From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Jan Beulich" Subject: Re: [PATCH v3 2/3] x86/ldt: Make modify_ldt optional Date: Thu, 23 Jul 2015 01:13:46 -0600 Message-ID: <55B0B04A02000078000945AF__1535.1346265108$1437635735$gmane$org@prv-mh.provo.novell.com> References: <7bfde005b84a90a83bf668a320c7d4ad1b940065.1437592883.git.luto@kernel.org> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <7bfde005b84a90a83bf668a320c7d4ad1b940065.1437592883.git.luto@kernel.org> Content-Disposition: inline List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Sender: xen-devel-bounces@lists.xen.org Errors-To: xen-devel-bounces@lists.xen.org To: Andy Lutomirski Cc: "security@kernel.org" , Peter Zijlstra , Andrew Cooper , X86 ML , linux-kernel@vger.kernel.org, Steven Rostedt , xen-devel , Borislav Petkov , Sasha Levin , Boris Ostrovsky List-Id: xen-devel@lists.xenproject.org >>> On 22.07.15 at 21:23, wrote: > --- a/arch/x86/Kconfig > +++ b/arch/x86/Kconfig > @@ -1015,6 +1015,7 @@ config VM86 > config X86_16BIT > bool "Enable support for 16-bit segments" if EXPERT > default y > + depends on MODIFY_LDT_SYSCALL > ---help--- > This option is required by programs like Wine to run 16-bit > protected mode legacy code on x86 processors. Disabling > @@ -2053,6 +2054,22 @@ config CMDLINE_OVERRIDE > This is used to work around broken boot loaders. This should > be set to 'N' under normal conditions. > > +config MODIFY_LDT_SYSCALL > + bool "Enable the LDT (local descriptor table)" if EXPERT > + default y > + ---help--- > + Linux can allow user programs to install a per-process x86 > + Local Descriptor Table (LDT) using the modify_ldt(2) system > + call. This is required to run 16-bit or segmented code such as > + DOSEMU or some Wine programs. It is also used by some very old > + threading libraries. > + > + Enabling this feature adds a small amount of overhead to > + context switches and increases the low-level kernel attack > + surface. Disabling it removes the modify_ldt(2) system call. > + > + Saying 'N' here may make sense for embedded or server kernels. > + I think it would be better to place this ahead of the one being made dependent on it, to avoid the user being prompted for X86_16BIT despite it possibly becoming unavailable (when this one gets set to n). Jan