ext4: avoid use-after-free in ext4_ext_show_leaf()

In ext4_find_extent(), path may be freed by error or be reallocated, so
using a previously saved *ppath may have been freed and thus may trigger
use-after-free, as follows:

ext4_split_extent
  path = *ppath;
  ext4_split_extent_at(ppath)
  path = ext4_find_extent(ppath)
  ext4_split_extent_at(ppath)
    // ext4_find_extent fails to free path
    // but zeroout succeeds
  ext4_ext_show_leaf(inode, path)
    eh = path[depth].p_hdr
    // path use-after-free !!!

Similar to ext4_split_extent_at(), we use *ppath directly as an input to
ext4_ext_show_leaf(). Fix a spelling error by the way.

Same problem in ext4_ext_handle_unwritten_extents(). Since 'path' is only
used in ext4_ext_show_leaf(), remove 'path' and use *ppath directly.

This issue is triggered only when EXT_DEBUG is defined and therefore does
not affect functionality.

Signed-off-by: Baokun Li <libaokun1@huawei.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Ojaswin Mujoo <ojaswin@linux.ibm.com>
Tested-by: Ojaswin Mujoo <ojaswin@linux.ibm.com>
Link: https://patch.msgid.link/20240822023545.1994557-5-libaokun@huaweicloud.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

Author: LiBaokun96

fs/ext4/extents.c

@@ -3327,7 +3327,7 @@ static int ext4_split_extent_at(handle_t *handle,
 }
 
 /*
- * ext4_split_extents() splits an extent and mark extent which is covered
+ * ext4_split_extent() splits an extent and mark extent which is covered
  * by @map as split_flags indicates
  *
  * It may result in splitting the extent into multiple extents (up to three)
@@ -3403,7 +3403,7 @@ static int ext4_split_extent(handle_t *handle,
 			goto out;
 	}
 
-	ext4_ext_show_leaf(inode, path);
+	ext4_ext_show_leaf(inode, *ppath);
 out:
 	return err ? err : allocated;
 }
@@ -3868,14 +3868,13 @@ ext4_ext_handle_unwritten_extents(handle_t *handle, struct inode *inode,
 			struct ext4_ext_path **ppath, int flags,
 			unsigned int allocated, ext4_fsblk_t newblock)
 {
-	struct ext4_ext_path __maybe_unused *path = *ppath;
 	int ret = 0;
 	int err = 0;
 
 	ext_debug(inode, "logical block %llu, max_blocks %u, flags 0x%x, allocated %u\n",
 		  (unsigned long long)map->m_lblk, map->m_len, flags,
 		  allocated);
-	ext4_ext_show_leaf(inode, path);
+	ext4_ext_show_leaf(inode, *ppath);
 
 	/*
 	 * When writing into unwritten space, we should not fail to
@@ -3972,7 +3971,7 @@ ext4_ext_handle_unwritten_extents(handle_t *handle, struct inode *inode,
 	if (allocated > map->m_len)
 		allocated = map->m_len;
 	map->m_len = allocated;
-	ext4_ext_show_leaf(inode, path);
+	ext4_ext_show_leaf(inode, *ppath);
 out2:
 	return err ? err : allocated;
 }