Commit a42aaad2 authored by Ricardo Ribalda's avatar Ricardo Ribalda Committed by Andrew Morton

kexec: introduce sysctl parameters kexec_load_limit_*

kexec allows replacing the current kernel with a different one.  This is
usually a source of concerns for sysadmins that want to harden a system.

Linux already provides a way to disable loading new kexec kernel via
kexec_load_disabled, but that control is very coard, it is all or nothing
and does not make distinction between a panic kexec and a normal kexec.

This patch introduces new sysctl parameters, with finer tuning to specify
how many times a kexec kernel can be loaded.  The sysadmin can set
different limits for kexec panic and kexec reboot kernels.  The value can
be modified at runtime via sysctl, but only with a stricter value.

With these new parameters on place, a system with loadpin and verity
enabled, using the following kernel parameters:
sysctl.kexec_load_limit_reboot=0 sysct.kexec_load_limit_panic=1 can have a
good warranty that if initrd tries to load a panic kernel, a malitious
user will have small chances to replace that kernel with a different one,
even if they can trigger timeouts on the disk where the panic kernel
lives.

Link: https://lkml.kernel.org/r/20221114-disable-kexec-reset-v6-3-6a8531a09b9a@chromium.orgSigned-off-by: default avatarRicardo Ribalda <ribalda@chromium.org>
Reviewed-by: default avatarSteven Rostedt (Google) <rostedt@goodmis.org>
Acked-by: default avatarBaoquan He <bhe@redhat.com>
Cc: Bagas Sanjaya <bagasdotme@gmail.com>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Guilherme G. Piccoli <gpiccoli@igalia.com> # Steam Deck
Cc: Joel Fernandes (Google) <joel@joelfernandes.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Philipp Rudo <prudo@redhat.com>
Cc: Ross Zwisler <zwisler@kernel.org>
Cc: Sergey Senozhatsky <senozhatsky@chromium.org>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
parent 7e99f8b6
......@@ -464,6 +464,24 @@ allowing a system to set up (and later use) an image without it being
altered.
Generally used together with the `modules_disabled`_ sysctl.
kexec_load_limit_panic
======================
This parameter specifies a limit to the number of times the syscalls
``kexec_load`` and ``kexec_file_load`` can be called with a crash
image. It can only be set with a more restrictive value than the
current one.
== ======================================================
-1 Unlimited calls to kexec. This is the default setting.
N Number of calls left.
== ======================================================
kexec_load_limit_reboot
=======================
Similar functionality as ``kexec_load_limit_panic``, but for a normal
image.
kptr_restrict
=============
......
......@@ -404,7 +404,7 @@ extern int kimage_crash_copy_vmcoreinfo(struct kimage *image);
extern struct kimage *kexec_image;
extern struct kimage *kexec_crash_image;
bool kexec_load_permitted(void);
bool kexec_load_permitted(int kexec_image_type);
#ifndef kexec_flush_icache_page
#define kexec_flush_icache_page(page)
......
......@@ -190,10 +190,12 @@ static int do_kexec_load(unsigned long entry, unsigned long nr_segments,
static inline int kexec_load_check(unsigned long nr_segments,
unsigned long flags)
{
int image_type = (flags & KEXEC_ON_CRASH) ?
KEXEC_TYPE_CRASH : KEXEC_TYPE_DEFAULT;
int result;
/* We only trust the superuser with rebooting the system. */
if (!kexec_load_permitted())
if (!kexec_load_permitted(image_type))
return -EPERM;
/* Permit LSMs and IMA to fail the kexec */
......
......@@ -920,10 +920,64 @@ int kimage_load_segment(struct kimage *image,
return result;
}
struct kexec_load_limit {
/* Mutex protects the limit count. */
struct mutex mutex;
int limit;
};
static struct kexec_load_limit load_limit_reboot = {
.mutex = __MUTEX_INITIALIZER(load_limit_reboot.mutex),
.limit = -1,
};
static struct kexec_load_limit load_limit_panic = {
.mutex = __MUTEX_INITIALIZER(load_limit_panic.mutex),
.limit = -1,
};
struct kimage *kexec_image;
struct kimage *kexec_crash_image;
static int kexec_load_disabled;
#ifdef CONFIG_SYSCTL
static int kexec_limit_handler(struct ctl_table *table, int write,
void *buffer, size_t *lenp, loff_t *ppos)
{
struct kexec_load_limit *limit = table->data;
int val;
struct ctl_table tmp = {
.data = &val,
.maxlen = sizeof(val),
.mode = table->mode,
};
int ret;
if (write) {
ret = proc_dointvec(&tmp, write, buffer, lenp, ppos);
if (ret)
return ret;
if (val < 0)
return -EINVAL;
mutex_lock(&limit->mutex);
if (limit->limit != -1 && val >= limit->limit)
ret = -EINVAL;
else
limit->limit = val;
mutex_unlock(&limit->mutex);
return ret;
}
mutex_lock(&limit->mutex);
val = limit->limit;
mutex_unlock(&limit->mutex);
return proc_dointvec(&tmp, write, buffer, lenp, ppos);
}
static struct ctl_table kexec_core_sysctls[] = {
{
.procname = "kexec_load_disabled",
......@@ -935,6 +989,18 @@ static struct ctl_table kexec_core_sysctls[] = {
.extra1 = SYSCTL_ONE,
.extra2 = SYSCTL_ONE,
},
{
.procname = "kexec_load_limit_panic",
.data = &load_limit_panic,
.mode = 0644,
.proc_handler = kexec_limit_handler,
},
{
.procname = "kexec_load_limit_reboot",
.data = &load_limit_reboot,
.mode = 0644,
.proc_handler = kexec_limit_handler,
},
{ }
};
......@@ -946,13 +1012,30 @@ static int __init kexec_core_sysctl_init(void)
late_initcall(kexec_core_sysctl_init);
#endif
bool kexec_load_permitted(void)
bool kexec_load_permitted(int kexec_image_type)
{
struct kexec_load_limit *limit;
/*
* Only the superuser can use the kexec syscall and if it has not
* been disabled.
*/
return capable(CAP_SYS_BOOT) && !kexec_load_disabled;
if (!capable(CAP_SYS_BOOT) || kexec_load_disabled)
return false;
/* Check limit counter and decrease it.*/
limit = (kexec_image_type == KEXEC_TYPE_CRASH) ?
&load_limit_panic : &load_limit_reboot;
mutex_lock(&limit->mutex);
if (!limit->limit) {
mutex_unlock(&limit->mutex);
return false;
}
if (limit->limit != -1)
limit->limit--;
mutex_unlock(&limit->mutex);
return true;
}
/*
......
......@@ -326,11 +326,13 @@ SYSCALL_DEFINE5(kexec_file_load, int, kernel_fd, int, initrd_fd,
unsigned long, cmdline_len, const char __user *, cmdline_ptr,
unsigned long, flags)
{
int ret = 0, i;
int image_type = (flags & KEXEC_FILE_ON_CRASH) ?
KEXEC_TYPE_CRASH : KEXEC_TYPE_DEFAULT;
struct kimage **dest_image, *image;
int ret = 0, i;
/* We only trust the superuser with rebooting the system. */
if (!kexec_load_permitted())
if (!kexec_load_permitted(image_type))
return -EPERM;
/* Make sure we have a legal set of flags */
......@@ -342,11 +344,12 @@ SYSCALL_DEFINE5(kexec_file_load, int, kernel_fd, int, initrd_fd,
if (!kexec_trylock())
return -EBUSY;
dest_image = &kexec_image;
if (flags & KEXEC_FILE_ON_CRASH) {
if (image_type == KEXEC_TYPE_CRASH) {
dest_image = &kexec_crash_image;
if (kexec_crash_image)
arch_kexec_unprotect_crashkres();
} else {
dest_image = &kexec_image;
}
if (flags & KEXEC_FILE_UNLOAD)
......
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