Fix corrupted OSF partition table parsing

The kernel automatically evaluates partition tables of storage devices.
The code for evaluating OSF partitions contains a bug that leaks data
from kernel heap memory to userspace for certain corrupted OSF
partitions.

In more detail:

  for (i = 0 ; i < le16_to_cpu(label->d_npartitions); i++, partition++) {

iterates from 0 to d_npartitions - 1, where d_npartitions is read from
the partition table without validation and partition is a pointer to an
array of at most 8 d_partitions.

Add the proper and obvious validation.
Signed-off-by: NTimo Warns <warns@pre-sense.de>
Cc: stable@kernel.org
[ Changed the patch trivially to not repeat the whole le16_to_cpu()
  thing, and to use an explicit constant for the magic value '8' ]
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

fs/partitions/osf.c

@@ -10,10 +10,13 @@
 #include "check.h"
 #include "osf.h"
 
+#define MAX_OSF_PARTITIONS 8
+
 int osf_partition(struct parsed_partitions *state)
 {
 	int i;
 	int slot = 1;
+	unsigned int npartitions;
 	Sector sect;
 	unsigned char *data;
 	struct disklabel {
@@ -45,7 +48,7 @@ int osf_partition(struct parsed_partitions *state)
 			u8  p_fstype;
 			u8  p_frag;
 			__le16 p_cpg;
-		} d_partitions[8];
+		} d_partitions[MAX_OSF_PARTITIONS];
 	} * label;
 	struct d_partition * partition;
 
@@ -63,7 +66,12 @@ int osf_partition(struct parsed_partitions *state)
 		put_dev_sector(sect);
 		return 0;
 	}
-	for (i = 0 ; i < le16_to_cpu(label->d_npartitions); i++, partition++) {
+	npartitions = le16_to_cpu(label->d_npartitions);
+	if (npartitions > MAX_OSF_PARTITIONS) {
+		put_dev_sector(sect);
+		return 0;
+	}
+	for (i = 0 ; i < npartitions; i++, partition++) {
 		if (slot == state->limit)
 		        break;
 		if (le32_to_cpu(partition->p_size))