[PATCH v7 4/7] crash_dump: reuse saved dm crypt keys for CPU/memory hot-plugging

[Coiby Xu <coxu@xxxxxxxxxx>]

When there are CPU and memory hot un/plugs, the dm crypt keys may need
to be reloaded again depending on the solution for crash hotplug
support. Currently, there are two solutions. One is to utilizes udev to
instruct user space to reload the kdump kernel image and initrd,
elfcorehdr and etc again. The other is to only update the elfcorehdr
segment introduced in commit 247262756121 ("crash:
add generic infrastructure for crash hotplug support").

For the 1st solution, the dm crypt keys need to be reloaded again. The
user space can write true to
/sys/kernel/config/crash_dm_crypt_key/reuse so the stored keys can be
re-used.

For the 2nd solution, the dm crypt keys don't need to be reloaded.
Currently, only x86 supports the 2nd solution. If the 2nd solution
gets extended to all arches, this patch can be dropped.

Signed-off-by: Coiby Xu <coxu@xxxxxxxxxx>
---
 Documentation/admin-guide/kdump/kdump.rst |  4 ++
 kernel/crash_dump_dm_crypt.c              | 52 +++++++++++++++++++++--
 2 files changed, 52 insertions(+), 4 deletions(-)

diff --git a/Documentation/admin-guide/kdump/kdump.rst b/Documentation/admin-guide/kdump/kdump.rst
index 192d6796ab94..cecfa5d34f01 100644
--- a/Documentation/admin-guide/kdump/kdump.rst
+++ b/Documentation/admin-guide/kdump/kdump.rst
@@ -574,6 +574,10 @@ encrypted disk volume. User space can interact with
     cat /sys/kernel/config/crash_dm_crypt_keys/count
     2
 
+    # To support CPU/memory hot-plugging, re-use keys already saved to reserved
+    # memory
+    echo true > /sys/kernel/config/crash_dm_crypt_key/reuse
+
 2. Load the dump-capture kernel
 
 3. After dump-capture kerne get booted, restore the keys to user keyring
diff --git a/kernel/crash_dump_dm_crypt.c b/kernel/crash_dump_dm_crypt.c
index 8c093c743d58..328d3dd0d8f6 100644
--- a/kernel/crash_dump_dm_crypt.c
+++ b/kernel/crash_dump_dm_crypt.c
@@ -28,6 +28,20 @@ static size_t get_keys_header_size(size_t total_keys)
 	return struct_size(keys_header, keys, total_keys);
 }
 
+static void get_keys_from_kdump_reserved_memory(void)
+{
+	struct keys_header *keys_header_loaded;
+
+	arch_kexec_unprotect_crashkres();
+
+	keys_header_loaded = kmap_local_page(pfn_to_page(
+		kexec_crash_image->dm_crypt_keys_addr >> PAGE_SHIFT));
+
+	memcpy(keys_header, keys_header_loaded, get_keys_header_size(key_count));
+	kunmap_local(keys_header_loaded);
+	arch_kexec_protect_crashkres();
+}
+
 static int read_key_from_user_keying(struct dm_crypt_key *dm_key)
 {
 	const struct user_key_payload *ukp;
@@ -150,8 +164,36 @@ static ssize_t config_keys_count_show(struct config_item *item, char *page)
 
 CONFIGFS_ATTR_RO(config_keys_, count);
 
+static bool is_dm_key_reused;
+
+static ssize_t config_keys_reuse_show(struct config_item *item, char *page)
+{
+	return sprintf(page, "%d\n", is_dm_key_reused);
+}
+
+static ssize_t config_keys_reuse_store(struct config_item *item,
+					   const char *page, size_t count)
+{
+	if (!kexec_crash_image || !kexec_crash_image->dm_crypt_keys_addr) {
+		kexec_dprintk(
+			"dm-crypt keys haven't be saved to crash-reserved memory\n");
+		return -EINVAL;
+	}
+
+	if (kstrtobool(page, &is_dm_key_reused))
+		return -EINVAL;
+
+	if (is_dm_key_reused)
+		get_keys_from_kdump_reserved_memory();
+
+	return count;
+}
+
+CONFIGFS_ATTR(config_keys_, reuse);
+
 static struct configfs_attribute *config_keys_attrs[] = {
 	&config_keys_attr_count,
+	&config_keys_attr_reuse,
 	NULL,
 };
 
@@ -233,10 +275,12 @@ int crash_load_dm_crypt_keys(struct kimage *image)
 		return -ENOENT;
 	}
 
-	image->dm_crypt_keys_addr = 0;
-	r = build_keys_header();
-	if (r)
-		return r;
+	if (!is_dm_key_reused) {
+		image->dm_crypt_keys_addr = 0;
+		r = build_keys_header();
+		if (r)
+			return r;
+	}
 
 	kbuf.buffer = keys_header;
 	kbuf.bufsz = get_keys_header_size(key_count);
-- 
2.47.1