Re: [PATCH v5 00/16] Enable Linear Address Space Separation support

[Matthew Wilcox <willy@infradead.org>]

On Mon, Oct 28, 2024 at 06:07:48PM +0200, Alexander Shishkin wrote:
> Linear Address Space Separation (LASS) is a security feature that intends to
> prevent malicious virtual address space accesses across user/kernel mode.
> 
> Such mode based access protection already exists today with paging and features
> such as SMEP and SMAP. However, to enforce these protections, the processor
> must traverse the paging structures in memory.  Malicious software can use
> timing information resulting from this traversal to determine details about the
> paging structures, and these details may also be used to determine the layout
> of the kernel memory.
> 
> The LASS mechanism provides the same mode-based protections as paging but
> without traversing the paging structures. Because the protections enforced by
> LASS are applied before paging, software will not be able to derive
> paging-based timing information from the various caching structures such as the
> TLBs, mid-level caches, page walker, data caches, etc. LASS can avoid probing
> using double page faults, TLB flush and reload, and SW prefetch instructions.
> See [2], [3] and [4] for some research on the related attack vectors.
> 
> In addition, LASS prevents an attack vector described in a Spectre LAM (SLAM)
> whitepaper [7].
> 
> LASS enforcement relies on the typical kernel implemetation to divide the
> 64-bit virtual address space into two halves:
>   Addr[63]=0 -> User address space
>   Addr[63]=1 -> Kernel address space
> Any data access or code execution across address spaces typically results in a
> #GP fault.
> 
> Kernel accesses usually only happen to the kernel address space. However, there
> are valid reasons for kernel to access memory in the user half. For these cases
> (such as text poking and EFI runtime accesses), the kernel can temporarily
> suspend the enforcement of LASS by toggling SMAP (Supervisor Mode Access
> Prevention) using the stac()/clac() instructions and in one instance a downright
> disabling LASS for an EFI runtime call.
> 
> User space cannot access any kernel address while LASS is enabled.
> Unfortunately, legacy vsyscall functions are located in the address range
> 0xffffffffff600000 - 0xffffffffff601000 and emulated in kernel.  To avoid
> breaking user applications when LASS is enabled, extend the vsyscall emulation
> in execute (XONLY) mode to the #GP fault handler.
> 
> In contrast, the vsyscall EMULATE mode is deprecated and not expected to be
> used by anyone.  Supporting EMULATE mode with LASS would need complex
> intruction decoding in the #GP fault handler and is probably not worth the
> hassle. Disable LASS in this rare case when someone absolutely needs and
> enables vsyscall=emulate via the command line.

I lack the wit to read & understand these patches to answer this
question, so I'll just ask it:

What happens when the kernel does a NULL pointer dereference (due to a
bug)?  It's not an attempt to access userspace, but it should result in
a good bug report.  Normally this would be outside a STAC/CLAC region,
but I suppose technically it could be within one.