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authorH. Peter Anvin <hpa@linux.intel.com>2014-03-16 15:31:54 -0700
committerH. Peter Anvin <hpa@linux.intel.com>2014-04-11 10:10:09 -0700
commitb3b42ac2cbae1f3cecbb6229964a4d48af31d382 (patch)
treeb611f0f4d789f79ef1b0b9056aae923327a18687 /arch
parent3151b942badd059431eff93833cc1e957195b53b (diff)
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 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 Cc: <stable@vger.kernel.org>
Diffstat (limited to 'arch')
-rw-r--r--arch/x86/kernel/ldt.c11
1 files changed, 11 insertions, 0 deletions
diff --git a/arch/x86/kernel/ldt.c b/arch/x86/kernel/ldt.c
index ebc98739892..af1d14a9ebd 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;
+ }
+#endif
+
fill_ldt(&ldt, &ldt_info);
if (oldmode)
ldt.avl = 0;