Re: [PATCH] x86/compressed: remove unnecessary check

[Alexander Kuleshov]

Hello Kees,

yes you're fully right.

sorry for noise.

2015-02-21 0:38 GMT+06:00 Kees Cook <keescook@xxxxxxxxxxxx>:
> On Fri, Feb 20, 2015 at 9:55 AM, Alexander Kuleshov
> <kuleshovmail@xxxxxxxxx> wrote:
>> region.start is entry->addr, but we already did check for case when
>> entry->addr + entry->size < minimum, so no need to check that
>> region.start is less than minimum.
>
> Nope, that's not correct. Those are separate checks. For example, a
> region 20..130 would be seen as:
>
> start = 20
> size = 110
>
> With minimum = 50, the first check eliminates anything with size <= 30
> (since the region would be entirely (20...50) below the minimum).
>
> The second check (the one you're suggesting to remove) raises the
> bottom of the region up to the minimum:
>
> start = 50
>
> And later on ("Reduce size by any delta from the original address."),
> the size is also adjusted to notice any changes:
>
> size = 80
>
> Now the region is 50...130: the bottom is cut off to adapt to the
> minimum location.
>
> Is there some situation you found where this logic isn't working?
>
> -Kees
>
>>
>> Signed-off-by: Alexander Kuleshov <kuleshovmail@xxxxxxxxx>
>> ---
>>  arch/x86/boot/compressed/aslr.c | 6 +-----
>>  1 file changed, 1 insertion(+), 5 deletions(-)
>>
>> diff --git a/arch/x86/boot/compressed/aslr.c b/arch/x86/boot/compressed/aslr.c
>> index bb13763..13424a9 100644
>> --- a/arch/x86/boot/compressed/aslr.c
>> +++ b/arch/x86/boot/compressed/aslr.c
>> @@ -249,11 +249,7 @@ static void process_e820_entry(struct e820entry *entry,
>>
>>         region.start = entry->addr;
>>         region.size = entry->size;
>> -
>> -       /* Potentially raise address to minimum location. */
>> -       if (region.start < minimum)
>> -               region.start = minimum;
>> -
>> +
>>         /* Potentially raise address to meet alignment requirements. */
>>         region.start = ALIGN(region.start, CONFIG_PHYSICAL_ALIGN);
>>
>> --
>> 2.3.0.80.g18d0fec
>>
>
>
>
> --
> Kees Cook
> Chrome OS Security



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