arm64: Obtain text offset from kernel image

Recent changes made to Linux 5.8 have outlined that kvmtool
hardcodes the text offset instead of reading it from the arm64
image itself.

To address this, import the image header structure into kvmtool
and do the right thing. 32bit guests are still loaded to their
usual locations.

While we're at it, check the image magic and default to the text
offset to be 0x80000 when image_size is 0, as described in the
kernel's booting.rst document.

Reported-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Cc: Alexandru Elisei <alexandru.elisei@arm.com>
Link: https://lore.kernel.org/r/20200608152801.1415902-1-maz@kernel.org
Signed-off-by: Will Deacon <will@kernel.org>

6 files changed, 107 insertions, 5 deletions

diff --git a/Makefile b/Makefile
index d27ff389..35bb1182 100644
--- a/Makefile
+++ b/Makefile
@@ -179,6 +179,7 @@ ifeq ($(ARCH), arm64)
 	OBJS		+= $(OBJS_ARM_COMMON)
 	OBJS		+= arm/aarch64/arm-cpu.o
 	OBJS		+= arm/aarch64/kvm-cpu.o
+	OBJS		+= arm/aarch64/kvm.o
 	ARCH_INCLUDE	:= $(HDRS_ARM_COMMON)
 	ARCH_INCLUDE	+= -Iarm/aarch64/include
 
diff --git a/arm/aarch32/include/kvm/kvm-arch.h b/arm/aarch32/include/kvm/kvm-arch.h
index cd31e729..a772bb1d 100644
--- a/arm/aarch32/include/kvm/kvm-arch.h
+++ b/arm/aarch32/include/kvm/kvm-arch.h
@@ -1,7 +1,7 @@
 #ifndef KVM__KVM_ARCH_H
 #define KVM__KVM_ARCH_H
 
-#define ARM_KERN_OFFSET(...)	0x8000
+#define kvm__arch_get_kern_offset(...)	0x8000
 
 #define ARM_MAX_MEMORY(...)	ARM_LOMAP_MAX_MEMORY
 
diff --git a/arm/aarch64/include/asm/image.h b/arm/aarch64/include/asm/image.h
new file mode 100644
index 00000000..c2b13213
--- /dev/null
+++ b/arm/aarch64/include/asm/image.h
@@ -0,0 +1,59 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+
+#ifndef __ASM_IMAGE_H
+#define __ASM_IMAGE_H
+
+#define ARM64_IMAGE_MAGIC	"ARM\x64"
+
+#define ARM64_IMAGE_FLAG_BE_SHIFT		0
+#define ARM64_IMAGE_FLAG_PAGE_SIZE_SHIFT	(ARM64_IMAGE_FLAG_BE_SHIFT + 1)
+#define ARM64_IMAGE_FLAG_PHYS_BASE_SHIFT \
+					(ARM64_IMAGE_FLAG_PAGE_SIZE_SHIFT + 2)
+#define ARM64_IMAGE_FLAG_BE_MASK		0x1
+#define ARM64_IMAGE_FLAG_PAGE_SIZE_MASK		0x3
+#define ARM64_IMAGE_FLAG_PHYS_BASE_MASK		0x1
+
+#define ARM64_IMAGE_FLAG_LE			0
+#define ARM64_IMAGE_FLAG_BE			1
+#define ARM64_IMAGE_FLAG_PAGE_SIZE_4K		1
+#define ARM64_IMAGE_FLAG_PAGE_SIZE_16K		2
+#define ARM64_IMAGE_FLAG_PAGE_SIZE_64K		3
+#define ARM64_IMAGE_FLAG_PHYS_BASE		1
+
+#ifndef __ASSEMBLY__
+
+#define arm64_image_flag_field(flags, field) \
+				(((flags) >> field##_SHIFT) & field##_MASK)
+
+/*
+ * struct arm64_image_header - arm64 kernel image header
+ * See Documentation/arm64/booting.rst for details
+ *
+ * @code0:		Executable code, or
+ *   @mz_header		  alternatively used for part of MZ header
+ * @code1:		Executable code
+ * @text_offset:	Image load offset
+ * @image_size:		Effective Image size
+ * @flags:		kernel flags
+ * @reserved:		reserved
+ * @magic:		Magic number
+ * @reserved5:		reserved, or
+ *   @pe_header:	  alternatively used for PE COFF offset
+ */
+
+struct arm64_image_header {
+	__le32 code0;
+	__le32 code1;
+	__le64 text_offset;
+	__le64 image_size;
+	__le64 flags;
+	__le64 res2;
+	__le64 res3;
+	__le64 res4;
+	__le32 magic;
+	__le32 res5;
+};
+
+#endif /* __ASSEMBLY__ */
+
+#endif /* __ASM_IMAGE_H */
diff --git a/arm/aarch64/include/kvm/kvm-arch.h b/arm/aarch64/include/kvm/kvm-arch.h
index 9de623ac..55ef8ed1 100644
--- a/arm/aarch64/include/kvm/kvm-arch.h
+++ b/arm/aarch64/include/kvm/kvm-arch.h
@@ -1,9 +1,8 @@
 #ifndef KVM__KVM_ARCH_H
 #define KVM__KVM_ARCH_H
 
-#define ARM_KERN_OFFSET(kvm)	((kvm)->cfg.arch.aarch32_guest	?	\
-				0x8000				:	\
-				0x80000)
+struct kvm;
+unsigned long long kvm__arch_get_kern_offset(struct kvm *kvm, int fd);
 
 #define ARM_MAX_MEMORY(kvm)	((kvm)->cfg.arch.aarch32_guest	?	\
 				ARM_LOMAP_MAX_MEMORY		:	\
diff --git a/arm/aarch64/kvm.c b/arm/aarch64/kvm.c
new file mode 100644
index 00000000..a46d438d
--- /dev/null
+++ b/arm/aarch64/kvm.c
@@ -0,0 +1,43 @@
+#include "kvm/kvm.h"
+
+#include <asm/image.h>
+
+#include <linux/byteorder.h>
+
+/*
+ * Return the TEXT_OFFSET value that the guest kernel expects. Note
+ * that pre-3.17 kernels expose this value using the native endianness
+ * instead of Little-Endian. BE kernels of this vintage may fail to
+ * boot. See Documentation/arm64/booting.rst in your local kernel tree.
+ */
+unsigned long long kvm__arch_get_kern_offset(struct kvm *kvm, int fd)
+{
+	struct arm64_image_header header;
+	off_t cur_offset;
+	ssize_t size;
+
+	/* the 32bit kernel offset is a well known value */
+	if (kvm->cfg.arch.aarch32_guest)
+		return 0x8000;
+
+	cur_offset = lseek(fd, 0, SEEK_CUR);
+	if (cur_offset == (off_t)-1 ||
+	    lseek(fd, 0, SEEK_SET) == (off_t)-1)
+		die("Failed to seek in image file");
+
+	size = xread(fd, &header, sizeof(header));
+	if (size < 0 || (size_t)size < sizeof(header))
+		die("Failed to read kernel image header");
+
+	lseek(fd, cur_offset, SEEK_SET);
+
+	if (memcmp(&header.magic, ARM64_IMAGE_MAGIC, sizeof(header.magic)))
+		pr_warning("Kernel image magic not matching");
+
+	if (le64_to_cpu(header.image_size))
+		return le64_to_cpu(header.text_offset);
+
+	pr_warning("Image size is 0, assuming TEXT_OFFSET to be 0x80000");
+	return 0x80000;
+}
+
diff --git a/arm/kvm.c b/arm/kvm.c
index 1f85fc60..5aea18fe 100644
--- a/arm/kvm.c
+++ b/arm/kvm.c
@@ -103,7 +103,7 @@ bool kvm__arch_load_kernel_image(struct kvm *kvm, int fd_kernel, int fd_initrd,
 	 */
 	limit = kvm->ram_start + min(kvm->ram_size, (u64)SZ_256M) - 1;
 
-	pos = kvm->ram_start + ARM_KERN_OFFSET(kvm);
+	pos = kvm->ram_start + kvm__arch_get_kern_offset(kvm, fd_kernel);
 	kvm->arch.kern_guest_start = host_to_guest_flat(kvm, pos);
 	file_size = read_file(fd_kernel, pos, limit - pos);
 	if (file_size < 0) {