[PATCH trusty SRU] x86-64, modify_ldt: Make support for 16-bit segments a runtime option

[Chris J Arges]

Will this patch be superseeded by espfix64 when that lands?
--chris

On 06/16/2014 11:17 AM, Tim Gardner wrote:
> From: Linus Torvalds <torvalds at linux-foundation.org>
> 
> BugLink: http://bugs.launchpad.net/bugs/1328965
> 
> Checkin:
> 
> b3b42ac2cbae x86-64, modify_ldt: Ban 16-bit segments on 64-bit kernels
> 
> disabled 16-bit segments on 64-bit kernels due to an information
> leak.  However, it does seem that people are genuinely using Wine to
> run old 16-bit Windows programs on Linux.
> 
> A proper fix for this ("espfix64") is coming in the upcoming merge
> window, but as a temporary fix, create a sysctl to allow the
> administrator to re-enable support for 16-bit segments.
> 
> It adds a "/proc/sys/abi/ldt16" sysctl that defaults to zero (off). If
> you hit this issue and care about your old Windows program more than
> you care about a kernel stack address information leak, you can do
> 
>    echo 1 > /proc/sys/abi/ldt16
> 
> as root (add it to your startup scripts), and you should be ok.
> 
> The sysctl table is only added if you have COMPAT support enabled on
> x86-64, but I assume anybody who runs old windows binaries very much
> does that ;)
> 
> Signed-off-by: H. Peter Anvin <hpa at linux.intel.com>
> Link: http://lkml.kernel.org/r/CA%2B55aFw9BPoD10U1LfHbOMpHWZkvJTkMcfCs9s3urPr1YyWBxw@mail.gmail.com
> Cc: <stable at vger.kernel.org>
> (cherry picked from commit fa81511bb0bbb2b1aace3695ce869da9762624ff)
> Signed-off-by: Tim Gardner <tim.gardner at canonical.com>
> ---
>  arch/x86/kernel/ldt.c        | 4 +++-
>  arch/x86/vdso/vdso32-setup.c | 8 ++++++++
>  2 files changed, 11 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kernel/ldt.c b/arch/x86/kernel/ldt.c
> index af1d14a..dcbbaa1 100644
> --- a/arch/x86/kernel/ldt.c
> +++ b/arch/x86/kernel/ldt.c
> @@ -20,6 +20,8 @@
>  #include <asm/mmu_context.h>
>  #include <asm/syscalls.h>
>  
> +int sysctl_ldt16 = 0;
> +
>  #ifdef CONFIG_SMP
>  static void flush_ldt(void *current_mm)
>  {
> @@ -234,7 +236,7 @@ static int write_ldt(void __user *ptr, unsigned long bytecount, int oldmode)
>  	 * IRET leaking the high bits of the kernel stack address.
>  	 */
>  #ifdef CONFIG_X86_64
> -	if (!ldt_info.seg_32bit) {
> +	if (!ldt_info.seg_32bit && !sysctl_ldt16) {
>  		error = -EINVAL;
>  		goto out_unlock;
>  	}
> diff --git a/arch/x86/vdso/vdso32-setup.c b/arch/x86/vdso/vdso32-setup.c
> index d6bfb87..f1d633a 100644
> --- a/arch/x86/vdso/vdso32-setup.c
> +++ b/arch/x86/vdso/vdso32-setup.c
> @@ -41,6 +41,7 @@ enum {
>  #ifdef CONFIG_X86_64
>  #define vdso_enabled			sysctl_vsyscall32
>  #define arch_setup_additional_pages	syscall32_setup_pages
> +extern int sysctl_ldt16;
>  #endif
>  
>  /*
> @@ -380,6 +381,13 @@ static struct ctl_table abi_table2[] = {
>  		.mode		= 0644,
>  		.proc_handler	= proc_dointvec
>  	},
> +	{
> +		.procname	= "ldt16",
> +		.data		= &sysctl_ldt16,
> +		.maxlen		= sizeof(int),
> +		.mode		= 0644,
> +		.proc_handler	= proc_dointvec
> +	},
>  	{}
>  };
>  
>