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

Thomas Gleixner
2020-07-23T20:23:34Z

Dave Hansen <dave.hansen@intel.com> 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 VMA’s .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
>> isn’t 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.