x86: Make IA32_EMULATION boot time configurable

Distributions would like to reduce their attack surface as much as
possible but at the same time they'd want to retain flexibility to cater
to a variety of legacy software. This stems from the conjecture that
compat layer is likely rarely tested and could have latent security
bugs. Ideally distributions will set their default policy and also
give users the ability to override it as appropriate.

To enable this use case, introduce CONFIG_IA32_EMULATION_DEFAULT_DISABLED
compile time option, which controls whether 32bit processes/syscalls
should be allowed or not. This option is aimed mainly at distributions
to set their preferred default behavior in their kernels.

To allow users to override the distro's policy, introduce the 'ia32_emulation'
parameter which allows overriding CONFIG_IA32_EMULATION_DEFAULT_DISABLED
state at boot time.

Signed-off-by: Nikolay Borisov <[email protected]>
Signed-off-by: Thomas Gleixner <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

Author: Nikolay Borisov

Documentation/admin-guide/kernel-parameters.txt

@@ -1893,6 +1893,12 @@
 			 0 -- machine default
 			 1 -- force brightness inversion
 
+	ia32_emulation=	[X86-64]
+			Format: <bool>
+			When true, allows loading 32-bit programs and executing 32-bit
+			syscalls, essentially overriding IA32_EMULATION_DEFAULT_DISABLED at
+			boot time. When false, unconditionally disables IA32 emulation.
+
 	icn=		[HW,ISDN]
 			Format: <io>[,<membase>[,<icn_id>[,<icn_id2>]]]
 

arch/x86/Kconfig

@@ -2954,6 +2954,15 @@ config IA32_EMULATION
 	  64-bit kernel. You should likely turn this on, unless you're
 	  100% sure that you don't have any 32-bit programs left.
 
+config IA32_EMULATION_DEFAULT_DISABLED
+	bool "IA32 emulation disabled by default"
+	default n
+	depends on IA32_EMULATION
+	help
+	  Make IA32 emulation disabled by default. This prevents loading 32-bit
+	  processes and access to 32-bit syscalls. If unsure, leave it to its
+	  default value.
+
 config X86_X32_ABI
 	bool "x32 ABI for 64-bit mode"
 	depends on X86_64

arch/x86/entry/common.c

@@ -19,6 +19,7 @@
 #include <linux/nospec.h>
 #include <linux/syscalls.h>
 #include <linux/uaccess.h>
+#include <linux/init.h>
 
 #ifdef CONFIG_XEN_PV
 #include <xen/xen-ops.h>
@@ -97,7 +98,13 @@ static __always_inline int syscall_32_enter(struct pt_regs *regs)
 }
 
 #ifdef CONFIG_IA32_EMULATION
-bool __ia32_enabled __ro_after_init = true;
+bool __ia32_enabled __ro_after_init = !IS_ENABLED(CONFIG_IA32_EMULATION_DEFAULT_DISABLED);
+
+static int ia32_emulation_override_cmdline(char *arg)
+{
+	return kstrtobool(arg, &__ia32_enabled);
+}
+early_param("ia32_emulation", ia32_emulation_override_cmdline);
 #endif
 
 /*