On 03/27/20 at 06:13am, Anshuman Khandual wrote:

On 03/26/2020 11:37 PM, James Morse wrote:

quoted

An image loaded for kexec is not stored in place, instead its segments
are scattered through memory, and are re-assembled when needed. In the
meantime, the target memory may have been removed.

Because mm is not aware that this memory is still in use, it allows it
to be removed.

Why the isolation process does not fail when these pages are currently
being used by kexec ?

That is trick of kexec implementaiton. When loading kexec-ed kernel, it
just reads in the kenrel image in user space, then searches a place
where it's going to put kernel image in the whole system RAM, but won't
put kernel image in the searched region immediately, just book ahead a
room. When you execute 'kexec -e' to trigger kexec jumping, it will copy
kernel image into the booked room. So the booking is only recorded in
kexec's own data structure.

quoted

Add a memory notifier to prevent the removal of memory regions that
overlap with a loaded kexec image segment. e.g., when triggered from the
Qemu console:
| kexec_core: memory region in use
| memory memory32: Offline failed.

Yes this is definitely an added protection for these kexec loaded kernels
memory areas from being offlined but I would have expected the preceding
offlining to have failed as well.

quoted

Signed-off-by: James Morse <james.morse@arm.com>
---
 kernel/kexec_core.c | 56 +++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 56 insertions(+)

diff --git a/kernel/kexec_core.c b/kernel/kexec_core.c
index c19c0dad1ebe..ba1d91e868ca 100644
--- a/kernel/kexec_core.c
+++ b/kernel/kexec_core.c

@@ -12,6 +12,7 @@
 #include <linux/slab.h>
 #include <linux/fs.h>
 #include <linux/kexec.h>
+#include <linux/memory.h>
 #include <linux/mutex.h>
 #include <linux/list.h>
 #include <linux/highmem.h>

@@ -22,10 +23,12 @@
 #include <linux/elf.h>
 #include <linux/elfcore.h>
 #include <linux/utsname.h>
+#include <linux/notifier.h>
 #include <linux/numa.h>
 #include <linux/suspend.h>
 #include <linux/device.h>
 #include <linux/freezer.h>
+#include <linux/pfn.h>
 #include <linux/pm.h>
 #include <linux/cpu.h>
 #include <linux/uaccess.h>

@@ -1219,3 +1222,56 @@ void __weak arch_kexec_protect_crashkres(void)
 
 void __weak arch_kexec_unprotect_crashkres(void)
 {}
+
+/*
+ * If user-space wants to offline memory that is in use by a loaded kexec
+ * image, it should unload the image first.
+ */

Probably this would need kexec user manual and related system call man pages
update as well.

quoted

+static int mem_remove_cb(struct notifier_block *nb, unsigned long action,
+			 void *data)
+{
+	int rv = NOTIFY_OK, i;
+	struct memory_notify *arg = data;
+	unsigned long pfn = arg->start_pfn;
+	unsigned long nr_segments, sstart, send;
+	unsigned long end_pfn = arg->start_pfn + arg->nr_pages;
+
+	might_sleep();

Required ?

quoted

+
+	if (action != MEM_GOING_OFFLINE)
+		return NOTIFY_DONE;
+
+	mutex_lock(&kexec_mutex);
+	if (kexec_image) {
+		nr_segments = kexec_image->nr_segments;
+
+		for (i = 0; i < nr_segments; i++) {
+			sstart = PFN_DOWN(kexec_image->segment[i].mem);
+			send = PFN_UP(kexec_image->segment[i].mem +
+				      kexec_image->segment[i].memsz);
+
+			if ((pfn <= sstart && sstart < end_pfn) ||
+			    (pfn <= send && send < end_pfn)) {
+				pr_warn("Memory region in use\n");
+				rv = NOTIFY_BAD;
+				break;
+			}
+		}
+	}
+	mutex_unlock(&kexec_mutex);
+
+	return rv;

Variable 'rv' is redundant, should use NOTIFY_[BAD|OK] directly instead.

quoted

+}
+
+static struct notifier_block mem_remove_nb = {
+	.notifier_call = mem_remove_cb,
+};
+
+static int __init register_mem_remove_cb(void)
+{
+	if (IS_ENABLED(CONFIG_MEMORY_HOTPLUG))

Should not all these new code here be wrapped with CONFIG_MEMORY_HOTREMOVE
to reduce the scope as well as final code size when the config is disabled.

quoted

+		return register_memory_notifier(&mem_remove_nb);
+
+	return 0;
+}
+device_initcall(register_mem_remove_cb);

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