block/partitions/efi.c: ensure that the GPT header is at least the size of the structure.

UEFI 2.3.1D will include a change to the spec language mandating that a
GPT header must be greater than *or equal to* the size of the defined
structure.  While verifying that this would work on Linux, I discovered
that we're not actually checking the minimum bound at all.

The result of this is that when we verify the checksum, it's possible that
on a malformed header (with header_size of 0), we won't actually verify
any data.

[akpm@linux-foundation.org: fix printk warning]
Signed-off-by: Peter Jones <pjones@redhat.com>
Acked-by: Matt Fleming <matt.fleming@intel.com>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

block/partitions/efi.c

@@ -310,15 +310,23 @@ static int is_gpt_valid(struct parsed_partitions *state, u64 lba,
 		goto fail;
 	}
 
-	/* Check the GUID Partition Table header size */
+	/* Check the GUID Partition Table header size is too big */
 	if (le32_to_cpu((*gpt)->header_size) >
 			bdev_logical_block_size(state->bdev)) {
-		pr_debug("GUID Partition Table Header size is wrong: %u > %u\n",
+		pr_debug("GUID Partition Table Header size is too large: %u > %u\n",
 			le32_to_cpu((*gpt)->header_size),
 			bdev_logical_block_size(state->bdev));
 		goto fail;
 	}
 
+	/* Check the GUID Partition Table header size is too small */
+	if (le32_to_cpu((*gpt)->header_size) < sizeof(gpt_header)) {
+		pr_debug("GUID Partition Table Header size is too small: %u < %zu\n",
+			le32_to_cpu((*gpt)->header_size),
+			sizeof(gpt_header));
+		goto fail;
+	}
+
 	/* Check the GUID Partition Table CRC */
 	origcrc = le32_to_cpu((*gpt)->header_crc32);
 	(*gpt)->header_crc32 = 0;