| From | Luis Henriques <> | Subject | [PATCH 3.11 144/182] x86-64, modify_ldt: Ban 16-bit segments on 64-bit kernels | Date | Thu, 24 Apr 2014 09:51:09 +0100 |
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3.11.10.9 -stable review patch. If anyone has any objections, please let me know.
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From: "H. Peter Anvin" <hpa@linux.intel.com>
commit b3b42ac2cbae1f3cecbb6229964a4d48af31d382 upstream.
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 Signed-off-by: Luis Henriques <luis.henriques@canonical.com> --- 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; -- 1.9.1
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