Re: [PATCH v4 29/30] x86/mm, mm/vmalloc: Defer flush_tlb_kernel_range() targeting NOHZ_FULL CPUs

Dave Hansen
2025-02-11T14:22:29Z

On 2/11/25 05:33, Valentin Schneider wrote:
>> 2. It's wrong to assume that TLB entries are only populated for
>> addresses you access - thanks to speculative execution, you have to
>> assume that the CPU might be populating random TLB entries all over
>> the place.
> Gotta love speculation. Now it is supposed to be limited to genuinely
> accessible data & code, right? Say theoretically we have a full TLBi as
> literally the last thing before doing the return-to-userspace, speculation
> should be limited to executing maybe bits of the return-from-userspace
> code?

In practice, it's mostly limited like that.

Architecturally, there are no promises from the CPU. It is within its
rights to cache anything from the page tables at any time. If it's in
the CR3 tree, it's fair game.

> Furthermore, I would hope that once a CPU is executing in userspace, it's
> not going to populate the TLB with kernel address translations - AIUI the
> whole vulnerability mitigation debacle was about preventing this sort of
> thing.

Nope, unfortunately. There's two big exception to this. First, "implicit
supervisor-mode accesses". There are structures for which the CPU gets a
virtual address and accesses it even while userspace is running. The LDT
and GDT are the most obvious examples, but there are some less
ubiquitous ones like the buffers for PEBS events.

Second, remember that user versus supervisor is determined *BY* the page
tables. Before Linear Address Space Separation (LASS), all virtual
memory accesses walk the page tables, even userspace accesses to kernel
addresses. The User/Supervisor bit is *in* the page tables, of course.

A userspace access to a kernel address results in a page walk and the
CPU is completely free to cache all or part of that page walk. A
Meltdown-style _speculative_ userspace access to kernel memory won't
generate a fault either. It won't leak data like it used to, of course,
but it can still walk the page tables. That's one reason LASS is needed.