Re: [tip:x86/urgent] x86-64, modify_ldt: Ban 16-bit segments on 64-bit kernels

[Linus Torvalds]

On Wed, May 7, 2014 at 2:18 AM, Sven Joachim <svenjoac@xxxxxx> wrote:
>
> It seems that at least some 32-bit programs are also broken, since after
> upgrading the kernel to 3.14.3 I can no longer start my old chess
> database program:

So for backporting (and for 3.15) maybe this (TOTALLY UNTESTED) patch
would be acceptable.

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.

Afaik, 16-bit programs under wine already need

  echo 0 > /proc/sys/vm/mmap_min_addr

because they want to map things at address 0, so this isn't a new concept.

I would like to repeat that this is totally untested. And the sysct
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 ;)

                        Linus
 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 af1d14a9ebda..dcbbaa165bde 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 00348980a3a6..e1f220e3ca68 100644
--- a/arch/x86/vdso/vdso32-setup.c
+++ b/arch/x86/vdso/vdso32-setup.c
@@ -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
+	},
 	{}
 };