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[RFC][patch 1/2] kdump: Add infrastructure for unmapping crashkernel memory



From: Michael Holzheu <holzheu@xxxxxxxxxxxxxxxxxx>

This patch introduces a mechanism that allows architecture backends to
remove page tables for the crashkernel memory. This can protect the loaded
kdump kernel from being overwritten by broken kernel code.
A new function crash_map_pages() is added that can be implemented by
architecture code. This function has the following syntax:

void crash_map_pages(int enable);

"enable" can be 0 for removing or 1 for adding page tables.  The function is
called before and after the crashkernel segments are loaded. It is also
called in crash_shrink_memory() to create new page tables when the
crashkernel memory size is reduced.

To support architectures that have large pages this patch also introduces
a new define KEXEC_CRASH_MEM_ALIGN. The crashkernel start and size must 
always be aligned with KEXEC_CRASH_MEM_ALIGN.

Signed-off-by: Michael Holzheu <holzheu@xxxxxxxxxxxxxxxxxx>
---
 include/linux/kexec.h |    5 +++++
 kernel/kexec.c        |   16 ++++++++++++++--
 2 files changed, 19 insertions(+), 2 deletions(-)

--- a/include/linux/kexec.h
+++ b/include/linux/kexec.h
@@ -37,6 +37,10 @@
 #define KEXEC_CRASH_CONTROL_MEMORY_LIMIT KEXEC_CONTROL_MEMORY_LIMIT
 #endif
 
+#ifndef KEXEC_CRASH_MEM_ALIGN
+#define KEXEC_CRASH_MEM_ALIGN PAGE_SIZE
+#endif
+
 #define KEXEC_NOTE_HEAD_BYTES ALIGN(sizeof(struct elf_note), 4)
 #define KEXEC_CORE_NOTE_NAME "CORE"
 #define KEXEC_CORE_NOTE_NAME_BYTES ALIGN(sizeof(KEXEC_CORE_NOTE_NAME), 4)
@@ -133,6 +137,7 @@ extern void crash_kexec(struct pt_regs *
 int kexec_should_crash(struct task_struct *);
 void crash_save_cpu(struct pt_regs *regs, int cpu);
 void crash_save_vmcoreinfo(void);
+void crash_map_pages(int enable);
 void arch_crash_save_vmcoreinfo(void);
 void vmcoreinfo_append_str(const char *fmt, ...)
 	__attribute__ ((format (printf, 1, 2)));
--- a/kernel/kexec.c
+++ b/kernel/kexec.c
@@ -999,6 +999,7 @@ SYSCALL_DEFINE4(kexec_load, unsigned lon
 			kimage_free(xchg(&kexec_crash_image, NULL));
 			result = kimage_crash_alloc(&image, entry,
 						     nr_segments, segments);
+			crash_map_pages(1);
 		}
 		if (result)
 			goto out;
@@ -1015,6 +1016,8 @@ SYSCALL_DEFINE4(kexec_load, unsigned lon
 				goto out;
 		}
 		kimage_terminate(image);
+		if (flags & KEXEC_ON_CRASH)
+			crash_map_pages(0);
 	}
 	/* Install the new kernel, and  Uninstall the old */
 	image = xchg(dest_image, image);
@@ -1026,6 +1029,13 @@ out:
 	return result;
 }
 
+/*
+ * provide an empty default implementation here -- architecture
+ * code may override this
+ */
+void __weak crash_map_pages(int enable)
+{}
+
 #ifdef CONFIG_COMPAT
 asmlinkage long compat_sys_kexec_load(unsigned long entry,
 				unsigned long nr_segments,
@@ -1134,14 +1144,16 @@ int crash_shrink_memory(unsigned long ne
 		goto unlock;
 	}
 
-	start = roundup(start, PAGE_SIZE);
-	end = roundup(start + new_size, PAGE_SIZE);
+	start = roundup(start, KEXEC_CRASH_MEM_ALIGN);
+	end = roundup(start + new_size, KEXEC_CRASH_MEM_ALIGN);
 
+	crash_map_pages(1);
 	crash_free_reserved_phys_range(end, crashk_res.end);
 
 	if ((start == end) && (crashk_res.parent != NULL))
 		release_resource(&crashk_res);
 	crashk_res.end = end - 1;
+	crash_map_pages(0);
 
 unlock:
 	mutex_unlock(&kexec_mutex);

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