ntfs: Fix index_root heap OOB write in ntfs_ir_to_ib()

ntfs_ir_to_ib copies all entries from index_root into a freshly allocated
index_block_size-byte buffer without verifying that the entries fit in the
available space. The entries in index_root may be larger than the usable
entry space in the index block.

This can cause OOB writes past the end of the allocation.

The validator ntfs_index_root_inconsistent() checks that entries are
self-consistent within the IR value, but never cross-checks them against
index_block_size. There is no bounds check in ntfs_ir_to_ib() before the
memcpy.

Fixing this at the sink in ntfs_ir_to_ib() since
ntfs_index_root_inconsistent() validates the logical consistency of
index_root as a structure and a root with large entries is a structurally
valid root. The bug is a size conflict of ntfs_ir_to_ib().
Also, the validator is called once per inode load in
ntfs_read_locked_inode() while ntfs_ir_to_ib() is only called during a
reparent, a check there adds no overhead to the common path.
Moreover, even a future call path that bypasses the validator would still
be protected.

With NULL as first parameter of ntfs_error(), the volume error flag is
never set by this call, so the device name will be absent from the error
message. In any case, that the caller, ntfs_ir_reparent(), prints an error
message that includes the device name on NULL returns.
I think this is the best solution available without adding
'struct super_block *sb' as a parameter to ntfs_ir_to_ib().

This heap out-of-bounds write is triggered by a crafted filesystem image,
which is not in the kernel threat model, anyway, fixing memory errors would
be nice to keep  things secure.

Fixes: 0a8ac0c1fa ("ntfs: update directory operations")
Signed-off-by: Alexandro Calo <alexandro.calo@nozominetworks.com>
Reviewed-by: Hyunchul Lee <hyc.lee@gmail.com>
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>

fs/ntfs/index.c

@@ -1112,6 +1112,7 @@ static struct index_block *ntfs_ir_to_ib(struct index_root *ir, s64 ib_vcn)
 	struct index_entry *ie_last;
 	char *ies_start, *ies_end;
 	int i;
+	u32 ib_cap;
 
 	ntfs_debug("Entering\n");
 
@@ -1127,6 +1128,16 @@ static struct index_block *ntfs_ir_to_ib(struct index_root *ir, s64 ib_vcn)
 	 * as well, which can never have any data.
 	 */
 	i = (char *)ie_last - ies_start + le16_to_cpu(ie_last->length);
+
+	/* Entries must fit in the allocated index block */
+	ib_cap = le32_to_cpu(ib->index.allocated_size) -
+			le32_to_cpu(ib->index.entries_offset);
+	if ((u32)i > ib_cap) {
+		ntfs_error(NULL, "Entries (%d B) exceed IB capacity", i);
+		kvfree(ib);
+		return NULL;
+	}
+
 	memcpy(ntfs_ie_get_first(&ib->index), ies_start, i);
 
 	ib->index.flags = ir->index.flags;