partitions: mac: fix handling of bogus partition table

Fix several issues in partition probing:

- The bailout for a bad partoffset must use put_dev_sector(), since the
preceding read_part_sector() succeeded.
- If the partition table claims a silly sector size like 0xfff bytes
(which results in partition table entries straddling sector boundaries),
bail out instead of accessing out-of-bounds memory.
- We must not assume that the partition table contains proper NUL
termination - use strnlen() and strncmp() instead of strlen() and
strcmp().

Cc: [e-mail]
Signed-off-by: Jann Horn <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jens Axboe <[e-mail]>

block/partitions/mac.c

···
 	}
 	secsize = be16_to_cpu(md->block_size);
 	put_dev_sector(sect);
+
+	/*
+	 * If the "block size" is not a power of 2, things get weird - we might
+	 * end up with a partition straddling a sector boundary, so we wouldn't
+	 * be able to read a partition entry with read_part_sector().
+	 * Real block sizes are probably (?) powers of two, so just require
+	 * that.
+	 */
+	if (!is_power_of_2(secsize))
+		return -1;
 	datasize = round_down(secsize, 512);
 	data = read_part_sector(state, datasize / 512, &sect);
 	if (!data)
 		return -1;
 	partoffset = secsize % 512;
-	if (partoffset + sizeof(*part) > datasize)
+	if (partoffset + sizeof(*part) > datasize) {
+		put_dev_sector(sect);
 		return -1;
+	}
 	part = (struct mac_partition *) (data + partoffset);
 	if (be16_to_cpu(part->signature) != MAC_PARTITION_MAGIC) {
 		put_dev_sector(sect);
···
 				int i, l;
 
 				goodness++;
-				l = strlen(part->name);
-				if (strcmp(part->name, "/") == 0)
+				l = strnlen(part->name, sizeof(part->name));
+				if (strncmp(part->name, "/", sizeof(part->name)) == 0)
 					goodness++;
 				for (i = 0; i <= l - 4; ++i) {
 					if (strncasecmp(part->name + i, "root",