linux-kernel - Re: [PATCH v3 1/3] x86/ldt: Make modify

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Date:	Sat, 25 Jul 2015 00:13:23 -0400
From:	Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:	Andy Lutomirski <luto@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>
CC:	"security@...nel.org" <security@...nel.org>,
	X86 ML <x86@...nel.org>, Borislav Petkov <bp@...en8.de>,
	Sasha Levin <sasha.levin@...cle.com>,
	linux-kernel@...r.kernel.org,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Andrew Cooper <andrew.cooper3@...rix.com>,
	Jan Beulich <jbeulich@...e.com>,
	xen-devel <xen-devel@...ts.xen.org>, stable@...r.kernel.org
Subject: Re: [PATCH v3 1/3] x86/ldt: Make modify_ldt synchronous



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.

-boris


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