[PATCH v1 2/2] kexec: Introduce kexec_reboot_disabled

Ricardo Ribalda
2022-11-14T13:20:03Z

Create a new toogle that disables LINUX_REBOOT_CMD_KEXEC, reducing the
attack surface to a system.

Without this toogle, an attacker can only reboot into a different kernel
if they can create a panic().

Signed-off-by: Ricardo Ribalda <ribalda@chromium.org>

diff --git a/Documentation/admin-guide/sysctl/kernel.rst b/Documentation/admin-guide/sysctl/kernel.rst
index 97394bd9d065..25d019682d33 100644
--- a/Documentation/admin-guide/sysctl/kernel.rst
+++ b/Documentation/admin-guide/sysctl/kernel.rst
@@ -462,6 +462,17 @@ altered.
Generally used together with the `modules_disabled`_ sysctl.

+kexec_reboot_disabled
+=====================
+
+A toggle indicating if ``LINUX_REBOOT_CMD_KEXEC`` has been disabled.
+This value defaults to 0 (false: ``LINUX_REBOOT_CMD_KEXEC`` enabled),
+but can be set to 1 (true: ``LINUX_REBOOT_CMD_KEXEC`` disabled).
+Once true, kexec can no longer be used for reboot and the toggle
+cannot be set back to false.
+This toggle does not affect the use of kexec during a crash.
+
+
kptr_restrict
=============

diff --git a/include/linux/kexec.h b/include/linux/kexec.h
index 41a686996aaa..15c3fad8918b 100644
--- a/include/linux/kexec.h
+++ b/include/linux/kexec.h
@@ -407,6 +407,7 @@ extern int kimage_crash_copy_vmcoreinfo(struct kimage *image);
extern struct kimage *kexec_image;
extern struct kimage *kexec_crash_image;
extern int kexec_load_disabled;
+extern int kexec_reboot_disabled;

#ifndef kexec_flush_icache_page
#define kexec_flush_icache_page(page)
diff --git a/kernel/kexec.c b/kernel/kexec.c
index cb8e6e6f983c..43063f803d81 100644
--- a/kernel/kexec.c
+++ b/kernel/kexec.c
@@ -196,6 +196,10 @@ static inline int kexec_load_check(unsigned long nr_segments,
if (!capable(CAP_SYS_BOOT) || kexec_load_disabled)
return -EPERM;

+ /* Check if the system admin has disabled kexec reboot. */
+ if (!(flags & KEXEC_ON_CRASH) && kexec_reboot_disabled)
+ return -EPERM;
+
/* Permit LSMs and IMA to fail the kexec */
result = security_kernel_load_data(LOADING_KEXEC_IMAGE, false);
if (result < 0)
diff --git a/kernel/kexec_core.c b/kernel/kexec_core.c
index ca2743f9c634..fe82e2525705 100644
--- a/kernel/kexec_core.c
+++ b/kernel/kexec_core.c
@@ -929,6 +929,7 @@ int kimage_load_segment(struct kimage *image,
struct kimage *kexec_image;
struct kimage *kexec_crash_image;
int kexec_load_disabled;
+int kexec_reboot_disabled;
#ifdef CONFIG_SYSCTL
static struct ctl_table kexec_core_sysctls[] = {
{
@@ -941,6 +942,16 @@ static struct ctl_table kexec_core_sysctls[] = {
.extra1 = SYSCTL_ONE,
.extra2 = SYSCTL_ONE,
},
+ {
+ .procname = "kexec_reboot_disabled",
+ .data = &kexec_reboot_disabled,
+ .maxlen = sizeof(int),
+ .mode = 0644,
+ /* only handle a transition from default "0" to "1" */
+ .proc_handler = proc_dointvec_minmax,
+ .extra1 = SYSCTL_ONE,
+ .extra2 = SYSCTL_ONE,
+ },
{ }
};

@@ -1138,7 +1149,7 @@ int kernel_kexec(void)

if (!kexec_trylock())
return -EBUSY;
- if (!kexec_image) {
+ if (!kexec_image || kexec_reboot_disabled) {
error = -EINVAL;
goto Unlock;
}
diff --git a/kernel/kexec_file.c b/kernel/kexec_file.c
index 45637511e0de..583fba6de5cb 100644
--- a/kernel/kexec_file.c
+++ b/kernel/kexec_file.c
@@ -333,6 +333,11 @@ SYSCALL_DEFINE5(kexec_file_load, int, kernel_fd, int, initrd_fd,
if (!capable(CAP_SYS_BOOT) || kexec_load_disabled)
return -EPERM;

+ /* Check if the system admin has disabled kexec reboot. */
+ if (!(flags & (KEXEC_FILE_ON_CRASH | KEXEC_FILE_UNLOAD))
+ && kexec_reboot_disabled)
+ return -EPERM;
+
/* Make sure we have a legal set of flags */
if (flags != (flags & KEXEC_FILE_FLAGS))
return -EINVAL;

--
b4 0.11.0-dev-d93f8