efi: fix potential NULL deref in efi_mem_reserve_persistent

When iterating on a linked list, a result of memremap is dereferenced
without checking it for NULL.

This patch adds a check that falls back on allocating a new page in
case memremap doesn't succeed.

Found by Linux Verification Center (linuxtesting.org) with SVACE.

Fixes: 18df7577 ("efi/memreserve: deal with memreserve entries in unmapped memory")
Signed-off-by: NAnton Gusev <aagusev@ispras.ru>
[ardb: return -ENOMEM instead of breaking out of the loop]
Signed-off-by: NArd Biesheuvel <ardb@kernel.org>

drivers/firmware/efi/efi.c

@@ -1007,6 +1007,8 @@ 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; ) {
 		rsv = memremap(prsv, sizeof(*rsv), MEMREMAP_WB);
+		if (!rsv)
+			return -ENOMEM;
 		index = atomic_fetch_add_unless(&rsv->count, 1, rsv->size);
 		if (index < rsv->size) {
 			rsv->entry[index].base = addr;