[PATCH 1/1] udf: fix silent AED tagLocation corruption

[Steve Magnani <magnani@ieee.org>]

From: Steven J. Magnani <magnani@ieee.org>

When extending a file, make sure that the pointer to the last written
extent does not get adjusted into the header (tag) portion of an
Allocation Extent Descriptor. Otherwise, a subsequent call to
udf_update_exents() can clobber the AED's tagLocation field, replacing
it with the logical block number of a file extent.

Signed-off-by: Steven J. Magnani <magnani@ieee.org>
---
--- a/fs/udf/inode.c	2020-12-28 20:51:29.000000000 -0600
+++ b/fs/udf/inode.c	2021-01-01 19:20:37.163767576 -0600
@@ -547,11 +547,14 @@ static int udf_do_extend_file(struct ino
 
 		udf_write_aext(inode, last_pos, &last_ext->extLocation,
 				last_ext->extLength, 1);
-		/*
-		 * We've rewritten the last extent but there may be empty
-		 * indirect extent after it - enter it.
-		 */
-		udf_next_aext(inode, last_pos, &tmploc, &tmplen, 0);
+
+		if (new_block_bytes || prealloc_len) {
+			/*
+			 * We've rewritten the last extent but there may be empty
+			 * indirect extent after it - enter it.
+			 */
+			udf_next_aext(inode, last_pos, &tmploc, &tmplen, 0);
+		}
 	}
 
 	/* Managed to do everything necessary? */