Commit b3b42ac2 authored Mar 16, 2014 by H. Peter Anvin

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>

parent 3151b942

arch/x86/kernel/ldt.c

+11 −0

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