x86/crash: make the page that stores the dm crypt keys inaccessible

This adds an addition layer of protection for the saved copy of dm crypt
key. Trying to access the saved copy will cause page fault.

Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: Coiby Xu <[e-mail]>
Suggested-by: Pingfan Liu <[e-mail]>
Acked-by: Baoquan He <[e-mail]>
Cc: "Daniel P. Berrange" <[e-mail]>
Cc: Dave Hansen <[e-mail]>
Cc: Dave Young <[e-mail]>
Cc: Jan Pazdziora <[e-mail]>
Cc: Milan Broz <[e-mail]>
Cc: Ondrej Kozina <[e-mail]>
Cc: Vitaly Kuznetsov <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

arch/x86/kernel/machine_kexec_64.c

···
 	kexec_mark_range(control, crashk_res.end, protect);
 }
 
+/* make the memory storing dm crypt keys in/accessible */
+static void kexec_mark_dm_crypt_keys(bool protect)
+{
+	unsigned long start_paddr, end_paddr;
+	unsigned int nr_pages;
+
+	if (kexec_crash_image->dm_crypt_keys_addr) {
+		start_paddr = kexec_crash_image->dm_crypt_keys_addr;
+		end_paddr = start_paddr + kexec_crash_image->dm_crypt_keys_sz - 1;
+		nr_pages = (PAGE_ALIGN(end_paddr) - PAGE_ALIGN_DOWN(start_paddr))/PAGE_SIZE;
+		if (protect)
+			set_memory_np((unsigned long)phys_to_virt(start_paddr), nr_pages);
+		else
+			__set_memory_prot(
+				(unsigned long)phys_to_virt(start_paddr),
+				nr_pages,
+				__pgprot(_PAGE_PRESENT | _PAGE_NX | _PAGE_RW));
+	}
+}
+
 void arch_kexec_protect_crashkres(void)
 {
 	kexec_mark_crashkres(true);
+	kexec_mark_dm_crypt_keys(true);
 }
 
 void arch_kexec_unprotect_crashkres(void)
 {
+	kexec_mark_dm_crypt_keys(false);
 	kexec_mark_crashkres(false);
 }
 #endif