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Subject: [PATCH 1/1] ext4: fix potential negative array index in do_split()
From:   Eric Sandeen <sandeen@redhat.com>
To:     "linux-ext4@vger.kernel.org" <linux-ext4@vger.kernel.org>
References: <d08d63e9-8f74-b571-07c7-828b9629ce6a@redhat.com>
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Message-ID: <f53e246b-647c-64bb-16ec-135383c70ad7@redhat.com>
Date:   Wed, 17 Jun 2020 14:19:04 -0500
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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>
---

diff --git a/fs/ext4/namei.c b/fs/ext4/namei.c
index a8aca4772aaa..8b60881f07ee 100644
--- a/fs/ext4/namei.c
+++ b/fs/ext4/namei.c
@@ -1858,7 +1858,7 @@ 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--) {
@@ -1868,8 +1868,18 @@ 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",