[PATCH v2 3/4] udf: fix preallocation discarding before an empty allocation extent

[Matthias Goergens <matthias.goergens@gmail.com>]

udf_discard_prealloc() finds the last extent of a file by peeking at
the next one with udf_next_aext() and stopping when that returns 0.
The peek reads into eloc and elen, and the code then uses them as the
last extent.  But when udf_next_aext() returns 0 they hold no defined
value: if the extent list ends in a continuation descriptor that leads
to an empty allocation extent, they describe that continuation, i.e.
the block of the empty allocation extent, one block long.

The kernel creates such lists itself: when udf_delete_aext() removes
the only descriptor of the last allocation extent in a list, it leaves
that allocation extent empty and the continuation to it in place.

If the last extent is a preallocation, udf_discard_prealloc() deletes
it with udf_delete_aext(), which finds the following allocation extent
empty, so it also removes the continuation and frees that allocation
extent's block.  udf_discard_prealloc() then frees eloc and elen: that
block a second time, instead of the preallocated blocks.  On a space
bitmap the preallocated blocks leak and the free block count drifts.
On an unallocated space table the second free adds a second free
extent for the same block, which is later handed out twice: fsx as run
by generic/091 and generic/263 ends up with two parts of its test file
in one block and reads back bad data.

Peek into separate variables, so that eloc and elen keep the last
extent that udf_next_aext() returned with 1.

Fixes: cfe4c1b25dd6 ("udf: Fix preallocation discarding at indirect extent boundary")
Cc: stable@vger.kernel.org
Suggested-by: Jan Kara <jack@suse.cz>
Signed-off-by: Matthias Goergens <matthias.goergens@gmail.com>
---
Reproducer, with fsx from fstests and mkudffs from udftools:

  truncate --size=2G udf.img
  mkudffs --blocksize=512 --space=unalloctable udf.img
  mount -t udf -o loop udf.img /mnt
  fsx -N 10000 -l 500000 -r 4096 -t 512 -w 512 -Z -R -W /mnt/junk

On the 2 October for_next base (4ad934e7a85e), fsx stopped with READ
BAD DATA after 9852 operations in five runs out of five; with this
patch alone, all 10000 operations completed in five runs out of
five.  With --space=unallocbitmap fsx completed either way.

 fs/udf/truncate.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/fs/udf/truncate.c b/fs/udf/truncate.c
index 0990f94..75151a5 100644
--- a/fs/udf/truncate.c
+++ b/fs/udf/truncate.c
@@ -124,8 +124,8 @@ void udf_discard_prealloc(struct inode *inode)
 {
 	struct extent_position epos = {};
 	struct extent_position prev_epos = {};
-	struct kernel_lb_addr eloc;
-	uint32_t elen;
+	struct kernel_lb_addr eloc, tmpeloc;
+	uint32_t elen, tmpelen;
 	uint64_t lbcount = 0;
 	int8_t etype = -1;
 	struct udf_inode_info *iinfo = UDF_I(inode);
@@ -141,7 +141,8 @@ void udf_discard_prealloc(struct inode *inode)
 
 	/* Find the last extent in the file */
 	while (1) {
-		ret = udf_next_aext(inode, &epos, &eloc, &elen, &tmpetype, 0);
+		ret = udf_next_aext(inode, &epos, &tmpeloc, &tmpelen,
+				    &tmpetype, 0);
 		if (ret < 0)
 			goto out;
 		if (ret == 0)
-- 
2.56.0