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

[Tim Gardner]

You're gonna have to provide a bit more detail 'cause I don't know
anything about espfix64.

rtg

On 06/16/2014 10:24 AM, Chris J Arges wrote:
> 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
>> +	},
>>  	{}
>>  };
>>  
>>


-- 
Tim Gardner tim.gardner at canonical.com