[x32] PATCH: Remove ix86_promote_function_mode

H. Peter Anvin hpa@zytor.com
Mon Jun 20 15:28:00 GMT 2011


On 06/20/2011 07:43 AM, H.J. Lu wrote:
> On Mon, Jun 20, 2011 at 7:33 AM, H. Peter Anvin <hpa@zytor.com> wrote:
>> On 06/20/2011 07:01 AM, H.J. Lu wrote:
>>> On Mon, Jun 20, 2011 at 6:53 AM, Bernd Schmidt <bernds@codesourcery.com> wrote:
>>>> On 06/20/2011 03:51 PM, H.J. Lu wrote:
>>>>> Promote pointers to Pmode when passing/returning in registers is
>>>>> a security concern.
>>
>> No.  Promoting *NON*-pointers (or rather, requiring non-pointers to
>> having already been zero extended) is a security concern.  I thought I'd
>> made that point clear already.  This is a hideously critical distinction.
> 
> I can promote pointers then.
> 

Yes.  The issue comes when promoting non-pointers, like "unsigned int".
 Pointers are the opposite because pointers in the kernel are 64 bits
and we'd like them to be pre-promoted.

>>> Peter, do you think it is safe to assume upper 32bits are zero in
>>> user space for x32? Kernel isn't a problem since pointer is 64bit
>>> in kernel and we don't pass pointers on stack to kernel.
>>
>> As I have already stated, if we *cannot* require pointers to be
>> zero-extended on entry to the kernel, we're going to have to have
>> special entry points for all the x32 system calls except the ones that
>> don't take pointers.
> 
> 32bit pointers are zero-extended to 64bit when passing in registers to
> kernel.

Excellent, sounds like we have converged.

I saw you posted something about pointers on the stack, but it sounds
like you already realized that we don't point any stack arguments
whatsoever to the kernel.

	-hpa


-- 
H. Peter Anvin, Intel Open Source Technology Center
I work for Intel.  I don't speak on their behalf.



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