ext4_split_extent_at() retries after ext4_ext_insert_extent() fails by
refinding the original extent and restoring its length. That recovery is
only safe for transient resource failures such as -ENOSPC, -EDQUOT, and
-ENOMEM.

When ext4_ext_insert_extent() fails because the extent tree is already
corrupted, ext4_find_extent() can return a leaf path without p_ext.
ext4_split_extent_at() then dereferences path[depth].p_ext while trying to
fix up the original extent length, causing a NULL pointer dereference while
handling a pre-existing filesystem corruption.

Do not enter the recovery path for corruption errors, and validate p_ext
after refinding the extent before touching it. This keeps the recovery path
limited to cases it can actually repair and turns the syzbot-triggered crash
into a proper corruption report.

Fixes: 716b9c23b862 ("ext4: refactor split and convert extents")
Reported-by: syzbot+1ffa5d865557e51cb604@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=1ffa5d865557e51cb604
Signed-off-by: hongao <hongao@uniontech.com>

diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c
index ae3804f36535..aba9947fd151 100644
--- a/fs/ext4/extents.c
+++ b/fs/ext4/extents.c
@@ -3239,6 +3239,9 @@ static struct ext4_ext_path *ext4_split_extent_at(handle_t *handle,
 
 	insert_err = PTR_ERR(path);
 	err = 0;
+	if (insert_err != -ENOSPC && insert_err != -EDQUOT &&
+	    insert_err != -ENOMEM)
+		goto out_path;
 
 	/*
 	 * Get a new path to try to zeroout or fix the extent length.
@@ -3261,6 +3264,13 @@ static struct ext4_ext_path *ext4_split_extent_at(handle_t *handle,
 
 	depth = ext_depth(inode);
 	ex = path[depth].p_ext;
+	if (!ex) {
+		EXT4_ERROR_INODE(inode,
+				 "bad extent address lblock: %lu, depth: %d pblock %lld",
+				 (unsigned long)ee_block, depth, path[depth].p_block);
+		err = -EFSCORRUPTED;
+		goto out;
+	}
 
 fix_extent_len:
 	ex->ee_len = orig_ex.ee_len;
-- 
2.51.0