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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives
Hash Suite: Windows password security audit tool. GUI, reports in PDF.
[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Message-ID: <ZyEX4cp-iiwiF_yX@casper.infradead.org>
Date: Tue, 29 Oct 2024 17:14:09 +0000
From: Matthew Wilcox <willy@...radead.org>
To: Alexander Shishkin <alexander.shishkin@...ux.intel.com>
Cc: Andy Lutomirski <luto@...nel.org>, Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
	"H. Peter Anvin" <hpa@...or.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ard Biesheuvel <ardb@...nel.org>,
	"Paul E. McKenney" <paulmck@...nel.org>,
	Josh Poimboeuf <jpoimboe@...nel.org>,
	Xiongwei Song <xiongwei.song@...driver.com>,
	Xin Li <xin3.li@...el.com>, "Mike Rapoport (IBM)" <rppt@...nel.org>,
	Brijesh Singh <brijesh.singh@....com>,
	Michael Roth <michael.roth@....com>,
	Tony Luck <tony.luck@...el.com>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Alexey Kardashevskiy <aik@....com>,
	Jonathan Corbet <corbet@....net>,
	Sohil Mehta <sohil.mehta@...el.com>, Ingo Molnar <mingo@...nel.org>,
	Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
	Daniel Sneddon <daniel.sneddon@...ux.intel.com>,
	Kai Huang <kai.huang@...el.com>,
	Sandipan Das <sandipan.das@....com>,
	Breno Leitao <leitao@...ian.org>,
	Rick Edgecombe <rick.p.edgecombe@...el.com>,
	Alexei Starovoitov <ast@...nel.org>, Hou Tao <houtao1@...wei.com>,
	Juergen Gross <jgross@...e.com>,
	Vegard Nossum <vegard.nossum@...cle.com>,
	Kees Cook <kees@...nel.org>, Eric Biggers <ebiggers@...gle.com>,
	Jason Gunthorpe <jgg@...pe.ca>,
	"Masami Hiramatsu (Google)" <mhiramat@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Luis Chamberlain <mcgrof@...nel.org>,
	Yuntao Wang <ytcoode@...il.com>,
	Rasmus Villemoes <linux@...musvillemoes.dk>,
	Christophe Leroy <christophe.leroy@...roup.eu>,
	Tejun Heo <tj@...nel.org>, Changbin Du <changbin.du@...wei.com>,
	Huang Shijie <shijie@...amperecomputing.com>,
	Geert Uytterhoeven <geert+renesas@...der.be>,
	Namhyung Kim <namhyung@...nel.org>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-efi@...r.kernel.org
Subject: Re: [PATCH v5 00/16] Enable Linear Address Space Separation support

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.