x86/ldt: Make modify_ldt() optional
The modify_ldt syscall exposes a large attack surface and is
unnecessary for modern userspace. Make it optional.
Signed-off-by: Andy Lutomirski <luto@kernel.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Cc: Andrew Cooper <andrew.cooper3@citrix.com>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Jan Beulich <jbeulich@suse.com>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Sasha Levin <sasha.levin@oracle.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: security@kernel.org <security@kernel.org>
Cc: xen-devel <xen-devel@lists.xen.org>
Link: http://lkml.kernel.org/r/a605166a771c343fd64802dece77a903507333bd.1438291540.git.luto@kernel.org
[ Made MATH_EMULATION dependent on MODIFY_LDT_SYSCALL. ]
Signed-off-by: Ingo Molnar <mingo@kernel.org>
diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index e26fe7a..79865804 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -1036,6 +1036,7 @@
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
@@ -1530,6 +1531,7 @@
config MATH_EMULATION
bool
+ depends on MODIFY_LDT_SYSCALL
prompt "Math emulation" if X86_32
---help---
Linux can emulate a math coprocessor (used for floating point
@@ -2074,6 +2076,22 @@
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.
+
source "kernel/livepatch/Kconfig"
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