x86-64, modify_ldt: Ban 16-bit segments on 64-bit kernels

commit b3b42ac2cbae1f3cecbb6229964a4d48af31d382 upstream.

The IRET instruction, when returning to a 16-bit segment, only
restores the bottom 16 bits of the user space stack pointer.  We have
a software workaround for that ("espfix") for the 32-bit kernel, but
it relies on a nonzero stack segment base which is not available in
32-bit mode.

Since 16-bit support is somewhat crippled anyway on a 64-bit kernel
(no V86 mode), and most (if not quite all) 64-bit processors support
virtualization for the users who really need it, simply reject
attempts at creating a 16-bit segment when running on top of a 64-bit
kernel.

Cc: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>
Link: http://lkml.kernel.org/n/tip-kicdm89kzw9lldryb1br9od0@git.kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/arch/x86/kernel/ldt.c b/arch/x86/kernel/ldt.c
index ebc9873989233dcba13d1fa1af14e3e921959a85..af1d14a9ebdae1ac2fddace4c15271babd1613a3 100644 (file)
--- a/arch/x86/kernel/ldt.c
+++ b/arch/x86/kernel/ldt.c
@@ -229,6 +229,17 @@ static int write_ldt(void __user *ptr, unsigned long bytecount, int oldmode)
                }
        }
 
+       /*
+        * On x86-64 we do not support 16-bit segments due to
+        * IRET leaking the high bits of the kernel stack address.
+        */
+#ifdef CONFIG_X86_64
+       if (!ldt_info.seg_32bit) {
+               error = -EINVAL;
+               goto out_unlock;
+       }
+#endif
+
        fill_ldt(&ldt, &ldt_info);
        if (oldmode)
                ldt.avl = 0;