158..5091255 100644
>> --- a/arch/x86/kvm/emulate.c
>> +++ b/arch/x86/kvm/emulate.c
>> @@ -5148,8 +5148,10 @@ int x86_decode_insn(struct x86_emulate_ctxt *ctxt, void *insn, int insn_len)
>>  		case 0x2e:	/* CS override */
>>  		case 0x36:	/* SS override */
>>  		case 0x3e:	/* DS override */
>> -			has_seg_override = true;
>> -			ctxt->seg_override = (ctxt->b >> 3) & 3;
>> +			if (mode != X86EMUL_MODE_PROT64) {
>> +				has_seg_override = true;
>> +				ctxt->seg_override = (ctxt->b >> 3) & 3;
>> +			}
>>  			break;
>>  		case 0x64:	/* FS override */
>>  		case 0x65:	/* GS override */
>>
> Testcase, please...

If you want to crib from one, this is the testcase I made for Xen.

http://xenbits.xen.org/docs/xtf/test-memop-seg.html

With the impending KVM/PVH work which is ongoing, it will soon be easy
to run Xen's HVM test suite unmodified under KVM, but we're not quite
there yet.

~Andrew