Re: [PATCH RFC V2 17/17] x86/entry: Preserve PKRS MSR across exceptions

[Thomas Gleixner]

Dave Hansen <dave.hansen@xxxxxxxxx> writes:

> On 7/23/20 10:08 AM, Andy Lutomirski wrote:
>> Suppose some kernel code (a syscall or kernel thread) changes PKRS
>> then takes a page fault. The page fault handler needs a fresh PKRS.
>> Then the page fault handler (say a VMAâs .fault handler) changes
>> PKRS.  The we get an interrupt. The interrupt *also* needs a fresh
>> PKRS and the page fault value needs to be saved somewhere.
>> 
>> So we have more than one saved value per thread, and thread_struct
>> isnât going to solve this problem.
>
> Taking a step back...  This is all true only if we decide that we want
> protection keys to provide protection during exceptions and interrupts.
>  Right now, the code supports nesting:
>
> 	kmap(foo);
> 		kmap(bar);
> 		kunmap(bar);
> 	kunmap(foo);
>
> with a reference count.  So, the nested kmap() will see the count
> elevated and do nothing.

Hopefully with a big fat warning if the nested map requires a different
key than the outer one.