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

Andy Lutomirski
2014-04-11T18:12:30Z

On 04/11/2014 10:36 AM, tip-bot for H. Peter Anvin wrote:
> Commit-ID: b3b42ac2cbae1f3cecbb6229964a4d48af31d382
> Gitweb: http://git.kernel.org/tip/b3b42ac2cbae1f3cecbb6229964a4d48af31d382
> Author: H. Peter Anvin <hpa@linux.intel.com>
> AuthorDate: Sun, 16 Mar 2014 15:31:54 -0700
> Committer: H. Peter Anvin <hpa@linux.intel.com>
> 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
> 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>

If this is what I think it is (hi, Spender), then it is probably only
useful for 3.14.y and not earlier kernels.

--Andy