Commit fa81511b authored May 14, 2014 by Linus Torvalds Committed by H. Peter Anvin May 14, 2014

x86-64, modify_ldt: Make support for 16-bit segments a runtime option



Checkin:

b3b42ac2 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@linux.intel.com>
Link: http://lkml.kernel.org/r/CA%2B55aFw9BPoD10U1LfHbOMpHWZkvJTkMcfCs9s3urPr1YyWBxw@mail.gmail.com
Cc: <stable@vger.kernel.org>

parent 9844f546

arch/x86/kernel/ldt.c

+3 −1

Original line number	Diff line number	Diff line
		@@ -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;
		}

arch/x86/vdso/vdso32-setup.c

+8 −0

Original line number	Diff line number	Diff line
		@@ -39,6 +39,7 @@
		#ifdef CONFIG_X86_64
		#define vdso_enabled sysctl_vsyscall32
		#define arch_setup_additional_pages syscall32_setup_pages
		extern int sysctl_ldt16;
		#endif

		/*
		@@ -249,6 +250,13 @@ 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
		},
		{}
		};