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


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