lkml.org 
[lkml]   [2017]   [Nov]   [23]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
SubjectRe: [PATCH 00/18] arm64: Unmap the kernel whilst running in userspace (KAISER)
From
Date


> On 22 Nov 2017, at 23:37, Pavel Machek <pavel@ucw.cz> wrote:
>
> Hi!
>
>>>>> If I'm willing to do timing attacks to defeat KASLR... what prevents
>>>>> me from using CPU caches to do that?
>>>>>
>>>>
>>>> Because it is impossible to get a cache hit on an access to an
>>>> unmapped address?
>>>
>>> Um, no, I don't need to be able to directly access kernel addresses. I
>>> just put some data in _same place in cache where kernel data would
>>> go_, then do syscall and look if my data are still cached. Caches
>>> don't have infinite associativity.
>>>
>>
>> Ah ok. Interesting.
>>
>> But how does that leak address bits that are covered by the tag?
>
> Same as leaking any other address bits? Caches are "virtually
> indexed",

Not on arm64, although I don’t see how that is relevant if you are trying to defeat kaslr.

> and tag does not come into play...
>

Well, I must be missing something then, because I don’t see how knowledge about which userland address shares a cache way with a kernel address can leak anything beyond the bits that make up the index (i.e., which cache way is being shared)

> Maybe this explains it?
>

No not really. It explains how cache timing can be used as a side channel, not how it defeats kaslr.

Thanks,
Ard.
\
 
 \ /
  Last update: 2017-11-23 07:51    [W:0.075 / U:28.732 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site