efi/memreserve: deal with memreserve entries in unmapped memory

Ensure that the EFI memreserve entries can be accessed, even if they
are located in memory that the kernel (e.g., a crashkernel) omits from
the linear map.

Fixes: 80424b02 ("efi: Reduce the amount of memblock reservations ...")
Cc: <stable@vger.kernel.org> # 5.0+
Reported-by: Jonathan Richardson <jonathan.richardson@broadcom.com>
Reviewed-by: Jonathan Richardson <jonathan.richardson@broadcom.com>
Tested-by: Jonathan Richardson <jonathan.richardson@broadcom.com>
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>

drivers/firmware/efi/efi.c

@@ -1009,14 +1009,16 @@ int __ref efi_mem_reserve_persistent(phys_addr_t addr, u64 size)
 
 	/* first try to find a slot in an existing linked list entry */
 	for (prsv = efi_memreserve_root->next; prsv; prsv = rsv->next) {
-		rsv = __va(prsv);
+		rsv = memremap(prsv, sizeof(*rsv), MEMREMAP_WB);
 		index = atomic_fetch_add_unless(&rsv->count, 1, rsv->size);
 		if (index < rsv->size) {
 			rsv->entry[index].base = addr;
 			rsv->entry[index].size = size;
 
+			memunmap(rsv);
 			return 0;
 		}
+		memunmap(rsv);
 	}
 
 	/* no slot found - allocate a new linked list entry */
@@ -1024,7 +1026,13 @@ int __ref efi_mem_reserve_persistent(phys_addr_t addr, u64 size)
 	if (!rsv)
 		return -ENOMEM;
 
-	rsv->size = EFI_MEMRESERVE_COUNT(PAGE_SIZE);
+	/*
+	 * The memremap() call above assumes that a linux_efi_memreserve entry
+	 * never crosses a page boundary, so let's ensure that this remains true
+	 * even when kexec'ing a 4k pages kernel from a >4k pages kernel, by
+	 * using SZ_4K explicitly in the size calculation below.
+	 */
+	rsv->size = EFI_MEMRESERVE_COUNT(SZ_4K);
 	atomic_set(&rsv->count, 1);
 	rsv->entry[0].base = addr;
 	rsv->entry[0].size = size;