riscv: Fix memblock reservation for device tree blob

This fixes an error with how the FDT blob is reserved in memblock.
An incorrect physical address calculation exposed the FDT header to
unintended corruption, which typically manifested with of_fdt_raw_init()
faulting during late boot after fdt_totalsize() returned a wrong value.
Systems with smaller physical memory sizes more frequently trigger this
issue, as the kernel is more likely to allocate from the DMA32 zone
where bbl places the DTB after the kernel image.

Commit 671f9a3e ("RISC-V: Setup initial page tables in two stages")
changed the mapping of the DTB to reside in the fixmap area.
Consequently, early_init_fdt_reserve_self() cannot be used anymore in
setup_bootmem() since it relies on __pa() to derive a physical address,
which does not work with dtb_early_va that is no longer a valid kernel
logical address.

The reserved[0x1] region shows the effect of the pointer underflow
resulting from the __pa(initial_boot_params) offset subtraction:

[    0.000000] MEMBLOCK configuration:
[    0.000000]  memory size = 0x000000001fe00000 reserved size = 0x0000000000a2e514
[    0.000000]  memory.cnt  = 0x1
[    0.000000]  memory[0x0]     [0x0000000080200000-0x000000009fffffff], 0x000000001fe00000 bytes flags: 0x0
[    0.000000]  reserved.cnt  = 0x2
[    0.000000]  reserved[0x0]   [0x0000000080200000-0x0000000080c2dfeb], 0x0000000000a2dfec bytes flags: 0x0
[    0.000000]  reserved[0x1]   [0xfffffff080100000-0xfffffff080100527], 0x0000000000000528 bytes flags: 0x0

With the fix applied:

[    0.000000] MEMBLOCK configuration:
[    0.000000]  memory size = 0x000000001fe00000 reserved size = 0x0000000000a2e514
[    0.000000]  memory.cnt  = 0x1
[    0.000000]  memory[0x0]     [0x0000000080200000-0x000000009fffffff], 0x000000001fe00000 bytes flags: 0x0
[    0.000000]  reserved.cnt  = 0x2
[    0.000000]  reserved[0x0]   [0x0000000080200000-0x0000000080c2dfeb], 0x0000000000a2dfec bytes flags: 0x0
[    0.000000]  reserved[0x1]   [0x0000000080e00000-0x0000000080e00527], 0x0000000000000528 bytes flags: 0x0

Fixes: 671f9a3e ("RISC-V: Setup initial page tables in two stages")
Signed-off-by: Albert Ou <aou@eecs.berkeley.edu>
Tested-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Anup Patel <anup@brainfault.org>
Signed-off-by: Paul Walmsley <paul.walmsley@sifive.com>

arch/riscv/mm/init.c

@@ -11,6 +11,7 @@
 #include <linux/swap.h>
 #include <linux/sizes.h>
 #include <linux/of_fdt.h>
+#include <linux/libfdt.h>
 
 #include <asm/fixmap.h>
 #include <asm/tlbflush.h>
@@ -82,6 +83,8 @@ static void __init setup_initrd(void)
 }
 #endif /* CONFIG_BLK_DEV_INITRD */
 
+static phys_addr_t dtb_early_pa __initdata;
+
 void __init setup_bootmem(void)
 {
 	struct memblock_region *reg;
@@ -117,7 +120,12 @@ void __init setup_bootmem(void)
 	setup_initrd();
 #endif /* CONFIG_BLK_DEV_INITRD */
 
-	early_init_fdt_reserve_self();
+	/*
+	 * Avoid using early_init_fdt_reserve_self() since __pa() does
+	 * not work for DTB pointers that are fixmap addresses
+	 */
+	memblock_reserve(dtb_early_pa, fdt_totalsize(dtb_early_va));
+
 	early_init_fdt_scan_reserved_mem();
 	memblock_allow_resize();
 	memblock_dump_all();
@@ -393,6 +401,8 @@ asmlinkage void __init setup_vm(uintptr_t dtb_pa)
 
 	/* Save pointer to DTB for early FDT parsing */
 	dtb_early_va = (void *)fix_to_virt(FIX_FDT) + (dtb_pa & ~PAGE_MASK);
+	/* Save physical address for memblock reservation */
+	dtb_early_pa = dtb_pa;
 }
 
 static void __init setup_vm_final(void)