linux-kernel - Re: [PATCH 4/7] x86/vsyscall: Setup vsyscall to compromise LASS protection

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Message-Id: <b2124803-edc2-44de-8980-11ed74dd883e@app.fastmail.com>
Date:   Fri, 20 Jan 2023 20:09:02 -0800
From:   "Andy Lutomirski" <luto@...nel.org>
To:     "Yian Chen" <yian.chen@...el.com>,
        "Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        "Dave Hansen" <dave.hansen@...ux.intel.com>,
        "Shankar, Ravi V" <ravi.v.shankar@...el.com>,
        "Tony Luck" <tony.luck@...el.com>,
        "Sohil Mehta" <sohil.mehta@...el.com>,
        "Paul Lai" <paul.c.lai@...el.com>
Subject: Re: [PATCH 4/7] x86/vsyscall: Setup vsyscall to compromise LASS protection



On Mon, Jan 9, 2023, at 9:52 PM, Yian Chen wrote:
> Kernel enables LASS automatically at starting time in LASS
> capable platforms. Any access to kernel addresses
> or upper half addresses from user space triggers a #GP
> fault.
>
> Legacy vsyscall does not comply with LASS, because
> the vsyscall functions are mapped in the range
> 0xffffffffff600000-0xffffffffff601000.
>
> In theory, it would be possible to write a #GP fault handler
> to emulate the old vsyscall behavior, but vsyscall has been
> deprecated for some time, so this has not been done.

The ISE docs are really quite explicit that #GP will have RIP pointing at the vDSO if LASS is on. Unless I’ve missed something, this should be quite straightforward to handle without breaking userspace compatibility.  Let’s please do this.

>
> Therefore, when kernel enforces LASS, vsyscall does not work
> and should be disabled. On the other hand, the user can relax
> the enforcement by clearing lass cpu id (clearcpuid=lass/390)
> or enabling vsyscall (vsyscall=xxx) from kernel command line.
> The user can also opt-out LASS in config file to build kernel
> binary.
>
> Signed-off-by: Yian Chen <yian.chen@...el.com>
> Reviewed-by: Tony Luck <tony.luck@...el.com>
> ---
>  Documentation/admin-guide/kernel-parameters.txt | 12 ++++++++----
>  arch/x86/entry/vsyscall/vsyscall_64.c           | 14 ++++++++++++++
>  2 files changed, 22 insertions(+), 4 deletions(-)
>
> diff --git a/Documentation/admin-guide/kernel-parameters.txt 
> b/Documentation/admin-guide/kernel-parameters.txt
> index 6cfa6e3996cf..3988e0c8c175 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -6755,10 +6755,14 @@
>  			versions of glibc use these calls.  Because these
>  			functions are at fixed addresses, they make nice
>  			targets for exploits that can control RIP.
> -
> -			emulate     [default] Vsyscalls turn into traps and are
> -			            emulated reasonably safely.  The vsyscall
> -				    page is readable.
> +			In newer versions of Intel platforms that come with
> +			LASS(Linear Address Space separation) protection,
> +			vsyscall is disabled by default. Enabling vsyscall
> +			via the parameter overrides LASS protection.
> +
> +			emulate     [default if not LASS capable] Vsyscalls
> +				    turn into traps and are emulated reasonably
> +				    safely.  The vsyscall page is readable.
> 
>  			xonly       Vsyscalls turn into traps and are
>  			            emulated reasonably safely.  The vsyscall
> diff --git a/arch/x86/entry/vsyscall/vsyscall_64.c 
> b/arch/x86/entry/vsyscall/vsyscall_64.c
> index 4af81df133ee..2691f26835d1 100644
> --- a/arch/x86/entry/vsyscall/vsyscall_64.c
> +++ b/arch/x86/entry/vsyscall/vsyscall_64.c
> @@ -63,6 +63,12 @@ static int __init vsyscall_setup(char *str)
>  		else
>  			return -EINVAL;
> 
> +		if (cpu_feature_enabled(X86_FEATURE_LASS) &&
> +		    vsyscall_mode != NONE) {
> +			setup_clear_cpu_cap(X86_FEATURE_LASS);
> +			pr_warn("LASS disabled by command line enabling vsyscall\n");
> +		}
> +
>  		return 0;
>  	}
> 
> @@ -379,6 +385,14 @@ void __init map_vsyscall(void)
>  	extern char __vsyscall_page;
>  	unsigned long physaddr_vsyscall = __pa_symbol(&__vsyscall_page);
> 
> +	/*
> +	 * When LASS is on, vsyscall triggers a #GP fault,
> +	 * so that force vsyscall_mode to NONE.
> +	 */
> +	if (cpu_feature_enabled(X86_FEATURE_LASS)) {
> +		vsyscall_mode = NONE;
> +		return;
> +	}
>  	/*
>  	 * For full emulation, the page needs to exist for real.  In
>  	 * execute-only mode, there is no PTE at all backing the vsyscall
> -- 
> 2.34.1