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Subject[tip:x86/urgent] x86-64, modify_ldt: Ban 16-bit segments on 64-bit kernels
Commit-ID:  b3b42ac2cbae1f3cecbb6229964a4d48af31d382
Author: H. Peter Anvin <>
AuthorDate: Sun, 16 Mar 2014 15:31:54 -0700
Committer: H. Peter Anvin <>
CommitDate: Fri, 11 Apr 2014 10:10:09 -0700

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

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

Cc: Linus Torvalds <>
Signed-off-by: H. Peter Anvin <>
Cc: <>
arch/x86/kernel/ldt.c | 11 +++++++++++
1 file changed, 11 insertions(+)

diff --git a/arch/x86/kernel/ldt.c b/arch/x86/kernel/ldt.c
index ebc9873..af1d14a 100644
--- 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;
+ }
fill_ldt(&ldt, &ldt_info);
if (oldmode)
ldt.avl = 0;

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