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SubjectRe: [RFC/PATCH v2 02/10] x86_64: add KASan support
On 09/11/2014 08:01 AM, H. Peter Anvin wrote:
> On 09/10/2014 07:31 AM, Andrey Ryabinin wrote:
>> This patch add arch specific code for kernel address sanitizer.
>>
>> 16TB of virtual addressed used for shadow memory.
>> It's located in range [0xffff800000000000 - 0xffff900000000000]
>> Therefore PAGE_OFFSET has to be changed from 0xffff880000000000
>> to 0xffff900000000000.
>
> NAK on this.
>
> 0xffff880000000000 is the lowest usable address because we have agreed
> to leave 0xffff800000000000-0xffff880000000000 for the hypervisor or
> other non-OS uses.
>
> Bumping PAGE_OFFSET seems needlessly messy, why not just designate a
> zone higher up in memory?
>

I already answered to Dave why I choose to place shadow bellow PAGE_OFFSET (answer copied bellow).
In short - yes, shadow could be higher. But for some sort of kernel bugs we could have confusing oopses in kasan kernel.

On 09/11/2014 12:30 AM, Andrey Ryabinin wrote:
> 2014-09-10 19:46 GMT+04:00 Dave Hansen <dave.hansen@intel.com>:
>>
>> Is there a reason this has to be _below_ the linear map? Couldn't we
>> just carve some space out of the vmalloc() area for the kasan area?
>>
>
> Yes, there is a reason for this. For inline instrumentation we need to
> catch access to userspace without any additional check.
> This means that we need shadow of 1 << 61 bytes and we don't have so
> many addresses available. However, we could use
> hole between userspace and kernelspace for that. For any address
> between [0 - 0xffff800000000000], shadow address will be
> in this hole, so checking shadow value will produce general protection
> fault (GPF). We may even try handle GPF in a special way
> and print more user-friendly report (this will be under CONFIG_KASAN of course).
>
> But now I realized that we even if we put shadow in vmalloc, shadow
> addresses corresponding to userspace addresses
> still will be in between userspace - kernelspace, so we also will get GPF.
> There is the only problem I see now in such approach. Lets consider
> that because of some bug in kernel we are trying to access
> memory slightly bellow 0xffff800000000000. In this case kasan will try
> to check some shadow which in fact is not a shadow byte at all.
> It's not a big deal though, kernel will crash anyway. In only means
> that debugging of such problems could be a little more complex
> than without kasan.
>
>



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