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    SubjectRe: [tip:x86/urgent] x86-64, modify_ldt: Ban 16-bit segments on 64-bit kernels
    From
    Is this bug really still present in modern CPUs?  This change breaks
    running 16-bit apps in Wine. I have a few really old games I like to
    play on occasion, and I don't have a copy of Win 3.11 to put in a VM.

    On Fri, Apr 11, 2014 at 1:36 PM, tip-bot for H. Peter Anvin
    <tipbot@zytor.com> 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>
    > ---
    > 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;
    > + }
    > +#endif
    > +
    > fill_ldt(&ldt, &ldt_info);
    > if (oldmode)
    > ldt.avl = 0;
    > --
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