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 <nik.borisov@suse.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20230623111409.3047467-7-nik.borisov@suse.com

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 0a1731a0f0ef373421c0594886c065461ade1064..45e34be4ed56a20e19caa5648dc5cdaff4642627 100644 (file)
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/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>]]]
 

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 982b777eadc7a16cc4b4b52f300b9c816f0cfd2f..c130bf3176fe2bbc8de63fb0d21db376bf5e2f3a 100644 (file)
--- a/arch/x86/Kconfig
+++ b/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

diff --git a/arch/x86/entry/common.c b/arch/x86/entry/common.c
index cfbd3aec3ddc7f0a996b1c7aaeffe12df9196cf2..a34e1a1adcf83caffc0b9a3bf89fc368946ad462 100644 (file)
--- a/arch/x86/entry/common.c
+++ b/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
 
 /*