From mboxrd@z Thu Jan  1 00:00:00 1970
From: Andy Lutomirski <luto@amacapital.net>
Subject: Re: [PATCH v3 1/3] x86/ldt: Make modify_ldt synchronous
Date: Fri, 24 Jul 2015 21:58:23 -0700
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To: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Cc: "security@kernel.org" <security@kernel.org>, Jan Beulich <jbeulich@suse.com>, Peter Zijlstra <peterz@infradead.org>, Andrew Cooper <andrew.cooper3@citrix.com>, X86 ML <x86@kernel.org>, "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>, Steven Rostedt <rostedt@goodmis.org>, xen-devel <xen-devel@lists.xen.org>, Borislav Petkov <bp@alien8.de>, stable <stable@vger.kernel.org>, Andy Lutomirski <luto@kernel.org>, Sasha Levin <sasha.levin@oracle.com>
List-Id: xen-devel@lists.xenproject.org

On Fri, Jul 24, 2015 at 9:13 PM, Boris Ostrovsky
<boris.ostrovsky@oracle.com> wrote:
>
>
> On 07/22/2015 06:20 PM, Boris Ostrovsky wrote:
>>
>> On 07/22/2015 03:23 PM, Andy Lutomirski wrote:
>>>
>>>
>>> +    error = -ENOMEM;
>>> +    new_ldt = alloc_ldt_struct(newsize);
>>> +    if (!new_ldt)
>>>           goto out_unlock;
>>> -    }
>>>   -    fill_ldt(&ldt, &ldt_info);
>>> -    if (oldmode)
>>> -        ldt.avl = 0;
>>> +    if (old_ldt) {
>>> +        memcpy(new_ldt->entries, old_ldt->entries,
>>> +               oldsize * LDT_ENTRY_SIZE);
>>> +    }
>>> +    memset(new_ldt->entries + oldsize * LDT_ENTRY_SIZE, 0,
>>> +           (newsize - oldsize) * LDT_ENTRY_SIZE);
>>
>>
>> We need to zero out full page (probably better in alloc_ldt_struct() with
>> vmzalloc/__GFP_ZERO) --- Xen checks whole page that is assigned to  G/LDT
>> and gets unhappy if an invalid descriptor is found there.
>>
>> This fixes one problem. There is something else that Xen gets upset about,
>> I haven't figured what it is yet (and I am out tomorrow so it may need to
>> wait until Friday).
>>
>
>
> What I thought was another problem turned out not to be one so both 64- and
> 32-bit tests passed on 64-bit PV (when allocated LDT is zeroed out)
>
> However, on 32-bit kernel the test is failing multicpu test, I don't know
> yet what it is.

Test case bug or unrelated kernel bug depending on your point of view.
I forgot that x86_32 and x86_64 have very different handling of IRET
faults.  Wait for v2 :)

--Andy