block/cloop: fix offsets[] size off-by-one

cloop stores the number of compressed blocks in the n_blocks header
field.  The file actually contains n_blocks + 1 offsets, where the extra
offset is the end-of-file offset.

The following line in cloop_read_block() results in an out-of-bounds
offsets[] access:

    uint32_t bytes = s->offsets[block_num + 1] - s->offsets[block_num];

This patch allocates and loads the extra offset so that
cloop_read_block() works correctly when the last block is accessed.

Notice that we must free s->offsets[] unconditionally now since there is
always an end-of-file offset.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

3 files changed, 14 insertions, 7 deletions

diff --git a/block/cloop.c b/block/cloop.c
index 55a804f1cc..b6ad50fbb4 100644
--- a/block/cloop.c
+++ b/block/cloop.c
@@ -99,14 +99,14 @@ static int cloop_open(BlockDriverState *bs, QDict *options, int flags,
     s->n_blocks = be32_to_cpu(s->n_blocks);
 
     /* read offsets */
-    if (s->n_blocks > UINT32_MAX / sizeof(uint64_t)) {
+    if (s->n_blocks > (UINT32_MAX - 1) / sizeof(uint64_t)) {
         /* Prevent integer overflow */
         error_setg(errp, "n_blocks %u must be %zu or less",
                    s->n_blocks,
-                   UINT32_MAX / sizeof(uint64_t));
+                   (UINT32_MAX - 1) / sizeof(uint64_t));
         return -EINVAL;
     }
-    offsets_size = s->n_blocks * sizeof(uint64_t);
+    offsets_size = (s->n_blocks + 1) * sizeof(uint64_t);
     if (offsets_size > 512 * 1024 * 1024) {
         /* Prevent ridiculous offsets_size which causes memory allocation to
          * fail or overflows bdrv_pread() size.  In practice the 512 MB
@@ -123,7 +123,7 @@ static int cloop_open(BlockDriverState *bs, QDict *options, int flags,
         goto fail;
     }
 
-    for(i=0;i<s->n_blocks;i++) {
+    for (i = 0; i < s->n_blocks + 1; i++) {
         uint64_t size;
 
         s->offsets[i] = be64_to_cpu(s->offsets[i]);
@@ -243,9 +243,7 @@ static coroutine_fn int cloop_co_read(BlockDriverState *bs, int64_t sector_num,
 static void cloop_close(BlockDriverState *bs)
 {
     BDRVCloopState *s = bs->opaque;
-    if (s->n_blocks > 0) {
-        g_free(s->offsets);
-    }
+    g_free(s->offsets);
     g_free(s->compressed_block);
     g_free(s->uncompressed_block);
     inflateEnd(&s->zstream);
diff --git a/tests/qemu-iotests/075 b/tests/qemu-iotests/075
index d74fb33272..40032c563d 100755
--- a/tests/qemu-iotests/075
+++ b/tests/qemu-iotests/075
@@ -52,6 +52,11 @@ _use_sample_img simple-pattern.cloop.bz2
 $QEMU_IO -c "read 0 512" $TEST_IMG 2>&1 | _filter_qemu_io | _filter_testdir
 
 echo
+echo "== check that the last sector can be read =="
+_use_sample_img simple-pattern.cloop.bz2
+$QEMU_IO -c "read $((1024 * 1024 - 512)) 512" $TEST_IMG 2>&1 | _filter_qemu_io | _filter_testdir
+
+echo
 echo "== block_size must be a multiple of 512 =="
 _use_sample_img simple-pattern.cloop.bz2
 poke_file "$TEST_IMG" "$block_size_offset" "\x00\x00\x02\x01"
diff --git a/tests/qemu-iotests/075.out b/tests/qemu-iotests/075.out
index 911cd3b4d8..5f1d6c120a 100644
--- a/tests/qemu-iotests/075.out
+++ b/tests/qemu-iotests/075.out
@@ -4,6 +4,10 @@ QA output created by 075
 read 512/512 bytes at offset 0
 512 bytes, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
 
+== check that the last sector can be read ==
+read 512/512 bytes at offset 1048064
+512 bytes, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+
 == block_size must be a multiple of 512 ==
 qemu-io: can't open device TEST_DIR/simple-pattern.cloop: block_size 513 must be a multiple of 512
 no file open, try 'help open'