Re: [RFC PATCH v0 0/6] x86/AMD: Userspace address tagging

[Bharata B Rao]

On 4/5/2022 1:44 PM, Peter Zijlstra wrote:
> On Wed, Mar 23, 2022 at 01:18:41PM +0530, Bharata B Rao wrote:
>> On 3/22/2022 3:59 AM, Andy Lutomirski wrote:
> 
>>> I hate to be a pain, but I'm really not convinced that this feature
>>> is suitable for Linux.  There are a few reasons:
>>>
>>> Right now, the concept that the high bit of an address determines
>>> whether it's a user or a kernel address is fairly fundamental to the
>>> x86_64 (and x86_32!) code.  It may not be strictly necessary to
>>> preserve this, but violating it would require substantial thought.
>>> With UAI enabled, kernel and user addresses are, functionally,
>>> interleaved.  This makes things like access_ok checks, and more
>>> generally anything that operates on a range of addresses, behave
>>> potentially quite differently.  A lot of auditing of existing code
>>> would be needed to make it safe.
>>
>> Ok got that. However can you point to me a few instances in the current
>> kernel code where such assumption of high bit being user/kernel address
>> differentiator exists so that I get some idea of what it takes to
>> audit all such cases?
> 
> The fact that you have to ask and can't readily find them should be a
> big honking clue on its own, no?
> 
> Anyway, see here:
> 
> arch/x86/events/perf_event.h:static inline bool kernel_ip(unsigned long ip)
> arch/x86/events/perf_event.h:{
> arch/x86/events/perf_event.h:#ifdef CONFIG_X86_32
> arch/x86/events/perf_event.h:   return ip > PAGE_OFFSET;
> arch/x86/events/perf_event.h:#else
> arch/x86/events/perf_event.h:   return (long)ip < 0;
> arch/x86/events/perf_event.h:#endif
> arch/x86/events/perf_event.h:}

That's a pretty good and clear example.

Thanks Peter. I do now see that auditing all such instances would be
an uphill task.

Regards,
Bharata.