[PATCH v8 1/7] kexec_file: allow to place kexec_buf randomly

[Coiby Xu <coxu@redhat.com>]

Currently, kexec_buf is placed in order which means for the same
machine, the info in the kexec_buf is always located at the same
position each time the machine is booted. This may cause a risk for
sensitive information like LUKS volume key. Now struct kexec_buf has a
new field random which indicates it's supposed to be placed in a random
position.

Note this feature is enabled only when CONFIG_CRASH_DUMP is enabled. So
it only takes effect for kdump and won't impact kexec reboot.

Suggested-by: Jan Pazdziora <jpazdziora@redhat.com>
Signed-off-by: Coiby Xu <coxu@redhat.com>
---
 include/linux/kexec.h | 30 ++++++++++++++++++++++++++++++
 kernel/kexec_file.c   |  3 +++
 2 files changed, 33 insertions(+)

diff --git a/include/linux/kexec.h b/include/linux/kexec.h
index f0e9f8eda7a3..61269e97502a 100644
--- a/include/linux/kexec.h
+++ b/include/linux/kexec.h
@@ -25,6 +25,10 @@
 
 extern note_buf_t __percpu *crash_notes;
 
+#ifdef CONFIG_CRASH_DUMP
+#include <linux/prandom.h>
+#endif
+
 #ifdef CONFIG_KEXEC_CORE
 #include <linux/list.h>
 #include <linux/compat.h>
@@ -171,6 +175,7 @@ int kexec_image_post_load_cleanup_default(struct kimage *image);
  * @buf_min:	The buffer can't be placed below this address.
  * @buf_max:	The buffer can't be placed above this address.
  * @top_down:	Allocate from top of memory.
+ * @random:	Place the buffer at a random position.
  */
 struct kexec_buf {
 	struct kimage *image;
@@ -182,8 +187,33 @@ struct kexec_buf {
 	unsigned long buf_min;
 	unsigned long buf_max;
 	bool top_down;
+#ifdef CONFIG_CRASH_DUMP
+	bool random;
+#endif
 };
 
+
+#ifdef CONFIG_CRASH_DUMP
+static inline void kexec_random_range_start(unsigned long start,
+					    unsigned long end,
+					    struct kexec_buf *kbuf,
+					    unsigned long *temp_start)
+{
+	unsigned short i;
+
+	if (kbuf->random) {
+		get_random_bytes(&i, sizeof(unsigned short));
+		*temp_start = start + (end - start) / USHRT_MAX * i;
+	}
+}
+#else
+static inline void kexec_random_range_start(unsigned long start,
+					    unsigned long end,
+					    struct kexec_buf *kbuf,
+					    unsigned long *temp_start)
+{}
+#endif
+
 int kexec_load_purgatory(struct kimage *image, struct kexec_buf *kbuf);
 int kexec_purgatory_get_set_symbol(struct kimage *image, const char *name,
 				   void *buf, unsigned int size,
diff --git a/kernel/kexec_file.c b/kernel/kexec_file.c
index 3eedb8c226ad..875fe108cc83 100644
--- a/kernel/kexec_file.c
+++ b/kernel/kexec_file.c
@@ -445,6 +445,7 @@ static int locate_mem_hole_top_down(unsigned long start, unsigned long end,
 
 	temp_end = min(end, kbuf->buf_max);
 	temp_start = temp_end - kbuf->memsz + 1;
+	kexec_random_range_start(temp_start, temp_end, kbuf, &temp_start);
 
 	do {
 		/* align down start */
@@ -483,6 +484,8 @@ static int locate_mem_hole_bottom_up(unsigned long start, unsigned long end,
 
 	temp_start = max(start, kbuf->buf_min);
 
+	kexec_random_range_start(temp_start, end, kbuf, &temp_start);
+
 	do {
 		temp_start = ALIGN(temp_start, kbuf->buf_align);
 		temp_end = temp_start + kbuf->memsz - 1;
-- 
2.48.1