The patch titled
Subject: kexec: introduce sysctl parameters kexec_load_limit_*
has been added to the -mm mm-nonmm-unstable branch. Its filename is
kexec-introduce-sysctl-parameters-kexec_load_limit_.patch
This patch will shortly appear at
https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/kexec-introduce-sysctl-parameters-kexec_load_limit_.patch
This patch will later appear in the mm-nonmm-unstable branch at
git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
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------------------------------------------------------
From: Ricardo Ribalda <ribalda@xxxxxxxxxxxx>
Subject: kexec: introduce sysctl parameters kexec_load_limit_*
Date: Wed, 04 Jan 2023 15:38:48 +0100
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 finner tunning 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@xxxxxxxxxxxx
Signed-off-by: Ricardo Ribalda <ribalda@xxxxxxxxxxxx>
Reviewed-by: Steven Rostedt (Google) <rostedt@xxxxxxxxxxx>
Acked-by: Baoquan He <bhe@xxxxxxxxxx>
Cc: Bagas Sanjaya <bagasdotme@xxxxxxxxx>
Cc: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
Cc: Guilherme G. Piccoli <gpiccoli@xxxxxxxxxx> # Steam Deck
Cc: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
Cc: Jonathan Corbet <corbet@xxxxxxx>
Cc: Philipp Rudo <prudo@xxxxxxxxxx>
Cc: Ross Zwisler <zwisler@xxxxxxxxxx>
Cc: Sergey Senozhatsky <senozhatsky@xxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
Documentation/admin-guide/sysctl/kernel.rst | 18 +++
include/linux/kexec.h | 2
kernel/kexec.c | 4
kernel/kexec_core.c | 87 +++++++++++++++++-
kernel/kexec_file.c | 11 +-
5 files changed, 114 insertions(+), 8 deletions(-)
--- a/Documentation/admin-guide/sysctl/kernel.rst~kexec-introduce-sysctl-parameters-kexec_load_limit_
+++ a/Documentation/admin-guide/sysctl/kernel.rst
@@ -464,6 +464,24 @@ allowing a system to set up (and later u
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
=============
--- a/include/linux/kexec.h~kexec-introduce-sysctl-parameters-kexec_load_limit_
+++ a/include/linux/kexec.h
@@ -404,7 +404,7 @@ extern int kimage_crash_copy_vmcoreinfo(
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)
--- a/kernel/kexec.c~kexec-introduce-sysctl-parameters-kexec_load_limit_
+++ a/kernel/kexec.c
@@ -190,10 +190,12 @@ out_unlock:
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 */
--- a/kernel/kexec_core.c~kexec-introduce-sysctl-parameters-kexec_load_limit_
+++ a/kernel/kexec_core.c
@@ -920,10 +920,64 @@ int kimage_load_segment(struct kimage *i
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_sysct
.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
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;
}
/*
--- a/kernel/kexec_file.c~kexec-introduce-sysctl-parameters-kexec_load_limit_
+++ a/kernel/kexec_file.c
@@ -326,11 +326,13 @@ SYSCALL_DEFINE5(kexec_file_load, int, ke
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, ke
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)
_
Patches currently in -mm which might be from ribalda@xxxxxxxxxxxx are
scripts-spellingtxt-add-permitted.patch
kvm-x86-fix-trivial-typo.patch
documentation-sysctl-correct-kexec_load_disabled.patch
kexec-factor-out-kexec_load_permitted.patch
kexec-introduce-sysctl-parameters-kexec_load_limit_.patch
