RISC-V: use memcpy for kexec_file mode

The pointer to buffer loading kernel binaries is in kernel space for
kexec_fil mode, When copy_from_user copies data from pointer to a block
of memory, it checkes that the pointer is in the user space range, on
RISCV-V that is:

static inline bool __access_ok(unsigned long addr, unsigned long size)
{
	return size <= TASK_SIZE && addr <= TASK_SIZE - size;
}

and TASK_SIZE is 0x4000000000 for 64-bits, which now causes
copy_from_user to reject the access of the field 'buf' of struct
kexec_segment that is in range [CONFIG_PAGE_OFFSET - VMALLOC_SIZE,
CONFIG_PAGE_OFFSET), is invalid user space pointer.

This patch fixes this issue by skipping access_ok(), use mempcy() instead.
Signed-off-by: Liao Chang <liaochang1@huawei.com>
Link: https://lore.kernel.org/r/20220408100914.150110-3-lizhengyu3@huawei.comSigned-off-by: Palmer Dabbelt <palmer@rivosinc.com>

arch/riscv/kernel/machine_kexec.c

@@ -65,7 +65,9 @@ machine_kexec_prepare(struct kimage *image)
 		if (image->segment[i].memsz <= sizeof(fdt))
 			continue;
 
-		if (copy_from_user(&fdt, image->segment[i].buf, sizeof(fdt)))
+		if (image->file_mode)
+			memcpy(&fdt, image->segment[i].buf, sizeof(fdt));
+		else if (copy_from_user(&fdt, image->segment[i].buf, sizeof(fdt)))
 			continue;
 
 		if (fdt_check_header(&fdt))