[PATCH 4.9 29/39] ext4: fix potential negative array index in do_split()

Greg Kroah-Hartman
2020-08-24T09:27:14Z

From: Eric Sandeen <sandeen@redhat.com>

[ Upstream commit 5872331b3d91820e14716632ebb56b1399b34fe1 ]

If for any reason a directory passed to do_split() does not have enough
active entries to exceed half the size of the block, we can end up
iterating over all "count" entries without finding a split point.

In this case, count == move, and split will be zero, and we will
attempt a negative index into map[].

Guard against this by detecting this case, and falling back to
split-to-half-of-count instead; in this case we will still have
plenty of space (> half blocksize) in each split block.

Fixes: ef2b02d3e617 ("ext34: ensure do_split leaves enough free space in both blocks")
Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Andreas Dilger <adilger@dilger.ca>
Reviewed-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/f53e246b-647c-64bb-16ec-135383c70ad7@redhat.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
fs/ext4/namei.c | 16 +++++++++++++---
1 file changed, 13 insertions(+), 3 deletions(-)

diff --git a/fs/ext4/namei.c b/fs/ext4/namei.c
index 2d918b43b536e..157dbbe235f90 100644
--- a/fs/ext4/namei.c
+++ b/fs/ext4/namei.c
@@ -1759,7 +1759,7 @@ static struct ext4_dir_entry_2 *do_split(handle_t *handle, struct inode *dir,
blocksize, hinfo, map);
map -= count;
dx_sort_map(map, count);
- /* Split the existing block in the middle, size-wise */
+ /* Ensure that neither split block is over half full */
size = 0;
move = 0;
for (i = count-1; i >= 0; i--) {
@@ -1769,8 +1769,18 @@ static struct ext4_dir_entry_2 *do_split(handle_t *handle, struct inode *dir,
size += map[i].size;
move++;
}
- /* map index at which we will split */
- split = count - move;
+ /*
+ * map index at which we will split
+ *
+ * If the sum of active entries didn't exceed half the block size, just
+ * split it in half by count; each resulting block will have at least
+ * half the space free.
+ */
+ if (i > 0)
+ split = count - move;
+ else
+ split = count/2;
+
hash2 = map[split].hash;
continued = hash2 == map[split - 1].hash;
dxtrace(printk(KERN_INFO "Split block %lu at %x, %i/%i\n",
--
2.25.1